Doppler effect

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Doppler_effect 

Change of wavelength caused by motion of the source.

An animation illustrating how the Doppler effect causes a car engine or siren to sound higher in pitch when it is approaching than when it is receding. The red circles represent sound waves.

The Doppler effect or Doppler shift (or simply Doppler, when in context) is the change in frequency of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.

A common example of Doppler shift is the change of pitch heard when a vehicle sounding a horn approaches and recedes from an observer. Compared to the emitted frequency, the received frequency is higher during the approach, identical at the instant of passing by, and lower during the recession.

The reason for the Doppler effect is that when the source of the waves is moving towards the observer, each successive wave crest is emitted from a position closer to the observer than the crest of the previous wave. Therefore, each wave takes slightly less time to reach the observer than the previous wave. Hence, the time between the arrivals of successive wave crests at the observer is reduced, causing an increase in the frequency. While they are traveling, the distance between successive wave fronts is reduced, so the waves "bunch together". Conversely, if the source of waves is moving away from the observer, each wave is emitted from a position farther from the observer than the previous wave, so the arrival time between successive waves is increased, reducing the frequency. The distance between successive wave fronts is then increased, so the waves "spread out".

For waves that propagate in a medium, such as sound waves, the velocity of the observer and of the source are relative to the medium in which the waves are transmitted. The total Doppler effect may therefore result from motion of the source, motion of the observer, or motion of the medium. Each of these effects is analyzed separately. For waves which do not require a medium, such as electromagnetic waves or gravitational waves, only the relative difference in velocity between the observer and the source needs to be considered. When this relative velocity is not negligible compared to the speed of light, a more complicated relativistic Doppler effect arises.

History

Experiment by Buys Ballot (1845) depicted on a wall in Utrecht (2019)

Doppler first proposed this effect in 1842 in his treatise "Über das farbige Licht der Doppelsterne und einiger anderer Gestirne des Himmels" (On the coloured light of the binary stars and some other stars of the heavens). The hypothesis was tested for sound waves by Buys Ballot in 1845. He confirmed that the sound's pitch was higher than the emitted frequency when the sound source approached him, and lower than the emitted frequency when the sound source receded from him. Hippolyte Fizeau discovered independently the same phenomenon on electromagnetic waves in 1848 (in France, the effect is sometimes called "effet Doppler-Fizeau" but that name was not adopted by the rest of the world as Fizeau's discovery was six years after Doppler's proposal). In Britain, John Scott Russell made an experimental study of the Doppler effect (1848).

General

In classical physics, where the speeds of source and the receiver relative to the medium are lower than the velocity of waves in the medium, the relationship between observed frequency ${\displaystyle f}$ and emitted frequency ${\displaystyle f_{\text{0}}}$ is given by:

$${\displaystyle f=\left({\frac {c\pm v_{\text{r}}}{c\pm v_{\text{s}}}}\right)f_{0}}$$

where

• ${\displaystyle c}$ is the propagation speed of waves in the medium;
• ${\displaystyle v_{\text{r}}}$ is the speed of the receiver relative to the medium, added to ${\displaystyle c}$ if the receiver is moving towards the source, subtracted if the receiver is moving away from the source;
• ${\displaystyle v_{\text{s}}}$ is the speed of the source relative to the medium, added to ${\displaystyle c}$ if the source is moving away from the receiver, subtracted if the source is moving towards the receiver.

Note this relationship predicts that the frequency will decrease if either source or receiver is moving away from the other.

Equivalently, under the assumption that the source is either directly approaching or receding from the observer:

$${\displaystyle {\frac {f}{v_{wr}}}={\frac {f_{0}}{v_{ws}}}={\frac {1}{\lambda }}}$$

where

• ${\displaystyle v_{wr}}$ is the wave's velocity relative to the receiver;
• ${\displaystyle v_{ws}}$ is the wave's velocity relative to the source;
• ${\displaystyle \lambda }$ is the wavelength.

If the source approaches the observer at an angle (but still with a constant velocity), the observed frequency that is first heard is higher than the object's emitted frequency. Thereafter, there is a monotonic decrease in the observed frequency as it gets closer to the observer, through equality when it is coming from a direction perpendicular to the relative motion (and was emitted at the point of closest approach; but when the wave is received, the source and observer will no longer be at their closest), and a continued monotonic decrease as it recedes from the observer. When the observer is very close to the path of the object, the transition from high to low frequency is very abrupt. When the observer is far from the path of the object, the transition from high to low frequency is gradual.

If the speeds ${\displaystyle v_{\text{s}}}$ and ${\displaystyle v_{\text{r}}\,}$ are small compared to the speed of the wave, the relationship between observed frequency ${\displaystyle f}$ and emitted frequency ${\displaystyle f_{\text{0}}}$ is approximately

Observed frequency	Change in frequency
${\displaystyle f=\left(1+{\frac {\Delta v}{c}}\right)f_{0}}$	${\displaystyle \Delta f={\frac {\Delta v}{c}}f_{0}}$

where

• ${\displaystyle \Delta f=f-f_{0}}$
• ${\displaystyle \Delta v=-(v_{\text{r}}-v_{\text{s}})}$ is the opposite of the velocity of the receiver relative to the source: it is positive when the source and the receiver are moving towards each other.

