The ventilatory recruitment threshold for carbon dioxide

We report our initial experience with a technique with which the chemoresponsiveness of the respiratory controller can be characterized in terms of an inspiratory on-switch threshold to CO₂. After suppression of phasic respiratory muscle activity by mechanical ventilation, a CO₂ recruitment threshold (PCO₂RT) was defined as the lowest alveolar CO₂ tension at which CO₂ supplementation to inspired gas caused a reappearance of inspiratory efforts. Because PCO₂RT can be determined in the absence of a mechanical load on the ventilatory pump, respiratory system mechanics and inspiratory muscle function should not influence the measurement itself. Thus, this technique may be helpful to study ventilatory requirements and load responses in critically ill patients with respiratory failure. We have shown that inspiratory muscle recruitment can be equally well-inferred from changes in the airway pressure and flow tracings during mechanical ventilation, from the pattern of chest wall displacement, and from the integrated diaphragm electromyogram. Within a subject, PCO₂RT is a reproducible measurement that is not influenced by ventilator settings and end-expiratory lung volume, provided that phasic respiratory muscle has been suppressed prior to CO₂ supplementation. Details of the methodology, the likely determinants of PCO₂RT, and the clinical utility of this technique are discussed.