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Anesth Analg 2007; 105:1079-1082 © 2007 International Anesthesia Research Society doi: 10.1213/01.ane.0000280440.08530.fb
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Abstract |
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 Top Abstract IntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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BACKGROUND: Use of the transpulmonary thermodilution technique has been suggested for extended hemodynamic monitoring in critically ill patients. However, many of these patients also require renal replacement therapy (RRT). Therefore, we analyzed the influence of venovenous RRT on measurement of cardiac index (CI), intrathoracic blood volume index (ITBVI), and extravascular lung water index (EVLWI).
METHODS: We studied 24 consecutive critically ill patients (15 males, 9 females; age 39–81, mean 62 yr) who had received a clinically indicated 5F femoral arterial catheter (PV2015L20, Pulsion Medical Systems, Germany), which was connected to a monitor (PiCCOplus, Pulsion Medical Systems, Germany). A 12F dialysis catheter (Trilyse Expert, Vygon) was either advanced from the vena femoralis into the vena cava inferior (n = 12) or placed into the superior vena cava (n = 12). Patients continuously received heparin for anticoagulation. Hemodynamic measurements were performed in triplicate by central venous injection of saline (15 mL, <8°C) during RRT, during a brief interruption in RRT (by disconnection, without retransfusion), and immediately after reconnection. Ventilator settings, fluid status, and vasoactive drugs remained unchanged.
RESULTS: RRT was associated with significant changes in CI (mean change, –0.1 L/min/m2, P = 0.003) and ITBVI (mean change, –18 mL/m2, P = 0.02), whereas EVLWI was unaffected (mean change, +0.1 mL/kg, P = 0.42). The influence of RRT on CI, ITBVI, and EVLWI was not statistically different in both subgroups.
CONCLUSIONS: RRT had no clinically relevant effect on measurement of CI, ITBVI, and EVLWI in patients with sepsis and maintained cardiac output. Furthermore, the dialysis catheter tip position had no significantly different influence under these conditions.
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Introduction |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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The transpulmonary indicator dilution technique is increasingly used for extended hemodynamic monitoring in the intensive care unit (ICU) (1,2). With this system, it is possible to measure cardiac output (CO), intrathoracic blood volume (ITBV), which is more appropriate than absolute cardiac filing pressures for assessment of cardiac preload (3), and extravascular lung water (EVLW) as a measure of lung edema. EVLW is of prognostic value in critically ill patients (4), and its use may reduce duration of mechanical ventilation and length of ICU stay (5).
The double-indicator (thermo-dye) dilution technique using indocyanine green is regarded as the clinical reference technique; however, experimental and clinical data have revealed that single transpulmonary thermodilution is accurate enough for estimating ITBV and EVLW (6–8).
However, many patients undergoing monitoring by the transpulmonary thermodilution technique also require renal replacement therapy (RRT). The influence of RRT on the accuracy of measured variables has not yet been studied. Therefore, we evaluated the influence of RRT on the measurement of cardiac index (CI), ITBV, and EVLW by the transpulmonary thermodilution technique. We also assessed whether the position of the tip of the dialysis catheter (superior vena cava versus inferior vena cava) had a significantly different influence on these measurements.
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METHODS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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After approval by our local ethics committee (the need for informed consent was waived), we studied 24 consecutive critically ill patients (15 males, 9 females; age 39–81, mean 62 yr) in a surgical ICU who underwent clinically indicated monitoring using the transpulmonary thermodilution technique. Patients’ mean body weight was 86 ± 16 kg, mean height was 172 ± 7 cm, and mean body surface area was 1.98 ± 0.24 m2. On average, the Acute Physiology and Chronic Health Evaluation score was 29 ± 8, the Simplified Acute Physiology score was 64 ± 15, and the Sequential Organ Failure Assessment score was 14 ± 3. All patients had already been hemodynamically stabilized, with 22 patients receiving norepinephrine (mean, 0.56 µg · kg–1 · min–1) and/or dobutamine (mean, 4.6 µg · kg–1 · min–1). All patients had a 5F femoral arterial thermistor catheter (PV2015L20, Pulsion Medical Systems, Germany), which was connected to a monitor (PiCCOplus, version 7.0 Pulsion Medical Systems, Germany). Furthermore, each patient had a 12F dialysis catheter in situ (12F-Trilyse Expert, length 20 cm, Vygon, Aachen, Germany), which was either completely advanced from the vena femoralis to the inferior vena cava (n = 12) or placed in the superior vena cava (n = 12) as confirmed by chest radiograph. All patients were being treated with venovenous RRT using BM11 (filtration) and BM14 (substitution) (Edwards Lifesciences Germany GmbH, Unterschleissheim, Germany).
