Clinical review: patient-ventilator interaction in chronic obstructive pulmonary disease

Abstract

Mechanically ventilated patients with chronic obstructive pulmonary disease often prove challenging to the clinician due to the complex pathophysiology of the disease and the high risk of patient-ventilator asynchrony. These problems are encountered in both intubated patients and those ventilated with noninvasive ventilation. Much knowledge has been gained over the years in our understanding of the mechanisms underlying the difficult interaction between these patients and the machines used to provide them with the ventilatory support they often require for prolonged periods. This paper attempts to summarize the various key issues of patient-ventilator interaction during pressure support ventilation, the most often used partial ventilatory support mode, and to draw clinicians' attention to the need for sufficient knowledge when setting the ventilator at the bedside, given the often conflicting goals that must be met.

Figure 1

Schematic tracing of a pressure support (PS) cycle, highlighting its four key phases.

Figure 2

Conceptual diagram illustrating the adverse effects of both insufficient and excessive levels of pressure support (PS) on the respiratory muscle workload. PEEPi, intrinsic positive end-expiratory pressure.

Figure 3

Mathematical modeling of the inspiratory instantaneous flow-time curve for progressively increasing levels of airway resistance (Rrs), from normal (5) to severe (20). The cross represents the point at which the inspiratory flow has decreased to 25% of its peak value, and corresponds to the default expiratory trigger (ET) on many ventilators.

Figure 4

Consequences of delayed cycling. PEEPi, intrinsic positive end-expiratory pressure.

Figure 5

Airway and flow-time tracings illustrating the concept of delayed cycling. (a) Normal mechanics. The expiratory trigger (ET) setting is 0.25. Cycling is ideal, that is, the inspiratory flow (V') decreases to the 0.25 cycling level at the end of the patient's neural inspiration (ti_n). (b) Obstructive mechanics. The change in inspiratory flow curve derived from Figure 3 leads to the 0.25 level being reached later, well after the end of ti_n. The magnitude of delayed cycling (ti_excess) is illustrated by the double arrow. Increasing the level of ET to 0.6 of peak inspiratory flow corrects this problem, and cycling occurs once more at the end of ti_n. Exp., expiration; Insp., inspiration; V'_peak, peak inspiratory flow.