Several years ago I found myself guiding a large group of climbers on Aconcagua (6962m). It goes without saying that summit day proved to be an enormous challenge and only a handful of us reached the top. Chatting a few days later at Base Camp we agreed that much of our success had been due to the strong wind that was blowing that day. Rather than holding us back, we all felt that opening our mouths and turning into the wind had made an enormous difference to our breathing. I'd forgotten about that until I recently came across this case study published in the New England Journal of Medicine...
"A 51 year old woman was admitted to our hospital with severe respiratory distress due to the acute onset of pulmonary oedema and bronchospasm. She was comatose and required emergency intubation. Arterial blood gas measurements before intubation showed the following (normal range in brackets):
pH 6.89 (7.35 - 7.45)
pO2 7.9 KPa (12 - 16)
pCO2 11.7 KPa (4.5 - 6)
The patient responded well to conventional treatment. Eventually, she recounted her harrowing trip to the hospital. Her husband had been driving the car. She said that she would have lost consciousness en route had she not held her head out the window to breathe. Her husband confirmed that her mental status and breathing had improved substantially with this manoeuvre. He stated that he was driving the vehicle at 80mph (130kph). He also noted that for his wife, the effect of holding her head out of the window quickly abated when he was forced to slow his vehicle near the hospital.
According to Bernoulli's Equation, the additional inspiratory pressure provided at this velocity is approximately 6mmHg or 8cm of water at sea level. Bernoulli's Equation (valid for steady, incompressible, non-viscous and irrotational flow) for the case of constant height is as follows:
Change in pressure = 0.5 x density x velocity2
The velocity of the vehicle was 80mph or 36 metres per second and the density of air at sea level is 1.21 kg per cubic metre. Thus, the change in pressure is approximately 784 Pa which is 5.9 mmHg or 7.8 cm of water.
On the basis of a review of the literature, both medical and automotive, we are unaware of previous reports of this type of auto-PEEP (positive end-expiratory pressure)."
How had positive end expiratory pressure (PEEP) helped this patient's lungs*? I reckon there's at least 3 different ways. First, it would have reduced the patient's work of breathing - it's easier to take a breath if gas is being blown into the nose and mouth! Second, PEEP encourages airways to stay open. In someone with pulmonary oedema there's a large quantity of fluid bearing down on the small airways causing them to close. PEEP keeps them open and encourages the movement of gas across the alveolar capillary membrane. Finally, oxygen in the body obeys Henry's Law. The greater the pressure of the gas, the more it dissolves in any neighbouring liquid. Therefore a PEEP of 8cm of water offers lots of encouragement for oxygen to cross the alveolar capillary membrane and enter the circulation.
Positive End Expiratory Pressure (PEEP) is delivered in hospital by a Continuous Positive Airway Pressure (CPAP) device. Most patients receive a PEEP of between 5 and 10 cm of water
Going back to Aconcagua, could we have enjoyed a similar range of benefits? If we substitute values for wind speed (40mph or 18 metres per second) and air density at 6000m (0.7 kg per cubic metre) into the Bernoulli Equation we get a change in pressure of 113 Pa which is 0.84 mmHg or 1.2 cm of water. Did this small increase in PEEP help? I'm not sure. Given the strength of the wind I suspect that any benefit would have been cancelled out by the extra effort needed to move against it. Perhaps the wind served as a distraction or something to liven us up? Who knows! Nevertheless, there seems to be a very good physiological reason for driving breathless patients to hospital quickly!
*As an aside, it's worth saying that PEEP can be particularly useful in those with heart failure since the added pressure within the chest compresses the large veins and reduces the amount of blood returning to the heart. In turn, this limits the flow of blood to failing heart chambers and may prevent the formation of pulmonary oedema.