Table 2 Summary of dead space ventilation-related indices

Bohr [8]

\({\text{VD}}_{{{\text{Bohr}}}} /{\text{VT}} = \frac{{{\text{PACO}}_{2} - {\text{P}}\overline{{\text{E}}} {\text{CO}}_{2} }}{{{\text{PACO}}_{2} }}\)

Direct and physiologically intuitive to calculate

Relies on Vcap; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Enghoff [9]

\({\text{VD}}_{{{\text{B}} - {\text{E}}}} /{\text{VT}} = \frac{{{\text{PACO}}_{2} - {\text{P}}\overline{{\text{E}}} {\text{CO}}_{2} }}{{{\text{PACO}}_{2} }}\)

Direct and physiologically intuitive to calculate; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Relies on Vcap; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Siddiki et al. [10] and Beitler et al. [11]

\(\frac{{{\text{VD}}}}{{{\text{VT}}}} = 1 - \frac{{\left( {{\dot{\text{V}}\text{CO}}_{2} \times 0.863} \right)}}{{\left( {{\text{RR}} \times {\text{VT}} \times {\text{PaCO}}_{2} } \right)}}\)

\({\dot{\text{V}}\text{CO}}_{2} = \frac{{{\text{REE}}}}{{\left( {\frac{5.616}{{{\text{RQ}}}} + 1.584} \right)}}\)

Uses a rearranged alveolar ventilation equation in combination with indirect measurements of \({\dot{\text{V}}\text{CO}}_{2}\), and modified Harris–Benedict equation, or unadjusted Harris–Benedict equation, or Penn state equation is used to estimate REE

Does not rely on Vcap; considers the influence of \({\dot{\text{V}}\text{CO}}_{2}\), especially suitable for patients treated with ECLS; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Predictive power is limited especially due to estimate \({\dot{\text{V}}\text{CO}}_{2}\) indirectly; changes in \({\dot{\text{V}}\text{CO}}_{2}\) due to ECLS procedures should be consider when make comparisons of VR among individuals; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Beitler et al. [11]

\(\begin{aligned} \frac{{{\text{VD}}}}{{{\text{VT}}}} = & 0.1726 + 0.0059 \times {\text{RR}} + 0.0054 \times {\text{PEEP}} + 0.0293 \\ & \times \;{\text{LIS}} + 0.0036 \times {\text{PaCO}}_{2} \times {\dot{\text{V}}\text{E}} + 0.000057 \\ & \times \;{\text{PaCO}}_{2} \times {\text{age}} \\ \end{aligned}\)

Does not rely on Vcap; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Predictive power is limited especially due to use physiological variables and a predictive equation; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Sinha et al. [6]

\({\text{VR}} = \frac{{{\dot{\text{V}}\text{E}}_{{{\text{measured}}}} \times {\text{PaCO}}_{{2{\text{measured}}}} }}{{{\dot{\text{V}}\text{E}}_{{{\text{predicted}}}} \times {\text{PaCO}}_{{2{\text{predicted}}}} }}\) or

\({\text{VR}} = \frac{{{\dot{\text{V}}\text{CO}}_{2} \times 0.863}}{{\left( {1 - \frac{{{\text{VD}}}}{{{\text{VT}}}}} \right) \times \left( {{\text{VE}}_{{{\text{predicted}}}} \times {\text{PaCO}}_{{2{\text{predicted}}}} } \right)}}\)

\({\text{VE}}_{{{\text{predicted}}}} = 100{\text{mL}}\cdot{\text{min}}^{ - 1} \times {\text{PBW}}\),

\({\text{PaCO}}_{{2{\text{predicted}}}} = 37.5{\text{mmHg}}\)

Does not rely on Vcap; considers the influence of \({\dot{\text{V}}\text{CO}}_{2}\), especially suitable for patients treated with ECLS; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Changes in \({\dot{\text{V}}\text{CO}}_{2}\) due to ECLS procedures should be consider when make comparisons of VR among individuals; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

The ARDS Berlin Definition Task Force [3]

\({\dot{\text{V}}\text{E}}_{{{\text{CORR}}}} = {\dot{\text{V}}\text{E}}_{{{\text{measured}}}} \times {\text{PaCO}}_{{2{\text{measured}}}} /40\) or

\({\dot{\text{V}}\text{E}}_{{{\text{CORR}}}} = \frac{{{\dot{\text{V}}\text{CO}}_{2} \times 0.863}}{{\left( {1 - \frac{{{\text{VD}}}}{{{\text{VT}}}}} \right) \times 40}}\)

Does not rely on Vcap; consider the influence of \({\dot{\text{V}}\text{CO}}_{2}\), especially suitable for patients treated with ECLS; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Does not standardize \({\dot{\text{V}}\text{E}}\) for patients’ weight, the accuracy to reflect \(\frac{{{\text{VD}}}}{{{\text{VT}}}}\) is limited; changes in \({\dot{\text{V}}\text{CO}}_{2}\) due to ECLS procedures should be considered when make comparisons of VR among individuals; does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Gattinoni et al. [7]

\(\frac{{{\text{PETCO}}_{2} }}{{{\text{PaCO}}_{2} }}\)

Does not rely on Vcap; a global index to assess \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch

Assumes \({\text{PETCO}}_{2} = {\text{PACO}}_{2}\); does not include \({\text{VD}}_{{{\text{aw}}}}\); neglects the influences of \({\dot{\text{V}}\text{CO}}_{2}\) and \({\dot{\text{V}}\text{E}}\); does not reflect regional \({\dot{\text{V}}\text{A}}/{\dot{\text{Q}}}\) mismatch