Hyperpnea limits the volume recruited by positive end-expiratory pressure.

Abstract

The effectiveness of positive end-expiratory pressure (PEEP) relates directly to alveolar recruitment. We tested the hypothesis that active use of expiratory muscles during labored breathing impairs the ability of PEEP to increase end-expiratory lung volume. Eight healthy volunteers naive to the purposes of our study were exposed to targeted end-expiratory pressures of 0, 5, and 10 cm H2O during mechanical ventilation applied by mouthpiece and noseclips at three levels of ventilation: resting and two levels (moderate and high) of CO2 stimulation (10.9 +/- 0.4, 19.9 +/- 0.5 and 27.5 +/- 0.5 L/min, respectively). Inductive plethysmography demonstrated that end-expiratory lung volume rose by an average of 98 +/- 5 ml/cm H2O PEEP during quiet breathing but by much less during the two levels (moderate and high) of CO2 stimulation: 78 +/- 6 ml/cm H2O and 47 +/- 5 ml/cm H2O (p < 0.05). Hyperpnea also shifted the distribution of the recruited volume toward regions sampled by the rib cage band of the plethysmograph. Whatever advantage expiratory muscle activity may have for minimizing the workload of the inspiratory muscles, the cost may be reduced effectiveness of PEEP in increasing lung volume and improving oxygen exchange.

Pathophysiology of changes in absolute lung volumes

“…This is a likely explanation for why muscle relaxants are useful in patients with respiratory failure treated with continuous positive airway pressure (CPAP), in whom expiratory muscle activation seems to limit the increases in lung volumes which would otherwise accompany CPAP [12].…”

“…Alterations in surface tension may have more impact on lung recoil [5,6] in patients with a variety of diseases than previously recognized. Reflexes [7][8][9][10][11][12] which modify the net inspiratory and expiratory muscle force are additional but poorly understood possible determinants of changes in lung volumes which occur with disease.…”

Interpretation of the pulmonary artery occlusion pressure in mechanically ventilated patients with large respiratory excursions in intrathoracic pressure

“…Errors of this magnitude may adversely influence clinical diagnosis and therapy. The presence of significant respiratory muscle activity in the assist-control mode of mechanical ventilation is not surprising in the light of previous studies [9][10][11][12][13]. Marini and co-workers have documented considerable inspiratory muscle effort during machine-assisted breaths in the assist-control mode [9,10].…”

Use of expiratory change in bladder pressure to assess expiratory muscle activity in patients with large respiratory excursions in central venous pressure

“…If the elevated CVP was shown to be influenced by significant expiratory muscle activity, there might be less likelihood that fluids would be inappropriately withheld. Forceful expiration could become important during CVP-guided fluid resuscitation of septic shock, because expiratory muscles are activated with hyperpnea, and a cardinal feature of severe sepsis is increased ventilatory drive [17][18][19]. Early goal-directed therapy of septic shock calls for rapid infusion of fluids to Fig.…”