Oxygen and Respiration — Page 51, Lesson 80

I want to walk you through two critical concepts in aviation physiology: Time of Useful Consciousness and Hyperventilation. Both are essential for you to understand as a professional pilot, because they directly affect your ability to think and act when oxygen becomes scarce. Let's start with Time of Useful Consciousness, or TUC. This is the time available from the moment oxygen supply drops until a pilot can no longer take effective action to help themselves. It is not the time until you pass out completely — it's shorter than that. TUC ends at the point where you reach a specific degree of impairment where you can no longer take steps to save yourself. So it's the window you have to recognise the problem and act — put on your oxygen mask, descend, or declare an emergency. The exact TUC depends on the individual and is affected by several factors: your individual fitness, your workload at the time, whether you smoke, whether you are overweight or obese, and whether the decompression is progressive (slow) or explosive (sudden). A fit, non-smoking pilot with a low workload will have a longer TUC than someone who is unfit, a smoker, or under heavy stress. Now, here are the average times at various altitudes. At 20,000 feet, a person seated at rest has about 30 minutes of useful consciousness. But with moderate activity, that drops to just 5 minutes. At 30,000 feet, you have only 1 to 2 minutes. At 35,000 feet, it's 30 to 90 seconds. And at 40,000 feet, you have just 15 to 20 seconds. Notice how dramatically the time shrinks as altitude increases — above 30,000 feet, you're counting in seconds, not minutes. Closely related is Effective Performance Time, or EPT. This is always within and shorter than TUC. EPT is the time you can actually perform a specific task — like reading an instrument or turning a knob — before impairment sets in. The problem is that EPT cannot be precisely quantified because it depends on the individual, the task at hand, physiological and mental stress, altitude, and the circumstances. It's highly variable and individualistic. However, above 40,000 feet, the EPT is approximately 5 to 6 seconds. That's barely enough time to recognise a problem, let alone solve it. Now let's move to Hyperventilation. This is a different problem, but it can mimic hypoxia. Hyperventilation is defined as lung ventilation in excess of the body's needs. In simple terms, it's overbreathing — breathing more than is required to remove carbon dioxide from your body. Normally, your brain automatically controls your breathing rate to keep carbon dioxide levels just right. Hyperventilation overrides that automatic control. When you overbreathe, you reduce the carbon dioxide in your blood, which decreases the carbonic acid balance of the blood. This disturbance of the acid balance has several effects. The major one is that haemoglobin gives up its oxygen readily only in an acid medium. So when the blood becomes too alkaline (less acidic), haemoglobin holds onto oxygen more tightly, and your tissues don't get the oxygen they need — even though you're breathing plenty of air. Now, here's an important point: hypoxia does cause hyperventilation, but it is far from the only cause. Many things can trigger hyperventilation: anxiety, motion sickness, shock, vibration, heat, high g-forces, and even pressure breathing from an oxygen mask. So as a pilot, you need to be aware that hyperventilation can occur in many situations, not just at altitude. The best prevention? A high standard of training. Training breeds confidence and decreases the chances of confronting unusual and stressful situations. That is, without doubt, the best means of preventing hyperventilation in aircrew. Finally, a practical warning: an anxious passenger boarding an aircraft must be closely watched, because hyperventilation may take place even while still on the ground. So don't assume it's only an in-flight issue. Let me show you the anatomy of the lungs and airways that make all this gas exchange possible.