Mass Spectrometry Monitoring of Respiratory Variables in an Intensive Care Unit

Abstract

Mass spectrometry is a technique for rapid analysis and continuous monitoring of selected respiratory variables in ICU patients. The commercially available Respiratory Intensive Care System (RICS), which includes a mass spectrometer, was used to study 12 ICU patients in acute respiratory failure. The difference between minimum expired and arterial oxygen tension values, P (Emin-a)02, agreed closely with the alveolar-arterial oxygen tension difference, P(A-a)02, as traditionally calculated by means of the alveolar gas equation. The gradient between arterial and maximum expired carbon dioxide tension, P(a-Emax)CO2, was also studied. This gradient approximated zero in 9 of the 12 patients; 2 patients showed a negative and 1 a positive gradient. Thus, PEmaxCO2, normally equal to PACO2, may sometimes be greater than PaCO2. This indicates that continuous or frequent monitoring of PEmaxCO2 can detect changes in blood gas variables caused by alterations in cardiopulmonary function that might otherwise remain undetected until deterioration has become clinically obvious and potentially life-threatening.