Proof

Given ${\displaystyle f=\left({\frac {c+v_{\text{r}}}{c+v_{\text{s}}}}\right)f_{0}}$

we divide for ${\displaystyle c}$

$${\displaystyle f=\left({\frac {1+{\frac {v_{\text{r}}}{c}}}{1+{\frac {v_{\text{s}}}{c}}}}\right)f_{0}=\left(1+{\frac {v_{\text{r}}}{c}}\right)\left({\frac {1}{1+{\frac {v_{\text{s}}}{c}}}}\right)f_{0}}$$

Since ${\displaystyle {\frac {v_{\text{s}}}{c}}\ll 1}$ we can substitute the geometric expansion:

$${\displaystyle {\frac {1}{1+{\frac {v_{\text{s}}}{c}}}}\approx 1-{\frac {v_{\text{s}}}{c}}}$$

• 

  Stationary sound source produces sound waves at a constant frequency f, and the wave-fronts propagate symmetrically away from the source at a constant speed c. The distance between wave-fronts is the wavelength. All observers will hear the same frequency, which will be equal to the actual frequency of the source where f = f₀.

• 

  The same sound source is radiating sound waves at a constant frequency in the same medium. However, now the sound source is moving with a speed υ_s = 0.7 c. Since the source is moving, the centre of each new wavefront is now slightly displaced to the right. As a result, the wave-fronts begin to bunch up on the right side (in front of) and spread further apart on the left side (behind) of the source. An observer in front of the source will hear a higher frequency f = c + 0/c – 0.7c f₀ = 3.33 f₀ and an observer behind the source will hear a lower frequency f = c − 0/c + 0.7c f₀ = 0.59 f₀.

• 

  Now the source is moving at the speed of sound in the medium (υ_s = c). The wave fronts in front of the source are now all bunched up at the same point. As a result, an observer in front of the source will detect nothing until the source arrives and an observer behind the source will hear a lower frequency f = c – 0/c + c f₀ = 0.5 f₀.

• 

  The sound source has now surpassed the speed of sound in the medium, and is traveling at 1.4 c. Since the source is moving faster than the sound waves it creates, it actually leads the advancing wavefront. The sound source will pass by a stationary observer before the observer hears the sound. As a result, an observer in front of the source will detect nothing and an observer behind the source will hear a lower frequency f = c – 0/c + 1.4c f₀ = 0.42 f₀.

Consequences

With an observer stationary relative to the medium, if a moving source is emitting waves with an actual frequency ${\displaystyle f_{0}}$ (in this case, the wavelength is changed, the transmission velocity of the wave keeps constant; note that the transmission velocity of the wave does not depend on the velocity of the source), then the observer detects waves with a frequency ${\displaystyle f}$ given by

$${\displaystyle f=\left({\frac {c}{c\pm v_{\text{s}}}}\right)f_{0}}$$

A similar analysis for a moving observer and a stationary source (in this case, the wavelength keeps constant, but due to the motion, the rate at which the observer receives waves and hence the transmission velocity of the wave [with respect to the observer] is changed) yields the observed frequency:

$${\displaystyle f=\left({\frac {c\pm v_{\text{r}}}{c}}\right)f_{0}}$$

Assuming a stationary observer and a source moving at the speed of sound, the Doppler equation predicts a perceived momentary infinite frequency by an observer in front of a source traveling at the speed of sound. All the peaks are at the same place, so the wavelength is zero and the frequency is infinite. This overlay of all the waves produces a shock wave which for sound waves is known as a sonic boom.

When the source moves faster than the wave speed the source outruns the wave. The equation gives negative frequency values, which have no physical sense in this context (no sound at all will be heard by the observer until the source passes past them).

Lord Rayleigh predicted the following effect in his classic book on sound: if the observer were moving from the (stationary) source at twice the speed of sound, a musical piece previously emitted by that source would be heard in correct tempo and pitch, but as if played backwards.

Applications

Acoustic Doppler current profiler

An acoustic Doppler current profiler (ADCP) is a hydroacoustic current meter similar to a sonar, used to measure water current velocities over a depth range using the Doppler effect of sound waves scattered back from particles within the water column. The term ADCP is a generic term for all acoustic current profilers, although the abbreviation originates from an instrument series introduced by RD Instruments in the 1980s. The working frequencies range of ADCPs range from 38 kHz to several Megahertz. The device used in the air for wind speed profiling using sound is known as SODAR and works with the same underlying principles.

Robotics

Dynamic real-time path planning in robotics to aid the movement of robots in a sophisticated environment with moving obstacles often take help of Doppler effect.^[10] Such applications are specially used for competitive robotics where the environment is constantly changing, such as robosoccer.

Sirens

A siren on a passing emergency vehicle will start out higher than its stationary pitch, slide down as it passes, and continue lower than its stationary pitch as it recedes from the observer. Astronomer John Dobson explained the effect thus:

The reason the siren slides is because it doesn't hit you.

