Lung Volume and VA/Q Distribution Response to Intravenous versus Inhalation Anesthesia in Sheep

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Abstract
The effects of i.v. pentobarbtial vs. N2O/halothane inhalation anesthesia on blood gases, distribution of ventilation-perfusion ratios (VA/Q) and lung volume (FRC) were compared in 8 tracheostomized sheep in the lateral decubitus position. Pentobarbital anesthesia produced no significant changes (from awake control) in arterial blood [partial pressure] PO2 [PaO2] or PCC2, ventilation-perfusion inequality, intrapulmonary shunt or FRC during either spontaneous breathing or mechanical ventilation with muscle paralysis. With inhalation anesthesia, PaO2 decreased from 132 .+-. 13 mmHg awake to 106 .+-. 1 mmHg and 104 .+-. 6 mmHg (FIO2 all 0.3) during spontaneous and mechanical ventilation. Shunt increased from 1.4 .+-. 1.0% awake to 10.6 .+-. 4.5 and 13.9 .+-. 5.3%, respectively. Mean VA/Q decreased from 0.39 .+-. 0.07 awake, to 0.21 .+-. 0.06 and 0.29 .+-. 0.07. Log standard deviation of VA/Q increased from 0.66 .+-. 0.12 awake to 0.83 .+-. 0.28 and 0.89 .+-. 0.15. FRC decreased from 1.66 .+-. 0.65 l to 1.46 .+-. 0.62 and 1.22 .+-. 0.63 l, respectively. Differences in response to i.v. vs. inhalation anesthesia for these variables all were statistically significant at P < 0.05. FRC and shunt changes with anesthesia showed significant correlation for both spontaneous (r = -0.80) and mechanical ventrilation (r = -0.77), P < 0.005 for both. Thus, the differences between lung volume and gas exchange effects of i.v. vs. inhalation anesthesia in sheep may have been related causally.