Abstract: A method and apparatus is described for preventing contamination of a gas analyzer of respiratory gas exchange analyzing equipment due to a patient's saliva. The method involves determining a resistance value of the inspired respiratory gas flow in a sample line leading to the gas analyzer. This is determined by calculating the absolute pressure in the line at the gas analyzer. The pressure difference from atmospheric pressure is then divided by the sample flow rate to obtain a resistance value. A microprocessor in the equipment compares the calculated resistance of the flow with a previously stored reference threshold resistance. If the calculated resistance exceeds the predetermined threshold, the microprocessor sends a signal to a vacuum pump used to draw the respiratory gases through the gas analyzer. The vacuum pump is immediately shut down when the calculated resistance exceeds the predetermined resistance threshold and, hence, any saliva in the sample line is not drawn into the gas analyzer.