Oxygen and Respiration — Page 51, Lesson 82

I want to walk you through a critical topic in aviation physiology: hyperventilation, its symptoms, its treatment, and how we distinguish it from hypoxia in the cockpit. Let's start with the symptoms of hyperventilation. Hyperventilation means you're breathing too fast and too deeply, which upsets the balance of carbon dioxide in your blood. The first symptom is dizziness and a feeling of unreality — you may feel lightheaded or detached from what's happening around you. Next comes tingling, especially in the extremities — that means your fingers and toes — and also around your lips. You may experience visual disturbances: blurred vision, tunnelling vision where your peripheral vision narrows, or clouding vision. Another symptom is hot or cold sensations; these can alternate in time and vary as to which parts of the body are affected. Anxiety is a key symptom, and here's the dangerous part — anxiety itself makes you breathe even faster, which establishes a vicious circle: hyperventilation causes anxiety, and anxiety worsens hyperventilation. You'll also see loss of muscular coordination and impaired performance — your fine motor control degrades, which is obviously dangerous when flying. Heart rate increases. Just prior to unconsciousness, the muscles of the hands, fingers, and feet may go into spasm — involuntary cramping. Finally, loss of consciousness: hyperventilation can lead to collapse, but here's the important distinction — once you lose consciousness, the body's automatic system will restore the normal respiration rate, and the individual will recover on their own. Now, treatment of hyperventilation. The classic method is to make the sufferer breathe into a paper bag. Why does this work? The person is forced to inhale the carbon dioxide that they have just exhaled. The immediate effect is to increase the carbonic acid level in the blood back to its normal value. The brain detects this and consequently reduces the breathing rate back to normal. In all cases, try to calm the patient and encourage them to slow down the rate of breathing — the symptoms themselves can be alarming, so reassurance is part of the treatment. Here's where it gets really important for pilots: distinguishing hypoxia from hyperventilation. The natural reaction to a shortage of oxygen — hypoxia — is for the body to try to obtain more air by breathing faster and deeper. So a hypoxic individual may hyperventilate in an effort to get more oxygen. But this is of little value when you're in an environment of low ambient pressure — at altitude, breathing faster doesn't help because the partial pressure of oxygen is too low. In flight, it can be difficult to distinguish the symptoms of hypoxia and hyperventilation because they overlap. So what must pilots do? The appropriate response is to assume the worst. If you are at an altitude where hypoxia is a possibility — typically above 10,000 feet — you must take that to be the cause and carry out your hypoxia drills immediately. That means putting on your oxygen mask and using 100% oxygen. If symptoms occur at an altitude at which hypoxia is not a consideration — that is, below 10,000 feet — then you should regulate the rate and depth of breathing to restore the normal acid/base balance of the blood and alleviate the symptoms. Below 10,000 feet, significant symptoms of hypoxia are unlikely, so hyperventilation may be assumed. Let me give you the critical rule that the book emphasises in bold capitals: DO NOT ASSUME HYPERVENTILATION IF IT COULD BE HYPOXIA. And here's the life-or-death difference: with hyperventilation, after unconsciousness, the outcome is recovery — your body's automatic systems restore normal breathing. With hypoxia, after unconsciousness, the outcome is death. That's why you always treat it as hypoxia first.