Here’s some potentially life-saving guidance from Dr TERRY MARTIN, Consultant Intensivist/Anaesthetist and Director, CCAT Aeromedical Training.

In flight, how much oxygen is ‘enough’?

The simple answer is, the amount needed to maintain normal activity of the cells without temporary or permanent damage. But the answer during aeromedical transport is a little more complex.

Few would disagree that oxygen is the first “drug” of choice in any medical emergency and nowhere is this more true than in an aircraft cabin where reduced ambient pressure means that fewer oxygen molecules are available for physiological use and they are dispersed in a larger volume.

However, the key is prevention – the best transfer is the one where problems are anticipated so that no interventions are required in the out-of-hospital environment.

So, how much is “enough”?

Calculating oxygen requirements for patients causes a lot of concern and rightly so. Get it wrong and the patient may suffer the ill effects of hypoxia for all or part of the transfer. I can’t cover all eventualities here, but a few points will help decision making.

First, though, there are a few key facts that are essential background knowledge for aeromedical personnel:

1. The fundamentals of altitude physiology

The proportions of gases in the mixture known as “air”
The relationship between altitude and air pressure
Boyle’s and Dalton’s laws
How the partial pressure of available oxygen in air varies with ambient pressure
How the oxygen cascade varies with altitude
The four major types of hypoxia
The respiratory and cardiovascular effects of hypoxia
The neurological effects of hypoxia.

2. The oxygen dissociation curve

Interpretation of the curve and its importance in ill patients
The causes and importance of curve shifts
How the curve is affected by altitude
How the curve is affected by acute and chronic anaemia.

3. An understanding of oxygen-related measurements

The definitions of oxygen content (CaO2), saturation (SpO2) and partial pressure (PaO2), and inspired oxygen fraction (FiO2)
The physiological relationship between haemoglobin (Hb), SpO2 and PaO2
The limitations of pulse oximetry
The value of blood gas analysis.

You will have studied this material during the IFNA or CCAT courses, but have you ever put the theory into practice? The issue is really about matching oxygen requirements to the patient’s pathophysiology, but with the added complexity of accounting for the hostile environment in the aircraft cabin. If your memory of any of these topics is vague, spend some time perusing the “further reading” list.

Arterial PO2 up to 20,000 ft (6,096 m), breathing air

Figure 1: Arterial PO2 up to 20,000 ft (6,096 m), breathing air

The key point to bear in mind

If you know the patient’s oxygen status when in a well oxygenated and stable condition on the ground before take-off, simply aim to replicate that status.

So what do I mean by “oxygen status”? This is combined knowledge of the Hb, FiO2, PaO2 and SpO2. Not all patients will have this information available prior to their transfer and, clearly, not all will need it. However, seriously ill patients will have higher oxygen demands because of their pathology and/or treatment. I therefore believe that the new hand-held blood gas analysers will become essential equipment for the assessment of aeromedical patients prior to departure and also en route.

With knowledge of the ground level oxygen status data, the aim is to maintain minimum PaO2 at that ground level equivalent, probably in the range 10.7-13.3 kPa.

Using figure 1, the patient’s PaO2 can be used to find the equivalent altitude. If the patient’s location is significantly higher than sea level, the height should be deducted from this equivalent altitude. The resultant final figure (the sea level compensated equivalent altitude) is plotted on figure 2 against the actual cabin equivalent altitude to give the fractional inspired oxygen needed to maintain a PaO2 of 13.3 kPa in flight.

Once the flow rate required to maintain an adequate FiO2 and PaO2 is known, you can determine the volume of oxygen required. For instance, if a constant flow of five litres per minute is required and the full oxygen cylinder contains 680 L (E size cylinder), then the contents will last for no more than 136 minutes. (Note: in practice, the effects of Boyle’s Law on the cylinder gas can be ignored at cabin altitudes up to 8,000 feet.)

Although the ventilation of ICU patients is more complex at altitude, their oxygen requirements are easier to estimate accurately. That is, (Minute volume x FiO2 x estimated transfer time in minutes) / cylinder capacity.

Remember also that some ventilators are gas driven and you must know how much oxygen is diverted for that purpose (usually around 20 ml/min).

Clearly, less oxygen will be needed for Pulse Dose Oxygen systems, but the key is to carry more than you think will be needed so that allowances are made for unforeseen delays and deterioration in the patient with subsequent increase in oxygen demand. It is wise, therefore, to take a 100 per cent reserve.

$The fractional concentration of oxygen required in the inspired gas to maintain alveolar PO2 of 13.3 kPa (100 mmHg)$

Figure 2: The fractional concentration of oxygen required in the inspired gas to maintain alveolar PO2 of 13.3 kPa (100 mmHg)

Why not 100 per cent throughout the journey?

Quite simply, as well as being uneconomical and incurring the weight penalty of unnecessary cylinders, 100 per cent inspired oxygen over long periods is an irritant, may be toxic to the respiratory tract and may contribute to delayed otic barotraumas.

So having elected to use supplemental oxygen in flight, clinical acumen and an understanding of the symptoms and signs of hypoxia are your best guide. Medical crew must always err on the side of caution. Constant reappraisal of the patient is essential and inspired oxygen should be titrated to match clinical and monitored responses.