Bedside Techniques for Calculating Dead Space/Tidal Volume Ratio (VD/VT) and Pulmonary Shunt (QS/QT)

Abstract

Calculations of VD/VT and $\dot{Q}s/\dot{Q}T$ are relatively simple bedside procedures used to evaluate progress of patients in respiratory failure. These calculations may be performed separately or simultaneously and whether the patient is breathing spontaneously or being mechanically ventilated. VD/VT determination requires measuring $P{a}_{C{O}_2}$ in an arterial blood sample and measuring ${\text{P}}_{{\overline{E}}_{C{O}_2}}$ in a mixed exhaled gas sample. Calculation of $\dot{Q}s/\dot{Q}T$ requires measuring hemoglobin, $P{a}_{C{O}_2}$ , $P{a}_{O_2}$ and $P{\overline{v}}_{O_2}$ while the patient breathes 100% oxygen long enough to clear nitrogen from the lungs. These procedures may be simplified by guidelines given here.