In other words, if the siren approached the observer directly, the pitch would remain constant, at a higher than stationary pitch, until the vehicle hit him, and then immediately jump to a new lower pitch. Because the vehicle passes by the observer, the radial velocity does not remain constant, but instead varies as a function of the angle between his line of sight and the siren's velocity:

$${\displaystyle v_{\text{radial}}=v_{\text{s}}\cos(\theta )}$$

where ${\displaystyle \theta }$ is the angle between the object's forward velocity and the line of sight from the object to the observer.

Astronomy

Main article: Relativistic Doppler effect

 

Redshift of spectral lines in the optical spectrum of a supercluster of distant galaxies (right), as compared to that of the Sun (left)

The Doppler effect for electromagnetic waves such as light is of widespread use in astronomy to measure the speed at which stars and galaxies are approaching or receding from us, resulting in so called blueshift or redshift, respectively. This may be used to detect if an apparently single star is, in reality, a close binary, to measure the rotational speed of stars and galaxies, or to detect exoplanets. This effect typically happens on a very small scale; there would not be a noticeable difference in visible light to the unaided eye. The use of the Doppler effect in astronomy depends on knowledge of precise frequencies of discrete lines in the spectra of stars.

Among the nearby stars, the largest radial velocities with respect to the Sun are +308 km/s (BD-15°4041, also known as LHS 52, 81.7 light-years away) and −260 km/s (Woolley 9722, also known as Wolf 1106 and LHS 64, 78.2 light-years away). Positive radial velocity means the star is receding from the Sun, negative that it is approaching.

Redshift is also used to measure the expansion of space, but this is not truly a Doppler effect. Rather, redshifting due to the expansion of space is known as cosmological redshift, which can be derived purely from the Robertson-Walker metric under the formalism of general relativity. Having said this, it also happens that there are detectable Doppler effects on cosmological scales, which, if incorrectly interpreted as cosmological in origin, lead to the observation of redshift-space distortions.

Radar

Main article: Doppler radar

 

U.S. Army soldier using a radar gun, an application of Doppler radar, to catch speeding violators.

The Doppler effect is used in some types of radar, to measure the velocity of detected objects. A radar beam is fired at a moving target — e.g. a motor car, as police use radar to detect speeding motorists — as it approaches or recedes from the radar source. Each successive radar wave has to travel farther to reach the car, before being reflected and re-detected near the source. As each wave has to move farther, the gap between each wave increases, increasing the wavelength. In some situations, the radar beam is fired at the moving car as it approaches, in which case each successive wave travels a lesser distance, decreasing the wavelength. In either situation, calculations from the Doppler effect accurately determine the car's velocity. Moreover, the proximity fuze, developed during World War II, relies upon Doppler radar to detonate explosives at the correct time, height, distance, etc.

Because the doppler shift affects the wave incident upon the target as well as the wave reflected back to the radar, the change in frequency observed by a radar due to a target moving at relative velocity ${\displaystyle \Delta v}$ is twice that from the same target emitting a wave:

$${\displaystyle \Delta f={\frac {2\Delta v}{c}}f_{0}.}$$

Medical

Main article: Doppler ultrasonography

 

Colour flow ultrasonography (Doppler) of a carotid artery – scanner and screen

An echocardiogram can, within certain limits, produce an accurate assessment of the direction of blood flow and the velocity of blood and cardiac tissue at any arbitrary point using the Doppler effect. One of the limitations is that the ultrasound beam should be as parallel to the blood flow as possible. Velocity measurements allow assessment of cardiac valve areas and function, abnormal communications between the left and right side of the heart, leaking of blood through the valves (valvular regurgitation), and calculation of the cardiac output. Contrast-enhanced ultrasound using gas-filled microbubble contrast media can be used to improve velocity or other flow-related medical measurements.

Although "Doppler" has become synonymous with "velocity measurement" in medical imaging, in many cases it is not the frequency shift (Doppler shift) of the received signal that is measured, but the phase shift (when the received signal arrives).

Velocity measurements of blood flow are also used in other fields of medical ultrasonography, such as obstetric ultrasonography and neurology. Velocity measurement of blood flow in arteries and veins based on Doppler effect is an effective tool for diagnosis of vascular problems like stenosis.

Flow measurement

Instruments such as the laser Doppler velocimeter (LDV), and acoustic Doppler velocimeter (ADV) have been developed to measure velocities in a fluid flow. The LDV emits a light beam and the ADV emits an ultrasonic acoustic burst, and measure the Doppler shift in wavelengths of reflections from particles moving with the flow. The actual flow is computed as a function of the water velocity and phase. This technique allows non-intrusive flow measurements, at high precision and high frequency.

Velocity profile measurement

Developed originally for velocity measurements in medical applications (blood flow), Ultrasonic Doppler Velocimetry (UDV) can measure in real time complete velocity profile in almost any liquids containing particles in suspension such as dust, gas bubbles, emulsions. Flows can be pulsating, oscillating, laminar or turbulent, stationary or transient. This technique is fully non-invasive.

Satellites

Possible Doppler shifts in dependence of the elevation angle (LEO: orbit altitude ${\displaystyle h}$ = 750 km). Fixed ground station.

Geometry for Doppler effects. Variables: ${\displaystyle v_{\text{mob}}}$ is the velocity of the mobile station, ${\displaystyle v_{\text{Sat}}}$ is the velocity of the satellite, ${\displaystyle v_{\text{rel,sat}}}$ is the relative speed of the satellite, ${\displaystyle \phi }$ is the elevation angle of the satellite and ${\displaystyle \theta }$ is the driving direction with respect to the satellite.

