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By James Meyer (PhD Physiology)

  • To skydive safely we rely not only on adequate blood flow to the brain (cerebral blood flow or CBF), but also on the correct amounts of oxygen and carbon dioxide in our blood.
  • Sometimes these are lacking to the extent that we experience varying degrees of diminished capacity for higher brain function and can lead to a complete loss of consciousness.
  • These events can occur during freefall or whilst under canopy.
  • These events can occur without warning.
  • Knowledge of the basic mechanisms involved may serve to increase the awareness for potential problems and to help personally manage the relevant risk factors.

Basic mechanisms

  • CBF is maintained by a process referred to as “autoregulation”.
  • Blood vessels are preferentially dilated or constricted to increase or reduce CBF as required.
  • Other factors, such as respiration rate and heart rate are also managed
  • The following example may help to explain the basic process:
    • If one hyperventilates on purpose dizziness is usually experienced. This is due to:
      • Increased breathing reducing blood CO2 levels
      • This causes CBF to decrease due to vessels being constricted
      • This causes oxygen levels in the brain to drop leading to the dizziness
  • This happens because CBF is more sensitive to CO2 than oxygen.
  • Another example indicates how this mechanism may have fatal consequences:
    • Hyperventilating before holding ones breath underwater can suppress the urge to breathe to the extent that you can lose consciousness whilst underwater without being aware of the need to surface for air.
    • This is known as “shallow water blackout” and unless rescued can be fatal.
  • In some individuals the nervous control of heart rate can lead to the same end result as the examples mentioned above by reducing arterial blood pressure.
  • This occurs as the initial response to stress is overcompensated for by a counteracting system (sympathetic nervous system – adrenalin versus parasympathetic system – vagus nerve).
  • This excessive response can diminish CBF to the extent that consciousness is lost. If of short duration this is referred to as syncope (fainting), if of long duration as a coma.
  • Usually someone who has fainted regains consciousness after moving from a vertical to a horizontal posture.  If suspended in a harness this benefit is obviously lost.
  • To summarize 3 key factors include:
    • CBF (vessels constricting or dilating)
    • Blood oxygen concentration
    • Arterial blood pressure
  • Take note of the key risk factors listed below.
  • Hypoxia – refer to safety section on PASA website “Basics of breathing”.

SUMMARY- mechanism

A loss of consciousness can occur due to a decreased CBF and/or brain oxygen concentrations caused by:

  • Decreased blood CO2 (hyperventilation)
  • Decreased blood O2 (hypoxia)
  • Decreased blood pressure (hypotension)
  • Vasovagal syncope = loss of blood pressure due to excessive dilation and slow heart rate
  • Emotions can also precipitate a vasovagal syncope (psychogenic syncope)

It is important to note that whilst non-cardiac causes of syncope do not usually require treatment as consciousness is usually regained by postural changes, consequences thereof are critical when skydiving.

Sequence experienced: dizziness, tingling of the fingers, clamminess, dimming of vision, tinnitus, weakness

Observed: pale complexion, hyperventilation, weakness (notably of legs), nausea.


SUMMARY- key risk factors

Alcohol (see ketoacidosis below)

Initial response to high altitudes in persons not adapted leads to hyperventilation and reduced CBF.
Be cautious of a cumulative effect on the next day and of increased time spent in slow-climbing aircraft or long run-ins.

Reduces cardiac output and can reduce arterial blood pressure leading to hypotension and reduced CBF.

A loss of blood volume and electrolytes due to diabetes can lead to dehydration and hypotension.
Diabetics also have elevated ketones in the blood stream that can cause metabolic acidosis leading to a diabetic coma.
This condition can also lead to increased loss of sodium that further reduces blood volume. 
The acids produced during some diabetic conditions reduce blood bicarbonate and the resultant metabolic acidosis can trigger a hyperventilation response in an attempt to compensate for the drop in blood pH (this will cause a loss of blood CO2).

Causes a decrease in sympathetic stimulation and consequently dilates peripheral blood vessels – think of sweating and heat loss.

Hypotension  (arterial)
Systolic value of < 100 mm Hg and being on antihypertensive medication (medications that lower blood pressure).

Ketoacidosis (& Alcohol)
Caused either by Type I Diabetes or Alcoholic Ketoacidosis. Consuming alcohol causes dehydration and reduces the ability of the liver to produce glucose (causes excessive ketone production – see Diabetes above).

