Paradoxical responses to positive end-expiratory pressure in patients with airway obstruction during controlled ventilation

Abstract

Objective: To reevaluate the clinical impact of external positive end-expiratory pressure (external-PEEP) application in patients with severe airway obstruction during controlled mechanical ventilation. The controversial occurrence of a paradoxic lung deflation promoted by PEEP was scrutinized.

Design: External-PEEP was applied stepwise (2 cm H(2)O, 5-min steps) from zero-PEEP to 150% of intrinsic-PEEP in patients already submitted to ventilatory settings minimizing overinflation. Two commonly used frequencies during permissive hypercapnia (6 and 9/min), combined with two different tidal volumes (VT: 6 and 9 mL/kg), were tested.

Setting: A hospital intensive care unit.

Patients: Eight patients were enrolled after confirmation of an obstructive lung disease (inspiratory resistance, >20 cm H(2)O/L per sec) and the presence of intrinsic-PEEP (> or =5 cm H(2)O) despite the use of very low minute ventilation.

Interventions: All patients were continuously monitored for intra-arterial blood gas values, cardiac output, lung mechanics, and lung volume with plethysmography.

Measurements and main results: Three different responses to external-PEEP were observed, which were independent of ventilatory settings. In the biphasic response, isovolume-expiratory flows and lung volumes remained constant during progressive PEEP steps until a threshold, beyond which overinflation ensued. In the classic overinflation response, any increment of external-PEEP caused a decrease in isovolume-expiratory flows, with evident overinflation. In the paradoxic response, a drop in functional residual capacity during external-PEEP application (when compared to zero-external-PEEP) was commonly accompanied by decreased plateau pressures and total-PEEP, with increased isovolume-expiratory flows. The paradoxic response was observed in five of the eight patients (three with asthma and two with chronic obstructive pulmonary disease) during at least one ventilator pattern.

Conclusions: External-PEEP application may relieve overinflation in selected patients with airway obstruction during controlled mechanical ventilation. No a priori information about disease, mechanics, or ventilatory settings was predictive of the response. An empirical PEEP trial investigating plateau pressure response in these patients appears to be a reasonable strategy with minimal side effects.

Figure 1

Changes in total intrinsic positive end-expiratory pressure (PEEP_i) and functional residual capacity (FRC) with increments in external-PEEP (mean ± SEM across patients). External-PEEP is shown as a percentage of external-PEEP_i measured at zero external-PEEP. The FRC measured at zero external-PEEP was considered as the reference. Gray squares represent a respiratory rate (RR) of 9 breaths/min and black squares, an RR of 6 breaths/min. The graphs at left represent trials with a small tidal volume (V_T; 6 mL/kg), and those at right represent trials with a high V_T (9 mL/kg).

Figure 2

Plateau pressures and peak airway pressures with increments in external positive end-expiratory pressure (PEEP; mean ± SEM across patients). External-PEEP is shown as a percentage of intrinsic-PEEP measured at zero external-PEEP. The gray squares represent a respiratory rate (RR) of 9 breaths/min and black squares, an RR of 6 breaths/min. The graphs at left represent trials with a small tidal volume (V_T; 6 mL/kg), and those at right represent trials with a high V_T (9 mL/kg).

Figure 3

Changes in functional residual capacity (FRC) assessed by inductive plethysmography. Individual responses along the incremental external positive end-expiratory pressure (PEEP) steps, from zero external-PEEP (reference) to a value matching 150% of intrinsic-PEEP measured at zero external-PEEP conditions. A, tidal volume (V_T) = 6 mL/kg, respiratory rate (RR) = 6 breaths/min; B, V_T = 6 mL/kg, RR = 9 breaths/min; C, V_T = 9 mL/kg, RR = 6 breaths/min; D, V_T = 9 mL/kg, RR = 9 breaths/min. The FRC measured at zero external-PEEP was considered as the reference. Points inside the gray zone indicate the occurrence of a paradoxic response.

Figure 4

Three of the possible responses observed in plateau pressure (P_PLAT), total intrinsic positive end-expiratory pressure (PEEP_i), and functional residual capacity (F.R.C.) with the application of external-PEEP (represented as percentage of PEEP_i measured at zero external-PEEP). The FRC measured at zero external-PEEP was considered as the reference. A, paradoxic response (patient 4), observed with a tidal volume (V_T) of 6 mL/kg and respiratory rate (RR) of 9 breaths/min; B, biphasic response (patient 7), observed with V_T of 9 mL/kg and RR of 6 breaths/min; C, classic overinflation response (patient 5), observed with a V_T of 9 mL/kg and RR of 9 breaths/min.

Figure 5

On the left, examples of flow-volume (F-V) loops, and on the right, driving pressure vs. expiratory flow relationships at isovolume conditions (from the same patients whose data are illustrated in Figure 4). A, paradoxic response, characterized by a rightward shift of the F-V loops at low external-PEEP followed by a leftward shift with high external-PEEP. The decrease in driving pressure resulted in a paradoxic increase in expiratory flow up to a critical pressure, beyond which expiratory flow decreased. B, biphasic response, characterized by nearly superimposed F-V loops at low external-PEEP levels, followed by a leftward shift at higher external-PEEP levels. C, classic overinflation response, characterized by a leftward shift of F-V loops with any external-PEEP increment; correspondingly, there was a quasilinear relationship between expiratory flow and driving pressures measured at isovolume conditions. Different loops represent data points collected when the external-PEEP was matching 40%, 80%, and 120% of baseline intrinsic-PEEP measure at zero end-expiratory pressure (ZEEP).