Sequential V˙a/Q˙ distributions in the normal rabbit by micropore membrane inlet mass spectrometry

We developed micropore membrane inlet mass spectrometer (MMIMS) probes to rapidly measure inert-gas partial pressuresin small blood samples. The mass spectrometer output was linearly related to inert-gas partial pressure( r 2 of 0.996–1.000) and was nearly independent of large variations in inert-gas solubility in liquid samples. We infused six inertgases into five pentobarbital-anesthetized New Zealand rabbits and used the MMIMS system to measure inert-gas partial pressuresin systemic and pulmonary arterial blood and in mixed expired gas samples. The retention and excretion data were transformed into distributions of ventilation-to-perfusion ratios (V˙a/Q˙) with the use of linear regression techniques. Distributions ofV˙a/Q˙ were unimodaland broad, consistent with prior reports in the normal rabbit. Total blood sample volume for eachV˙a/Q˙ distribution was 4 ml, and analysis time was 8 min. MMIMS provides a convenient method to perform the multiple inert-gas elimination techniquerapidly and with small blood sample volumes.