Multiple inert gas elimination technique for determining ventilation/perfusion distributions in rat during normoxia, hypoxia and hyperoxia

1. The use of the multiple inert gas elimination technique (MIGET) in quant ifying ventilation/perfusion distributions ((V) over dot A/(Q) over dot) in small animals, such as the rat, may cause results to be biased due to haem odilution produced by the large volume of liquid infused intravenously. 2. We tested two methods of administering inert gases in rats using the MIG ET: (i) standard continuous intravenous administration of inert gases (meth od A); and (ii) a new method based on the physicochemical properties of eac h inert gas (method B). This method included acute simultaneous inert gas a dministration using three pathways: inhalation, intravenous infusion and re ctal infusion. Both MIGET methods were applied to obtain data while breathi ng three different inspiratory fractions of oxygen (F-IO2): normoxia, hypox ia and hyperoxia. 3. Inert gas levels obtained from blood or expired air samples were suffici ent for chromatographic measurement, at least during a 2 h period. The (V) over dot A/(Q) over dot distributions reported using both methods were acce ptable for all the physiological conditions studied; therefore, the alterna tive method used here may be useful in further MIGET studies in rats becaus e haemodilution resulting from continuous intravenous infusion of less-solu ble gases can be avoided. 4. Normoxic rats showed lower mean values of the (V) over dot A/(Q) over do t ratio of ventilation distribution and higher mean values of the (V) over dot A/(Q) over dot ratio of perfusion distribution with the usual method of inert gas administration (method A). These non-significant differences wer e observed under almost all physiological conditions studied and they could be caused by haemodilution. Nevertheless, the effect of interindividual di fferences cannot be discarded. An additional effect of the low haematocrit on cardiovascular changes due to low FIO2, such as pulmonary vasoconstricti on or increased cardiac output, may explain the lower dispersion of perfusi on distributions found in group A during hypoxia.