Doppler effect on the mobile channel. Variables: ${\displaystyle f_{c}={\frac {c}{\lambda _{\rm {c}}}}}$ is the carrier frequency, ${\displaystyle f_{\rm {D,max}}={\frac {v_{\rm {mob}}}{\lambda _{\rm {c}}}}}$ is the maximum Doppler shift due to the mobile station moving (see Doppler Spread) and ${\displaystyle f_{\rm {D,Sat}}}$ is the additional Doppler shift due to the satellite moving.

Satellite navigation

Main article: Satellite navigation

The Doppler shift can be exploited for satellite navigation such as in Transit and DORIS.

Satellite communication

Main article: Satellite communication

Doppler also needs to be compensated in satellite communication. Fast moving satellites can have a Doppler shift of dozens of kilohertz relative to a ground station. The speed, thus magnitude of Doppler effect, changes due to earth curvature. Dynamic Doppler compensation, where the frequency of a signal is changed progressively during transmission, is used so the satellite receives a constant frequency signal. After realizing that the Doppler shift had not been considered before launch of the Huygens probe of the 2005 Cassini–Huygens mission, the probe trajectory was altered to approach Titan in such a way that its transmissions traveled perpendicular to its direction of motion relative to Cassini, greatly reducing the Doppler shift.

Doppler shift of the direct path can be estimated by the following formula:

$${\displaystyle f_{\rm {D,dir}}={\frac {v_{\rm {mob}}}{\lambda _{\rm {c}}}}\cos \phi \cos \theta }$$

where ${\displaystyle v_{\text{mob}}}$ is the velocity of the mobile station, ${\displaystyle \lambda _{\rm {c}}}$ is the wavelength of the carrier, ${\displaystyle \phi }$ is the elevation angle of the satellite and ${\displaystyle \theta }$ is the driving direction with respect to the satellite.

The additional Doppler shift due to the satellite moving can be described as:

$${\displaystyle f_{\rm {D,sat}}={\frac {v_{\rm {rel,sat}}}{\lambda _{\rm {c}}}}}$$

where ${\displaystyle v_{\rm {rel,sat}}}$ is the relative speed of the satellite.

Audio

The Leslie speaker, most commonly associated with and predominantly used with the famous Hammond organ, takes advantage of the Doppler effect by using an electric motor to rotate an acoustic horn around a loudspeaker, sending its sound in a circle. This results at the listener's ear in rapidly fluctuating frequencies of a keyboard note.

Vibration measurement

A laser Doppler vibrometer (LDV) is a non-contact instrument for measuring vibration. The laser beam from the LDV is directed at the surface of interest, and the vibration amplitude and frequency are extracted from the Doppler shift of the laser beam frequency due to the motion of the surface.

Developmental biology

During the segmentation of vertebrate embryos, waves of gene expression sweep across the presomitic mesoderm, the tissue from which the precursors of the vertebrae (somites) are formed. A new somite is formed upon arrival of a wave at the anterior end of the presomitic mesoderm. In zebrafish, it has been shown that the shortening of the presomitic mesoderm during segmentation leads to a Doppler-like effect as the anterior end of the tissue moves into the waves. This effect contributes to the period of segmentation.

Inverse Doppler effect

Since 1968 scientists such as Victor Veselago have speculated about the possibility of an inverse Doppler effect. The size of the Doppler shift depends on the refractive index of the medium a wave is traveling through. But some materials are capable of negative refraction, which should lead to a Doppler shift that works in a direction opposite that of a conventional Doppler shift. First experiment that detected this effect was conducted by Nigel Seddon and Trevor Bearpark in Bristol, United Kingdom in 2003. Later inverse Doppler effect was observed in some inhomogeneous materials and predicted inside Vavilov–Cherenkov cone.

Coma

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Coma

Coma
Specialty	Neurology, psychiatry
Symptoms	Unconsciousness
Complications	Persistent vegetative state, death
Duration	Can vary from a few days to several years (longest recorded is 42 years)

A coma is a deep state of prolonged unconsciousness in which a person cannot be awakened, fails to respond normally to painful stimuli, light, or sound, lacks a normal wake-sleep cycle and does not initiate voluntary actions. Coma patients exhibit a complete absence of wakefulness and are unable to consciously feel, speak or move. Comas can be derived by natural causes, or can be medically induced.

Clinically, a coma can be defined as the inability consistently to follow a one-step command. It can also be defined as a score of ≤ 8 on the Glasgow Coma Scale (GCS) lasting ≥ 6 hours. For a patient to maintain consciousness, the components of wakefulness and awareness must be maintained. Wakefulness describes the quantitative degree of consciousness, whereas awareness relates to the qualitative aspects of the functions mediated by the cortex, including cognitive abilities such as attention, sensory perception, explicit memory, language, the execution of tasks, temporal and spatial orientation and reality judgment. From a neurological perspective, consciousness is maintained by the activation of the cerebral cortex—the gray matter that forms the outer layer of the brain—and by the reticular activating system (RAS), a structure located within the brainstem.

