‘Auto’ Positive End Expiratory Pressure
“It is a narrow mind which cannot look at a subject from various points of view.”
On background reading for the recent entry on ‘Asthmatic Mechanics,’ I happened across a delightful case report submitted as a letter to the editor of the New England Journal of Medicine in 1993. In it, the authors describe the continuous positive airway pressure [CPAP] that is generated by placing one’s head out of the window of a speeding automobile – hence, true ‘auto’ PEEP.
The authors briefly describe a middle-aged woman who presented to the emergency department with combined pulmonary edema and obstructive airways disease; she required immediate intubation given her comatose state as well as her arterial blood gas which revealed a pH of 6.89, PaCO2 of 89 mmHg, PaO2 of 60 mmHg and an oxygen saturation of 70%!
Notably, she was raced to the hospital by her husband and she described great improvement in her mental state when she stuck her head out of the window of their speeding car. Her husband stated that he was driving 80 miles per hour [130 km/h] and that as he slowed upon hospital arrival, his wife’s mental status worsened.
The authors used Bernoulli’s equation to estimate the gauge pressure generated by air moving at 80 mph. The equation is, essentially, the kinetic energy of air:
½ • density of air • (velocity of air)2
The assumptions here are that air is incompressible, non-viscous and irrotational. Given that the vehicle was moving at 36 metres per second and that the density of air is 1.21 kg per m3, the pressure applied to the patient’s face was roughly 8 cm H2O.
As elaborated in the vodcast accompanying ‘Asthmatic Mechanics’ the application of CPAP has salutary inspiratory and expiratory effects on the lungs of a patient with air-trapping and expiratory flow limitation.
The inspiratory threshold load generated by dynamic hyperinflation is immediate and a function of respiratory system compliance. The patient in the case was described as having co-morbid pulmonary edema meaning that she required extra inspiratory effort to overcome her intrinsic positive end-expiratory pressure [PEEP] and trigger airflow. Technically, this initial load upon the respiratory system is not truly ‘work’ [in Joules] because no change in lung volume occurs; it is an undesirable isometric exercise for the respiratory system and it diminishes the energetic efficiency of inspiratory muscles. Critically, as illustrated in the vodcast, CPAP carries this inspiratory load for the patient, improves inspiratory energetic efficiency and, therefore, reduces the oxygen demands of the respiratory pump.
The expiratory effects of CPAP may be less intuitive, but if the patient has expiratory flow limitation it suggests that there is a ‘choke point’ pressure acting as the effective downstream pressure of expiration – not the pressure at the mouth. If, for example, the patient described in the case above had large segments of her lung with an intrinsic pressure of 10 cm H2O at the end of expiration [i.e. PEEPi], this suggests that those lung segments were limited by a ‘choke point’ pressure of approximately 10 cm H2O. The 8 cm H2O of CPAP provided by the automobile would, therefore, be functionally absent from those ‘choked’ lung units. Thus, and somewhat paradoxically, CPAP acts in a manner akin to pressure support in that it promotes inspiration beyond that of expiration!
Fortunately, the patient in the report did well in hospital with traditional mechanical ventilation.
Dr. Kenny is the cofounder and Chief Medical Officer of Flosonics Medical; he also the creator and author of a free hemodynamic curriculum at heart-lung.org