Normalizing Low Arterial CO₂ Tension During Mechanical Ventilation

¹ Department of Anesthesiology (Critical Care Medicine), University of Pittsburgh School of Medicine; Presbyterian-University Hospital, Pittsburgh

In ten patients with progressive pulmonary consolidation (PPC) during controlled mechanical ventilation with or without positive end expiratory pressure (PEEP) and in five patients with decompensated chronic obstructive lung disease (COLD) during patient triggered assisted respiration, low PaCO₂ was normalized by adding 1 percent to 3 percent CO₂ to the inhaled gas or by adding 200 to 300 milliliters of mechanical dead space to the ventilator tubing. As low arterial Pco₂ and high pH were normalized, significant increases occurred in arterial Po₂, cardiac output and arterial O₂ transport, while both O₂ utilization coefficient and systemic vascular resistance (SVR) decreased. Venous admixture (Q_S/Q_T) increased in seven of ten patients and decreased in three of ten with PPC. This variable decreased in the three COLD patients in whom it was measured. The total amount of blood flowing through ventilated areas of the lungs (QT-QS) increased in all patients. The venous O₂ content also increased in all and thereby shunted less desaturated blood to the arterial side. The beneficial circulatory effects of normalizing low Paco₂ values in part offset the circulatory depressant effects of PEEP. The increase in Pao₂ resulting from normalizing Paco₂ allowed lowering of inspired O₂ concentrations from toxic to nontoxic levels even when this was not possible with PEEP alone. Controlling Paco₂ by regulating Flco₂ during controlled and assisted ventilation with use of a prototype CO₂ mixer (attached to a commercially available air-O₂ mixer) seems more predictable and easier to adjust than the use of mechanical dead space.