For anticoagulation of the extracorporeal circuit, all patients continuously received heparin. The dialysis system comprised an AN69ST Nephral membrane (Hospal, Meyzien, France) and a tube system (Hemotronic, Edwards Lifesciences GmbH, Unterschleissheim, Germany). Hemodynamic measurements were made in triplicate during RRT, during a brief interruption in RRT, and immediately after restarting RRT. At each time point, 15 mL of cooled 0.9% saline (<8°C) was injected manually through the distal lumen of a central venous catheter (Certofix Trio, Braun, Melsungen, Germany). The correct position of this catheter in the superior vena cava was checked by radiograph.
In detail, RRT was briefly interrupted by connecting both parts of the circuit (i.e., red and blue lines) to a stopcock without retransfusion of the blood. Subsequently, the circuit was allowed to function as a closed circuit. Adjustments of RRT remained totally unchanged and the system was reconnected to the patient after the second time point.
Blood flow rates were between 80 and 150 mL/min: 80 (n = 1 patient), 100 (n = 9 patients), 120 (n = 12 patients), 140 (n = 1 patient), and 150 mL/min (n = 1 patient), respectively. Body temperature, as continuously indicated by the thermistor catheter, remained unchanged during the study (36.7°C ± 0.8°C). No infusions were administered to keep fluid status unchanged.
Before each hemodynamic measurement, arterial blood samples were drawn for determination of blood gases and hemoglobin concentration to verify isovolemia. At baseline, mean Pao2 was 12.7 kPa, Paco2 was 5.7 kPa, and lactate was 5.6 mmol/L. Dosages of vasoactive and sedative drugs and ventilator settings remained absolutely constant during the study. Fio2 was 45% ± 10%, inspiratory peak pressure was 26 ± 6 mbar, and positive end-expiratory pressure (PEEP) was 10 ± 4 mbar. All measurements were performed within approximately 15 min and studies were completed without negative effects.
Raw data on CO, ITBV, and EVLW were indexed by individual body measures. We calculated CI, ITBVI (ITBV divided by body surface area), and EVLWI (EVLW divided by body weight). Hemodynamic data at the three time points were compared by ANOVA for repeated measurements and an all-pair-wise comparison procedure. By using data from all 24 patients, we used a mixed linear regression model to analyze the effect of RRT on CI, ITBVI, and EVLWI. Variable estimates of means and variances or standard deviations are reported, as well as P values and 95% confidence intervals. In the two different patient populations (dialysis catheter tip in the superior vena cava or inferior vena cava), for each of the three variables (CI, ITBVI, and EVLWI), a mixed linear regression model was fitted to the data, with three nested random effects (residual variance of repetitions at measurement times, variance between measurement times in the same patient, variance between patients) and RRT as a fixed effect. To determine whether blood flow relates to the changes in CI, ITBVI, and EVLWI, we added the blood flow rate as a covariate to the models in an additional analysis. Statistical analysis was performed using the software SPSS for Windows (version 12.0). Statistical significance was considered at P < 0.05.
