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Effect of pulmonary perfusion on the slopes of single-breath test of CO₂

Departments of ¹Anesthesiology, ²Cardiovascular Surgery, and ³Critical Care Medicine, Hospital Privado de Comunidad, Mar del Plata, Argentina; Departments of ⁴Critical Care Medicine and ⁵Pneumonology, Fundación Jimenez Díaz, Madrid, Spain; and ⁶Respiratory Intensive Care Unit, Pulmonary Division, Hospital das Clínicas, University of Sao Paulo, Brazil

Submitted 5 October 2004 ; accepted in final form 28 March 2005

The objective of this study was to evaluate the effects of lung perfusion on the slopes of phases II (S_II) and III (S_III) of a single-breath test of CO₂ (SBT-CO₂). Fourteen patients submitted to cardiac surgery were studied during weaning from cardiopulmonary bypass (CPB). Pump flow was decreased in 20% steps, from 100% (total CPB = 2.5 l·min^–1·m^–2) to 0%. This maneuver resulted in a progressive and opposite increase in pulmonary blood flow (PBF) while maintaining ventilator settings constant. SBT-CO₂, respiratory, and hemodynamic variables remained unchanged before and after CPB, reflecting a constant condition at those stages. S_III was similar before and after CPB (19.6 ± 2.8 and 18.7 ± 2.1 mmHg/l, respectively). S_III was lowest during 20% PBF (8.6 ± 1.9 mmHg/l) and increased in proportion to PBF until exit from CPB (15.6 ± 2.2 mmHg/l; P < 0.05). Similarly, S_II and the CO₂ area under the curve increased from 163 ± 41 mmHg/l and 4.7 ± 0.6 ml, respectively, at 20% PBF to 313 ± 32 mmHg/l and 7.9 ± 0.6 ml (P < 0.05) at CPB end. When S_II and S_III were normalized by the mean percent expired CO₂, they remained unchanged during the protocol. In summary, the changes in PBF affect the slopes of the SBT-CO₂. Normalizing S_II and S_III eliminated the effect of changes in the magnitude of PBF on the shape of the SBT-CO₂ curve.

cardiopulmonary bypass; carbon dioxide; capnography

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