Positive Pressure and Positive End-expiratory Pressure (PEEP)
Contrary to popular delusion, there are difference between positive airway pressure, positive end-expiratory pressure(PEEP) and CPAP. These terms are not interchangeable and their misuse tends to enrage the ventilation pedant. Once the important distinction between them is well understood, the shared features of these concepts can be explored together.
- Positive pressure is all respiratory pressure above atmospheric pressure
- PEEP refers to the positive airway pressure at the end of expiration only.
- CPAP refers to a particular spontaneous mode of ventilation.
Positive airway pressure
An airway disconnected from the corrugated tubing of the ventilator is open to atmospheric pressure. Though used as the "zero" reference point, of course this pressure is not under "zero pressure". A "zero pressure" would be hard space vacuum. The disconnected airway is under atmospheric pressure, around 760 mm Hg or 1033 cm H2O.
For some reason, we measure gas partial pressure in mmHg, and ventilator gas pressures in cmH2O. In the mercury manometer, pressure which results in the displacement of one millimetre of mercury corresponds to the displacement of 1.36 cm of water in the water manometer.
The tradition of measuring respiratory pressure in terms of water rather than mercury does not appear to have a well-reasoned origin. Some textbooks suggest that the respiratory pressure changes are so small that their measurement technique requires a factor-of-ten increase in scale in order to generate conveniently large changes. If one finds oneself measuring a pleural pressure from a mercury manometer, a change in height of 0.37 mm is much less visible to the naked eye than a change of 0.5 cm H2O.
This is not a good argument, as a sane person would immediately object to it. Long gone are the days when physiologists would need to squint at menisci in graduated cylinders; this is the era of electronic pressure measurement. The modern intensivist is not at all inconvenienced by small numbers. In any case there is a conceivable range of inconveniently small numbers no matter what liquid is in your manometer, and if convenience were the real issue one could point to the inconvenience of comparing water pressure measurements with mercury measurements of practically every other pressure variable in physiology (gas partial pressure, blood pressure, intracranial pressure, and so on).
So, with the normal" atmospheric pressure of 760 mm Hg, one's entire native respiratory circuit is under 10 metres of water pressure. Similarly, 10 metres below the surface of the sea one's lung experiences an extra 1 atmosphere of pressure, and with deeper submersion the pressure increases by 1 atmosphere for every 10 metres. Thus, we ventilate our patients with pressure differences which are minute in comparison to the total pressures in play in the environment around them, or even to the pressures in their own cardiovascular system.
Anyway, I thought this was interesting.
Back to positive pressure. "Positive" pressure is any airway pressure above the atmospheric. On the ventilator monitor, airway pressure is offered as a graph of pressure over time. The characteristic pattern of a breath can be seen in the diagram below.
The shape of the pressure waveform is a luxurious banquet of physiological information, and its interpretation is discussed elsewhere, as it would be unwise to digress yet further in this chapter.
Without further ado:
Positive pressure is not PEEP.
But PEEP is positive pressure.
Positive airway pressure is any pressure above atmospheric pressure applied to the airway.
Positive pressure ventilation is the convention of modern ventilation, in contrast to the old-school "iron lung" which was a negative pressure ventilator. Negative airway pressure is also generated during physiologically normal respiration.
In short, positive pressure is a physical variable which we manipulate to change the gradient of gas flow into (and out of) the patient's lung. PEEP, on the other hand, is Positive End Expiratory Pressure. It is a variable of the 4th phase of ventilation - the pressure at the end of expiration. This is a precise definition. When we discuss the effects of PEEP, we discuss the effects of positive pressure specifically in the relaxation period of the respiratory cycle; for example, PEEP influences the effort required to generate a breath. In contrast, when we discuss the effects of positive pressure ventilation, we refer to all the effects which are generated by increased pulmonary and intrathoracic pressure. Positive pressure applied to the respiratory system has numerous consequences ranging far beyond the end-expiratory phase, and these are discussed in subsequent chapters.
CPAP is not PEEP.
CPAP is a mode of ventilation consisting of Continuous Positive Airway Pressure. It is not PEEP, because PEEP is a phase variable of ventilation. CPAP incorporates PEEP (strictly speaking) because at the end of a CPAP breath there is end-expiratory pressure; however CPAP is not synonymous with PEEP, and the two terms cannot be used interchangeably.
One might argue that in any ventilator mode the presence of any PEEP above zero classifies the ventilation as CPAP. Because throughout the respiratory cycle the pressure in the respiratory circuit is continuously above atmospheric, the circuit can be said to be under Continuous Positive Airway Pressure. This is technically correct, but nonetheless confusing. Literary convention has left the two concepts distinct. When discussed in literature, CPAP typically implies a mode of ventilation where the ventilator does not cycle in inspiration, offering only one level of pressure.
In summary, CPAP is a mode of ventilation in which triggering is disabled and the entire respiratory cycle is exposed to a uniform pressure from the ventilator, which corresponds to the PEEP setting. The actual respiratory circuit pressure will vary throughout the cycle because the patient will generate a negative pressure to inhale a breath, and a positive pressure to exhale; but at the end of the breath the pressure will revert to the PEEP setting.
CPAP as a mode of ventilation (along with its advantages and disadvantages) is discussed elsewhere in these chapters.