States of low pulmonary blood flow can be detected non-invasively at the bedside measuring alveolar dead space

Tusman, Gerardo; Suarez-Sipmann, Fernando; Paez, Gabriel; Alvarez, Jorge; Bohm, Stephan
June 2012
Journal of Clinical Monitoring & Computing;Jun2012, Vol. 26 Issue 3, p183
Academic Journal
We tested whether the ratio of alveolar dead space to alveolar tidal volume (VD/VT) can detect states of low pulmonary blood flow (PBF) in a non-invasive way. Fifteen patients undergoing cardiovascular surgeries with cardiopulmonary bypass (CPB) were studied. CPB is a technique that excludes the lungs from the general circulation. The weaning of CPB is a model that manipulates PBF in vivo because each time blood flow through the CPB decreases, expected PBF (ePBF) increases. Patients were liberated from CPB in steps of 20 % every 2′ starting from 100 % CPB (very low ePBF) to 0 % CPB (100 % ePBF). During constant ventilation, volumetric capnograms were recorded and Bohr's dead space ratio (VD/VT), VD/VT and the ratio of airway dead space to tidal volume (VD/VT) were calculated. Before CPB, VD/VT was 0.36 ± 0.05, VD/VT 0.21 ± 0.04 and VD/VT 0.18 ± 0.06 (mean ± SD). During weaning from CPB, VD/VT remained unchanged while VD/VT and VD/VT decreased with increasing ePBF. At CPB of 80, 60, 40 and 20 % VD/VT was 0.64 ± 0.06, 0.55 ± 0.06, 0.47 ± 0.05 and 0.40 ± 0.04, respectively; p < 0.001 and VD/VT 0.53 ± 0.07, 0.40 ± 0.07, 0.29 ± 0.06 and 0.25 ± 0.04, respectively; p < 0.001). After CPB, VD/VT and VD/VT reached values similar to baseline (0.37 ± 0.04 and 0.19 ± 0.06, respectively). At constant ventilation the alveolar component of VD/VT increased in proportion to the deficit in lung perfusion.


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