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Impact of CVVHD on pulmonary gas exchange measurement


To quantify extrapulmonal gas exchange during CVVHD.

Patients and methods

Ten indirect calorimetry (IC) measurements (Deltatrac, Datex, Finland) were done in five stable mechanically ventilated patients requiring treatment by CVVHD (Prisma, Hospal, UK) because of MODS. Dialysate and substitution solutions did not contain bicarbonate. During each IC measurement CVVHD setting was change in 30 min intervals as shown in the table.

Estimated extracorporeal CO2 losses by CVVHD were calculated as a difference between VCO2 measured by IC when no CVVHD was performed (period 5) and other periods. The study was approved by Ethics Committee of University Hospital. ANOVA for repeated measurements, paired-t test were used for statistical analysis. Values are provided as means ± SD, P < 0.05 was considered significant.


Amount of CO2 removed by pulmonary gas exchange (CO2 pulm) during the study is shown in the Figure.

Maximal difference in CO2pulm was measured between no CVVHD (period 5) and CO2pulm during CVVHD running at maximal parameters (period 3) (292 ± 26 and 263 ± 37 ml/min, respectively, P < 0.01). The difference in C02pulm between periods 5 (no CVVHD) and standard CVVHD setting (period 4) was also significant (292 ± 26 and 268 ± 40 ml/min, respectively, P < 0.01). There was no difference in CO2pulm between periods when no CVVHD was performed (period 5) and only blood was running through the extracorporeal circuit (period 2) (292 ± 26 and 286 ± 46 ml/min, respectively; P = 0.33). There was no impact of CVVHD on VO2 measurement during the study.


During CVVHD treatment indirect calorimetry measurement underestimates VCO2 due to significant losses of CO2 into ultradiafiltrate (up to 10% of VCO2). Measurement of VO2 by IC seems to be reliable.

figure 1


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Srámek, V., Rokyta, R., Novák, I. et al. Impact of CVVHD on pulmonary gas exchange measurement. Crit Care 1 (Suppl 1), P069 (1997).

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