Spontaneous Breathing Patterns During Maximum Extracorporeal CO 2 Removal in Subjects With Early Severe ARDS

Abstract

BACKGROUND:

Switching patients affected by early severe ARDS and undergoing extracorporeal membrane oxygenation (ECMO) from controlled ventilation to spontaneous breathing can be either beneficial or harmful, depending on how effectively the breathing pattern is controlled with ECMO. Identifying the factors associated with ineffective control of spontaneous breathing with ECMO may advance our pathophysiologic understanding of this syndrome.

METHODS:

We conducted a prospective study in subjects with severe ARDS who were on ECMO support ≤ 7 d. Subjects were switched to minimal sedation and pressure-support ventilation while extracorporeal CO₂ removal was increased to approximate the subject’s total CO₂ production ( ${\dot{\mathbf{V}}}_{\mathbf{\text{C}}{\mathbf{\text{O}}}_{\mathbf{2}}}$ ). We calculated the rapid shallow breathing index (RSBI) as breathing frequency divided by tidal volume. We explored the correlation between certain characteristics recorded during pretest controlled ventilation and the development of apnea (ie, expiratory pause lasting > 10 s; n = 3), normal breathing pattern (ie, apnea to RSBI ≤ 105 breaths/min/L; n = 6), and rapid shallow breathing (RSBI > 105 breaths/min/L; n = 6) that occurred during the test study.

RESULTS:

The ratio of extracorporeal CO₂ removal to the subjects’ ${\dot{\mathbf{V}}}_{\mathbf{\text{C}}{\mathbf{\text{O}}}_{\mathbf{2}}}$ was >90% in all 15 subjects, and arterial blood gases remained within normal ranges. Baseline pretest Sequential Organ Failure Assessment score, total ${\dot{\mathbf{V}}}_{\mathbf{\text{C}}{\mathbf{\text{O}}}_{\mathbf{2}}}$ and ventilatory ratio increased steadily, whereas ${\mathbf{\text{P}}}_{{\mathbf{\text{aO}}}_{\mathbf{2}}}$ / ${\mathbf{\text{F}}}_{{\mathbf{\text{IO}}}_{\mathbf{2}}}$ was higher in subjects with apnea compared to intermediate RSBI ≤105 breaths/min/L and elevated RSBI >105 breaths/min/L. In subjects with rapid shallow breathing, baseline lung weight measured with quantitative computed tomography scored higher, as well.

CONCLUSIONS:

In early severe ARDS, the factors associated with rapid shallow breathing despite maximum extracorporeal CO₂ extraction include less efficient CO₂ and O₂ exchange by the natural lung, higher severity of organ failure, and greater magnitude of lung edema.