Paying the piper — the linkage of alveolar ventilation to alveolar pressure

Abstract

The most basic aspects of positive pressure ventilation are generally well appreciated by modern practitioners of critical care. Yet, even after decades of application, many nuances of mechanical ventilation (MV) are incompletely understood. In this issue of Intensive Care Medicine, Conti et al. [1] have provided convincing evidence that rethinking and additional research are needed to consolidate our understanding. In studying a sample of patients with severe chronic airflow obstruction, these authors demonstrated that high-frequency jet ventilation (HFJV) at 120 breaths per minute produced similar levels of end-expiratory alveolar pressure (auto-PEEP) to those yielded by conventiona] MV accomplishing similar CO 2 elimination. In view of the apparent simplicity of MV, it is interesting to note that this work is based on two concepts high frequency ventilation (HFV) and autoPEEP that were essentially unknown or unrecognized by the medical practitioner of 10 years ago. In clinical practice, three forms of ventilation are in widespread use: conventional (volume-cycled) ventilation, pressure-preset ventilation (PPV), and HFJV. Clinicians tend to have a good intuitive sense regarding volume-cycled ventilation, understanding that this mode guarantees the ventilation that occurs per unit time, but at the cost of raising alveolar pressure. Pressure-preset ventilation (pressure support, pressure control, inverse ratio, airway pressure release) and HFJV are less intuitive and less well understood. Yet a firm understanding of their ventilatory characteristics is crucial to scientific management. Perhaps no aspect of MV engenders quite as much confusion and uncertainty as high-frequency ventilation. The flurry of interest in HFV that began about 1980 peaked mid-decade, spurred by the recognition that effective gas exchange could be maintained by machines that rapidly cycle with a tidal volume smaller than the calculated anatomical deadspace. As investigators explored this phenomenon, exotic mechanisms to supplement convection were suggested to account for effective alveolar ventilation and gas transfer. Most such mechanisms, however, apply only at very high cycling frequencies. During HFJV at rates of 100-200/min (a range incorporating the frequency studied by Conti), the primary mechanism of alveolar ventilation is likely to be that of bulk (convective) flow [2]. It is important to know this fact, because as long as convective mechanisms prevail, the relationship of alveolar pressure to alveolar ventilation is both intimate and predictable. The parallel between HFJV and pressure-preset ventilation at conventional rates may not be obvious, but it is well worth considering. When HFJV is applied to the airway, approximately square waves of airway pressure (similar in form to those of PPV) are generated a short distance downstream of the injector and, as already noted, the primary mechanism of alveolar ventilation is convective for both modes. In a real sense, therefore, HFJV applied at moderate frequencies (~100/min) closely resembles PPV with small tidal volumes. The similarities extend further. In both HFJV and PPV, the clinician selects a fixed pressure with which to work, and once this is chosen, the clinician varies only two other parameters the inspiratory time fraction ( t i / t t o t ) and the cycling frequency. The other crucial determinants of alveolar ventilation and alveolar pressure (the key outcome variables of the ventilatory process) are a function of the patient's impedance to volume change the elastance of the respiratory system (the reciprocal of compliance) and airflow resistance. As cycling frequency increases at a fixed ti/ttot, the durations of inspiration and expiration progressively shorten. However, assuming that total alveolar ventilation (~ralv) remains the same and that purely convective mechanisms apply, the average rates of expiratory and inspiratoy flow are in-