Etymology

The term 'coma', from the Greek κῶμα koma, meaning deep sleep, had already been used in the Hippocratic corpus (Epidemica) and later by Galen (second century AD). Subsequently, it was hardly used in the known literature up to the middle of the 17th century. The term is found again in Thomas Willis' (1621–1675) influential De anima brutorum (1672), where lethargy (pathological sleep), 'coma' (heavy sleeping), carus (deprivation of the senses) and apoplexy (into which carus could turn and which he localized in the white matter) are mentioned. The term carus is also derived from Greek, where it can be found in the roots of several words meaning soporific or sleepy. It can still be found in the root of the term 'carotid'. Thomas Sydenham (1624–89) mentioned the term 'coma' in several cases of fever (Sydenham, 1685).

Signs and symptoms

General symptoms of a person in a comatose state are:

• Inability to voluntarily open the eyes
• A non-existent sleep-wake cycle
• Lack of response to physical (painful) or verbal stimuli
• Depressed brainstem reflexes, such as pupils not responding to light
• Irregular breathing
• Scores between 3 and 8 on the Glasgow Coma Scale

Causes

Many types of problems can cause a coma. Forty percent of comatose states result from drug poisoning. Certain drug use under certain conditions can damage or weaken the synaptic functioning in the ascending reticular activating system (ARAS) and keep the system from properly functioning to arouse the brain. Secondary effects of drugs, which include abnormal heart rate and blood pressure, as well as abnormal breathing and sweating, may also indirectly harm the functioning of the ARAS and lead to a coma. Given that drug poisoning is the cause for a large portion of patients in a coma, hospitals first test all comatose patients by observing pupil size and eye movement, through the vestibular-ocular reflex. (See Diagnosis below.)

The second most common cause of coma, which makes up about 25% of cases, is lack of oxygen, generally resulting from cardiac arrest. The Central Nervous System (CNS) requires a great deal of oxygen for its neurons. Oxygen deprivation in the brain, also known as hypoxia, causes sodium and calcium from outside of the neurons to decrease and intracellular calcium to increase, which harms neuron communication. Lack of oxygen in the brain also causes ATP exhaustion and cellular breakdown from cytoskeleton damage and nitric oxide production.

Twenty percent of comatose states result from the side effects of a stroke. During a stroke, blood flow to part of the brain is restricted or blocked. An ischemic stroke, brain hemorrhage, or tumor may cause restriction of blood flow. Lack of blood to cells in the brain prevents oxygen from getting to the neurons, and consequently causes cells to become disrupted and die. As brain cells die, brain tissue continues to deteriorate, which may affect the functioning of the ARAS.

The remaining 15% of comatose cases result from trauma, excessive blood loss, malnutrition, hypothermia, hyperthermia, hyperammonemia, abnormal glucose levels, and many other biological disorders. Furthermore, studies show that 1 out of 8 patients with traumatic brain injury experience a comatose state.

Pathophysiology

Injury to either or both of the cerebral cortex or the reticular activating system (RAS) is sufficient to cause a person to enter coma.

The cerebral cortex is the outer layer of neural tissue of the cerebrum of the brain. The cerebral cortex is composed of gray matter which consists of the nuclei of neurons, whereas the inner portion of the cerebrum is composed of white matter and is composed of the axons of neuron. White matter is responsible for perception, relay of the sensory input via the thalamic pathway, and many other neurological functions, including complex thinking.

The RAS, on the other hand, is a more primitive structure in the brainstem which includes the reticular formation (RF). The RAS has two tracts, the ascending and descending tract. The ascending tract, or ascending reticular activating system (ARAS), is made up of a system of acetylcholine-producing neurons, and works to arouse and wake up the brain. Arousal of the brain begins from the RF, through the thalamus, and then finally to the cerebral cortex. Any impairment in ARAS functioning, a neuronal dysfunction, along the arousal pathway stated directly above, prevents the body from being aware of its surroundings. Without the arousal and consciousness centers, the body cannot awaken, remaining in a comatose state.

The severity and mode of onset of coma depends on the underlying cause. There are two main subdivisions of a coma: structural and diffuse neuronal. A structural cause, for example, is brought upon by a mechanical force that brings about cellular damage, such as physical pressure or a blockage in neural transmission. While a diffuse cause is limited to aberrations of cellular function, that fall under a metabolic or toxic subgroup. Toxin-induced comas are caused by extrinsic substances, whereas metabolic-induced comas are caused by intrinsic processes, such as body thermoregulation or ionic imbalances(e.g. sodium). For instance, severe hypoglycemia (low blood sugar) or hypercapnia (increased carbon dioxide levels in the blood) are examples of a metabolic diffuse neuronal dysfunction. Hypoglycemia or hypercapnia initially cause mild agitation and confusion, but progress to obtundation, stupor, and finally, complete unconsciousness. In contrast, coma resulting from a severe traumatic brain injury or subarachnoid hemorrhage can be instantaneous. The mode of onset may therefore be indicative of the underlying cause.

Structural and diffuse causes of coma are not isolated from one another, as one can lead to the other in some situations. For instance, coma induced by a diffuse metabolic process, such as hypoglycemia, can result in a structural coma if it is not resolved. Another example is if cerebral edema, a diffuse dysfunction, leads to ischemia of the brainstem, a structural issue, due to the blockage of the circulation in the brain.

