Effect of non-invasive ventilation on the measurement of ventilatory and metabolic variables

• Non invasive ventilation can be used in conjunction with cardio pulmonary exercise testing equipment.
• Cardio pulmonary exercise testing metabolic variables are unaffected by pressure support with provided the gas density changed is accounted for.
• Oxygen uptake can be measured after correcting for gas density. This should be applied to future studies examining oxygen uptake with non invasive ventilation.

The effect of non-invasive ventilation (NIV) on the accuracy of measurements of ventilation, oxygen consumption (V˙O2) and carbon dioxide production (V˙CO2) was examined using a simulator. Known gas volumes of oxygen and carbon dioxide were delivered to a metabolic system that measured tidal volume, respiratory rate, V˙O2 and V˙CO2, both with and without NIV. Bland-Altman analyses were used to compare between conditions. NIV at pressure support (PS) 20 cm H2O compared to without NIV showed: VT, mean difference (MD) 0 mL (limits of agreement (LOA) −21 to 21) mL; V˙O2 MD −413 (LOA −810 to 16) mL/min; and V˙CO2 MD 32 (LOA −32 to 97) mL/min. For V˙O2 measurements during NIV, a correction was applied to account for increased air density due to PS. After correction, V˙O2 measurement accuracy improved; MD −46 (LOA −108 to 17) mL/min. Tidal volume and metabolic variables can be measured with acceptable accuracy during NIV, providing V˙O2 is corrected for altered gas density.