Previous studies have established the role of various tissue compartments in the kinetics of inhaled anesthetic uptake and elimination. The role of normal lungs in inhaled anesthetic kinetics is less understood. In juvenile pigs with normal lungs, the authors measured desflurane and sevoflurane washin and washout kinetics at three different ratios of alveolar minute ventilation to cardiac output value. The main hypothesis was that the ventilation/perfusion ratio (VA/Q) of normal lungs influences the kinetics of inhaled anesthetics.
Seven healthy pigs were anesthetized with intravenous anesthetics and mechanically ventilated. Each animal was studied under three different VA/Q conditions: normal, low, and high. For each VA/Q condition, desflurane and sevoflurane were administered at a constant, subanesthetic inspired partial pressure (0.15 volume% for sevoflurane and 0.5 volume% for desflurane) for 45 min. Pulmonary arterial and systemic arterial blood samples were collected at eight time points during uptake, and then at these same times during elimination, for measurement of desflurane and sevoflurane partial pressures. The authors also assessed the effect of VA/Q on paired differences in arterial and mixed venous partial pressures.
For desflurane washin, the scaled arterial partial pressure differences between 5 and 0 min were 0.70 ± 0.10, 0.93 ± 0.08, and 0.82 ± 0.07 for the low, normal, and high VA/Q conditions (means, 95% CI). Equivalent measurements for sevoflurane were 0.55 ± 0.06, 0.77 ± 0.04, and 0.75 ± 0.08. For desflurane washout, the scaled arterial partial pressure differences between 0 and 5 min were 0.76 ± 0.04, 0.88 ± 0.02, and 0.92 ± 0.01 for the low, normal, and high VA/Q conditions. Equivalent measurements for sevoflurane were 0.79 ± 0.05, 0.85 ± 0.03, and 0.90 ± 0.03.
Kinetics of inhaled anesthetic washin and washout are substantially altered by changes in the global VA/Q ratio for normal lungs.
The role of lung gas exchange in whole-body kinetics can be studied by direct measurements of anesthetic partial pressures in mixed venous (Pmv) and arterial (Part) blood samples
Washin and washout of desflurane should be more rapid than sevoflurane because sevoflurane has higher gas solubilities in blood and tissues
The washin and washout kinetics of simultaneously administered desflurane and sevoflurane were assessed in seven piglets by measuring Pmv and Part during uptake and elimination under normal, low, and high ventilation/perfusion ratio (VA/Q) conditions
Faster arterial kinetics for desflurane were generally maintained for both washin and washout under all VA/Q conditions
The low VA/Q condition decreased the differences in scaled Part between 0 and 5 min; the high VA/Q condition increased these differences from the low VA/Q value to a value approaching or exceeding the value for normal VA/Q
Mixed venous kinetics were slower than arterial kinetics for washin and washout and were less influenced by VA/Q