Diagnosis

Although diagnosis of coma is simple, investigating the underlying cause of onset can be rather challenging. As such, after gaining stabilization of the patient's airways, breathing and circulation (the basic ABCs) various diagnostic tests, such as physical examinations and imaging tools (CT scan, MRI, etc.) are employed to access the underlying cause of the coma.

When an unconscious person enters a hospital, the hospital utilizes a series of diagnostic steps to identify the cause of unconsciousness. According to Young, the following steps should be taken when dealing with a patient possibly in a coma:

1. Perform a general examination and medical history check
2. Make sure the patient is in an actual comatose state and is not in a locked-in state or experiencing psychogenic unresponsiveness. Patients with locked-in syndrome present with voluntary movement of their eyes, whereas patients with psychogenic comas demonstrate active resistance to passive opening of the eyelids, with the eyelids closing abruptly and completely when the lifted upper eyelid is released (rather than slowly, asymmetrically and incompletely as seen in comas due to organic causes).
3. Find the site of the brain that may be causing coma (e.g., brainstem, back of brain...) and assess the severity of the coma with the Glasgow Coma Scale
4. Take blood work to see if drugs were involved or if it was a result of hypoventilation/hyperventilation
5. Check for levels of serum glucose, calcium, sodium, potassium, magnesium, phosphate, urea, and creatinine
6. Perform brain scans to observe any abnormal brain functioning using either CT or MRI scans
7. Continue to monitor brain waves and identify seizures of patient using EEGs

Initial evaluation

In the initial assessment of coma, it is common to gauge the level of consciousness on the AVPU (alert, vocal stimuli, painful stimuli, unresponsive) scale by spontaneously exhibiting actions and, assessing the patient's response to vocal and painful stimuli. More elaborate scales, such as the Glasgow Coma Scale, quantify an individual's reactions such as eye opening, movement and verbal response in order to indicate their extent of brain injury. The patient's score can vary from a score of 3 (indicating severe brain injury and death) to 15 (indicating mild or no brain injury).

In those with deep unconsciousness, there is a risk of asphyxiation as the control over the muscles in the face and throat is diminished. As a result, those presenting to a hospital with coma are typically assessed for this risk ("airway management"). If the risk of asphyxiation is deemed high, doctors may use various devices (such as an oropharyngeal airway, nasopharyngeal airway or endotracheal tube) to safeguard the airway.

Imaging and testing

Imaging basically encompasses computed tomography (CAT or CT) scan of the brain, or MRI for example, and is performed to identify specific causes of the coma, such as hemorrhage in the brain or herniation of the brain structures. Special tests such as an EEG can also show a lot about the activity level of the cortex such as semantic processing, presence of seizures, and are important available tools not only for the assessment of the cortical activity but also for predicting the likelihood of the patient's awakening. The autonomous responses such as the skin conductance response may also provide further insight on the patient's emotional processing.

In the treatment of traumatic brain injury (TBI), there are 4 examination methods that have proved useful: skull x-ray, angiography, computed tomography (CT), and magnetic resonance imaging (MRI). The skull x-ray can detect linear fractures, impression fractures (expression fractures) and burst fractures. Angiography is used on rare occasions for TBIs i.e. when there is suspicion of an aneurysm, carotid sinus fistula, traumatic vascular occlusion, and vascular dissection. A CT can detect changes in density between the brain tissue and hemorrhages like subdural and intracerebral hemorrhages. MRIs are not the first choice in emergencies because of the long scanning times and because fractures cannot be detected as well as CT. MRIs are used for the imaging of soft tissues and lesions in the posterior fossa which cannot be found with the use of CT.

Body movements

Assessment of the brainstem and cortical function through special reflex tests such as the oculocephalic reflex test (doll's eyes test), oculovestibular reflex test (cold caloric test), corneal reflex, and the gag reflex. Reflexes are a good indicator of what cranial nerves are still intact and functioning and is an important part of the physical exam. Due to the unconscious status of the patient, only a limited number of the nerves can be assessed. These include the cranial nerves number 2 (CN II), number 3 (CN III), number 5 (CN V), number 7 (CN VII), and cranial nerves 9 and 10 (CN IX, CN X).

