Impact of CVVHD on pulmonary gas exchange measurement

Aim

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.

Results

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.

Conclusion

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.

Table

Figure