A Graphic Approach to Assessing Interrelationships among Minute Ventilation, Arterial Carbon Dioxide Tension, and Ratio of Physiologic Dead Space to Tidal Volume in Patients on Respirators1

A graphic analysis of the relationships among minute ventilation, arterial Pco2, and the ratio of physiologic dead space to tidal volume is presented for the bedside clinical evaluation of patients receiving continuous respiratory support. With simultaneous measurement of minute ventilation and arterial Pco2, the graph can be used to determine the patient's ratio of dead space to tidal volume. This information can subsequently be used (1) for respirator adjustments to obtain the desired arterial Pco2; (2) to assess the severity of ventilation/ perfusion unevenness; and (3) to follow the patient's response to respiratory therapy. It provides the clinician with a conceptual tool with which he can assess the status of a patient's gas exchange and more accurately evaluate and plan his course of treatment.