Type of reflex	Description
Oculocephalic reflex	Oculocephalic reflex, also known as the doll's eye, is performed to assess the integrity of the brainstem. Patient's eyelids are gently elevated and the cornea is visualized. The patient's head is then moved to the patient's left, to observe whether the eyes stay or deviate toward the patient's right; same maneuver is attempted on the opposite side. If the patient's eyes move in a direction opposite to the direction of the rotation of the head, then the patient is said to have an intact brainstem. However, failure of both eyes to move to one side can indicate damage or destruction of the affected side. In special cases, where only one eye deviates and the other does not, this often indicates a lesion (or damage) of the medial longitudinal fasciculus (MLF), which is a brainstem nerve tract.
Pupillary light reflex	Pupil reaction to light is important because it shows an intact retina, and cranial nerve number 2 (CN II) If pupils are reactive to light, then that also indicates that the cranial nerve number 3 (CN III) (or at least its parasympathetic fibers) are intact.
Oculovestibular reflex (Cold Caloric Test)	Caloric reflex test also evaluates both cortical and brainstem function Cold water is injected into one ear and the patient is observed for eye movement If the patient's eyes slowly deviate toward the ear where the water was injected, then the brainstem is intact, however failure to deviate toward the injected ear indicates damage of the brainstem on that side. The cortex is responsible for a rapid nystagmus away from this deviated position and is often seen in patients who are conscious or merely lethargic.
Corneal reflex	The corneal reflex assesses the proper function of the trigeminal nerve (CN 5) and facial nerve (CN 7), and is present at infancy. Lightly touching the cornea with a tissue or cotton swab induces a rapid blink reflex of both eyes. Touching the sclera or eyelashes, presenting a light flash, or stimulating the supraorbital nerve will induce a less rapid but still reliable response. Those in a comatose state will have altered corneal reflex depending on the severity of their unconscious and the location of their lesion.
Gag reflex	The gag, or pharyngeal, reflex is centered in the medulla and consists of the reflexive motor response of pharyngeal elevation and constriction with tongue retraction in response to sensory stimulation of the pharyngeal wall, posterior tongue, tonsils, or faucial pillars. This reflex is examined by touching the posterior pharynx with the soft tip of a cotton applicator and visually inspecting for elevation of the pharynx. Those in comatose states will often demonstrate poor gag reflexes if there has been damage to their glossopharyngeal (CN 9) or vagus nerve (CN 10).

Decorticate posturing, indicating a lesion at the red nucleus or above. This positioning is stereotypical for upper brain stem, or cortical damage. The other variant is decerebrate posturing, not seen in this picture.

Assessment of posture and physique is the next step. It involves general observation about the patient's positioning. There are often two stereotypical postures seen in comatose patients. Decorticate posturing is a stereotypical posturing in which the patient has arms flexed at the elbow, and arms adducted toward the body, with both legs extended. Decerebrate posturing is a stereotypical posturing in which the legs are similarly extended (stretched), but the arms are also stretched (extended at the elbow). The posturing is critical since it indicates where the damage is in the central nervous system. A decorticate posturing indicates a lesion (a point of damage) at or above the red nucleus, whereas a decerebrate posturing indicates a lesion at or below the red nucleus. In other words, a decorticate lesion is closer to the cortex, as opposed to a decerebrate posturing which indicates that the lesion is closer to the brainstem.

Pupil size

Pupil assessment is often a critical portion of a comatose examination, as it can give information as to the cause of the coma; the following table is a technical, medical guideline for common pupil findings and their possible interpretations:

Pupil sizes (left eye vs. right eye)	Possible interpretation
	Normal eye with two pupils equal in size and reactive to light. This means that the patient is probably not in a coma and is probably lethargic, under influence of a drug, or sleeping.
	"Pinpoint" pupils indicate heroin or opiate overdose, which can be responsible for a patient's coma. The pinpoint pupils are still reactive to light bilaterally (in both eyes, not just one). Another possibility is damage to the pons.
	One pupil is dilated and unreactive, while the other is normal (in this case, the right eye is dilated, while the left eye is normal in size). This could mean damage to the oculomotor nerve (cranial nerve number 3, CN III) on the right side, or indicate the possibility of vascular involvement.
	Both pupils are dilated and unreactive to light. This could be due to overdose of certain medications, hypothermia or severe anoxia (lack of oxygen).

Severity

Main article: Coma scale

A coma can be classified as (1) supratentorial (above Tentorium cerebelli), (2) infratentorial (below Tentorium cerebelli), (3) metabolic or (4) diffused. This classification is merely dependent on the position of the original damage that caused the coma, and does not correlate with severity or the prognosis. The severity of coma impairment however is categorized into several levels. Patients may or may not progress through these levels. In the first level, the brain responsiveness lessens, normal reflexes are lost, the patient no longer responds to pain and cannot hear.

The Rancho Los Amigos Scale is a complex scale that has eight separate levels, and is often used in the first few weeks or months of coma while the patient is under closer observation, and when shifts between levels are more frequent.

Treatment

Treatment for people in a coma will depend on the severity and cause of the comatose state. Upon admittance to an emergency department, coma patients will usually be placed in an Intensive Care Unit (ICU) immediately, where maintenance of the patient's respiration and circulation become a first priority. Stability of their respiration and circulation is sustained through the use of intubation, ventilation, administration of intravenous fluids or blood and other supportive care as needed.

Continued care

Once a patient is stable and no longer in immediate danger, there may be a shift of priority from stabilizing the patient to maintaining the state of their physical wellbeing. Moving patients every 2–3 hours by turning them side to side is crucial to avoiding bed sores as a result of being confined to a bed. Moving patients through the use of physical therapy also aids in preventing atelectasis, contractures or other orthopedic deformities which would interfere with a coma patient's recovery.

Pneumonia is also common in coma patients due to their inability to swallow which can then lead to aspiration. A coma patient's lack of a gag reflex and use of a feeding tube can result in food, drink or other solid organic matter being lodged within their lower respiratory tract (from the trachea to the lungs). This trapping of matter in their lower respiratory tract can ultimately lead to infection, resulting in aspiration pneumonia.