Increased intracranial pressure
This compresses the brain capillaries and can thus reduce CBF.

Strenuous exercise
This can lead to increased lactic acid that can have a similar effect to diabetic ketoacidosis.
Can also lead to a loss of sodium and consumption of just water can aggravate the condition by further diluting the body’s sodium concentration.  Mental proficiency can be compromised.

Summary - Management

Take note of the risk factors.
Consult your doctor if you are on antihypertensive drugs.
Stay Hydrated – consume sports drinks and note that water alone will not remedy the situation and electrolytes (specifically sodium) are also required

Breathe normally – do not hyperventilate!
Relax as much as possible and allow blood O2 and CO2 to get to normal levels.
If excited take extra care to calm down – adrenalin encourages hyperventilation and can result in a vasovagal syncope.

Being thirsty can indicate dehydration of either cellular water and, in more advanced dehydration, a loss of blood volume. 

The consequence is a decrease in arterial pressure. 
Take extra caution if you suffer from respiratory and/or cardiac insufficiency.

Do not hold your breath during the skydive!

Skydive within your limits – be cautious when performing more jumps than you are used to.

Background information

Autoregulation of CBF

Although it may seem self-evident that skydivers rely on higher brain functions in order to safely perform parachute jumps, many may be unaware of how reduced Cerebral Blood Flow (CBF) can not only impair judgment and performance during a skydive and canopy flight, but also result in a complete loss of consciousness either during freefall or once under canopy.

Unlike blood capillaries elsewhere in the body those in the brain are relatively impermeable to most substances and hence a blood-brain barrier is often referred to.  Notable exceptions are fat-soluble drugs, glucose, O2 and CO2, which can cross from the blood capillaries into brain tissue with relative ease.

CO2 and O2 Pressures

Not surprisingly the brain has a high O2 demand and the control of CBF is critical to normal function.
Autoregulation assists greatly in maintaining a stable O2 supply.  Some relevant ranges are:

  • Blood O2 partial pressure of 100 mm Hg = normal arterial value
  • Blood O2 partial pressure of 50 mm Hg = hypoxia

Below the hypoxia value brain capillaries are very sensitive to O2 and respond by dilating in order to increase CBF.

However, these capillaries are much more sensitive to CO2 fluctuations and blood pH.

If the CO2 value drops (40 – 20 mm Hg) a strong constriction effect occurs.  To experience this effect all you need to do is hyperventilate on purpose.  Typically dizziness will be experienced. This is caused by the lowering of blood CO2 due to the increased ventilation, consequent constriction of the brain capillaries and subsequent reduction in CBF and thence brain O2.
Normally this means that CBF is controlled by CO2.  However, an additional complication occurs when altitude is factored in as the O2 value may also change.  Any respiratory disorder or disease can have a similar effect (asthma, infections, emphysema, etc.).

Blood Pressure

This whole autoregulation system also works within a range of arterial blood pressure, namely:

Mean Arterial Pressure = 50 – 150 mm Hg

Should either the O2 value or the arterial pressure drop significantly,
the autoregulation can fail to function sufficiently and
CBF can drop enough to reduce the O2 supply to the brain.

This can lead to a sudden state of unconsciousness.

If this is of short duration it is referred to as a syncope (faint). If this is prolonged it is referred to as a coma.

A significant reduction in blood pressure and CBF can occur as a vasovagal response.

Most people will be familiar with the “fight or flight” response and effects of adrenalin on increased heart rate and constriction of peripheral blood vessels (pale complexion) in an attempt to deliver more blood to the brain and muscles. 
This is mediated by the sympathetic system and is countered by the parasympathetic system which acts to dilate certain blood vessels and slow down the heart rate. 
This response can be excessive and a vasovagal (vessel and vagus nerve) block can occur with a disruption to the normal heat beat.  This can decrease CBF significantly.
Some people are prone to fainting due to emotional factors (psychogenic syncope).
The most common manifestation to an acute drop in blood pressure (acute hypotension) is fainting.
Once fainted, the posture typically changes from standing to horizontal which assists in increasing blood flow to the brain and hence recovery is usually quick.

If you are suspended in a HARNESS this benefit due to a change in posture and thus blood flow

Tension on leg-straps can also reduce the return of blood from the legs to the heart, further reducing CBF.