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RESULTS |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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Heart rate, mean arterial blood pressure, central venous pressure, and hemoglobin concentration remained unchanged (Table 1). Over all 24 patients, RRT was associated with a statistically significant change in mean CI of –0.1 [95% confidence interval: –0.16 to –0.03 L/min/m2] (P = 0.003) and –18 for ITBVI (P = 0.02) [95% confidence interval: –33 to –3 mL/m2] whereas measurement of EVLWI was not influenced by RRT (mean change, +0.1) [95% confidence interval: 0.3 to –0.1 mL/kg] (P = 0.42) (Table 2). The position of the dialysis catheter tip had no significantly different effect on the changes by RRT in CI, ITBVI, and EVLWI. Although statistically significant in both subgroups, there were smaller variations between measurement times than between patients or repetitions within the same time point (Table 3). This was also found when analyzing both subgroups separately from each other. When blood flow was added to the model, it was not associated with CI and EVLWI, but revealed a weak negative correlation with ITBVI (P = 0.034). However, the RRT effects reported in Table 2 could not be explained by blood flow variations.
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DISCUSSION |
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TopAbstractIntroductionMETHODSRESULTSDISCUSSIONREFERENCES |
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We found that RRT does not influence measurements by transpulmonary thermodilution in a clinically relevant manner and that the position of the dialysis catheter tip (inferior vena cava vs superior vena cava) had no significantly different effect. The statistically significant changes by RRT on CI and ITBVI can be considered as clinically irrelevant, especially as long as variability in CO measurement exceeds 10% (9). There was no influence of RRT on measurement of EVLWI and no correlation between blood flow rates and differences in CI. Accordingly, the extracorporeal system does not need to be disconnected to measure CI, ITBVI, and EVLWI.
Previous studies found good agreement between CO obtained by transpulmonary thermodilution and pulmonary artery thermodilution (10–12). However, the effects of extracorporeal systems on the accuracy of CO measurement per se are limited. In theory, the higher the flow of the extracorporeal circuit the higher the overestimation of CO by the thermodilution technique (13). For instance, determination of CO is crucially dependent on the correct mass of indicator injected, and any loss may result in an error in the determination of CO (14,15). Further factors may be manual injection, injection port, indicator amount, and loss of indicator during injection (dead-space) (16–18). However, in our study, we tried to control for those factors, as they were present in all our measurements. We used one catheter type and site (superior vena cava) and we kept the injection volume constant. However, because of manual injections, we cannot exclude some of these influencing factors. Furthermore, we could show that the position of the dialysis catheter tip has no significantly different impact, and the effect of a probably higher loss of indicator at the superior vena cava site when compared with the inferior vena cava was not confirmed.
Our results may be explained by the fact that no "loss of indicator" occurred independently where the catheter tip was placed. However, as indicated clinically, blood flow rates were relatively low when compared with global flow (CO). In our patients, CI was high (mean, 3.8 L/min/m2, lowest 2.2 L/min/m2), and we can only speculate that values for CI, ITBVI, and EVLWI were reliable as long as the relation of extracorporeal blood flow to global flow (i.e., cardiac output) was actually very low. However, further clinical or experimental studies would be required to elucidate the relevance of RRT in settings other than ours, including higher extracorporeal flow rates and patients with low CO states.
Independently, we found that variability of results was more pronounced between patients than between time points, and this finding can be interpreted as a marker of good acquisition quality. More clearly, the differences we measured between the different time points are likely to be real changes. Of note, we did not find a difference between both catheter tip position sites, indicating that this point is irrelevant for clinical purposes.
Our study has several limitations. First, we did not use the thermo-dye technique as the clinical reference standard for measurement of ITBV and EVLW. However, our intention was not to validate the system itself, but to assess the accuracy of transpulmonary thermodilution during RRT. Furthermore, we studied patients with relatively high CO while using relatively low extracorporeal flow rates. Data in patients with low-CO states and/or higher blood flow rates would be interesting, as the influence on the reliability of the results obtained by the transpulmonary thermodilution would be greater.
In conclusion, RRT with low blood flow rates has no clinically relevant effect on the accuracy of the measurement of CI, ITBVI, and EVLWI in patients with sepsis and maintained CI. Furthermore, the venous access port used for RRT does not significantly influence measurement under these conditions.
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Footnotes |
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Accepted for publication June 28, 2007.
This work was presented, in part, at the 26th International Symposium on Intensive Care and Emergency Medicine (ISICEM), Brussels, Belgium, March 21–24, 2006.
Dr. Samir Sakka has received honoraria from Pulsion Medical Systems AG for giving lectures.
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REFERENCES |
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