Coma patients may also deal with restlessness or seizures. As such, soft cloth restraints may be used to prevent them from pulling on tubes or dressings and side rails on the bed should be kept up to prevent patients from falling.

Caregivers

Coma has a wide variety of emotional reactions from the family members of the affected patients, as well as the primary care givers taking care of the patients. Research has shown that the severity of injury causing coma was found to have no significant impact compared to how much time has passed since the injury occurred. Common reactions, such as desperation, anger, frustration, and denial are possible. The focus of the patient care should be on creating an amicable relationship with the family members or dependents of a comatose patient as well as creating a rapport with the medical staff. Although there is heavy importance of a primary care taker, secondary care takers can play a supporting role to temporarily relieve the primary care taker's burden of tasks.

Prognosis

Comas can last from several days to, in particularly extreme cases, years. Some patients eventually gradually come out of the coma, some progress to a vegetative state, and others die. Some patients who have entered a vegetative state go on to regain a degree of awareness; and in some cases may remain in vegetative state for years or even decades (the longest recorded period is 42 years).

Predicted chances of recovery will differ depending on which techniques were used to measure the patient's severity of neurological damage. Predictions of recovery are based on statistical rates, expressed as the level of chance the person has of recovering. Time is the best general predictor of a chance of recovery. For example, after four months of coma caused by brain damage, the chance of partial recovery is less than 15%, and the chance of full recovery is very low.

The outcome for coma and vegetative state depends on the cause, location, severity and extent of neurological damage. A deeper coma alone does not necessarily mean a slimmer chance of recovery; similarly, a milder coma does not indicate a higher chance of recovery. The most common cause of death for a person in a vegetative state is secondary infection such as pneumonia, which can occur in patients who lie still for extended periods.

Recovery

People may emerge from a coma with a combination of physical, intellectual, and psychological difficulties that need special attention. It is common for coma patients to awaken in a profound state of confusion and experience dysarthria, the inability to articulate any speech. Recovery is usually gradual. In the first days, the patient may only awaken for a few minutes, with increased duration of wakefulness as their recovery progresses, and they may eventually recover full awareness. That said, some patients may never progress beyond very basic responses.

There are reports of people coming out of a coma after long periods of time. After 19 years in a minimally conscious state, Terry Wallis spontaneously began speaking and regained awareness of his surroundings.

A man with brain-damage and trapped in a coma-like state for six years, was brought back to consciousness in 2003 by doctors who planted electrodes deep inside his brain. The method, called deep brain stimulation (DBS), successfully roused communication, complex movement and eating ability in the 38-year-old American man with a traumatic brain injury. His injuries left him in a minimally conscious state, a condition akin to a coma but characterized by occasional, but brief, evidence of environmental and self-awareness that coma patients lack.

Society and culture

Research by Dr. Eelco Wijdicks on the depiction of comas in movies was published in Neurology in May 2006. Dr. Wijdicks studied 30 films (made between 1970 and 2004) that portrayed actors in prolonged comas, and he concluded that only two films accurately depicted the state of a coma patient and the agony of waiting for a patient to awaken: Reversal of Fortune (1990) and The Dreamlife of Angels (1998). The remaining 28 were criticized for portraying miraculous awakenings with no lasting side effects, unrealistic depictions of treatments and equipment required, and comatose patients remaining muscular and tanned.

Bioethics

A person in a coma is said to be in an unconscious state. Perspectives on personhood, identity and consciousness come into play when discussing the metaphysical and bioethical views on comas.

It has been argued that unawareness should be just as ethically relevant and important as a state of awareness and that there should be metaphysical support of unawareness as a state.

In the ethical discussions about disorders of consciousness (DOCs), two abilities are usually considered as central: experiencing well-being and having interest. Well-being can broadly be understood as the positive effect related to what makes life good (according to specific standards) for the individual in question. The only condition for well-being broadly considered is the ability to experience its ‘positiveness’. That said, because experiencing positiveness is a basic emotional process with phylogenetic roots, it is likely to occur at a completely unaware level and therefore, introduces the idea of an unconscious well-being. As such, the ability of having interests, is crucial for describing two abilities which those with comas are deficient in. Having an interest in a certain domain can be understood as having a stake in something that can affect what makes our life good in that domain. An interest is what directly and immediately improves life from a certain point of view or within a particular domain, or greatly increases the likelihood of life improvement enabling the subject to realize some good. That said, sensitivity to reward signals is a fundamental element in the learning process, both consciously and unconsciously. Moreover, the unconscious brain is able to interact with its surroundings in a meaningful way and to produce meaningful information processing of stimuli coming from the external environment, including other people.

According to Hawkins, "1. A life is good if the subject is able to value, or more basically if the subject is able to care. Importantly, Hawkins stresses that caring has no need for cognitive commitment, i.e. for high-level cognitive activities: it requires being able to distinguish something, track it for a while, recognize it over time, and have certain emotional dispositions vis-à-vis something. 2. A life is good if the subject has the capacity for relationship with others, i.e. for meaningfully interacting with other people." This suggests that unawareness may (at least partly) fulfill both conditions identified by Hawkins for life to be good for a subject, thus making the unconscious ethically relevant.