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Department of Anesthesiology; University of Hirosaki School of Medicine; Hirosaki, Japan; ishihara{at}cc.hirosaki-u.ac.jp
To the Editor:
I appreciate the interest shown by van Tulder et al. (1) in our fluid volume study (2) using initial distribution volume of glucose (IDVG) as an indicator. They reported that IDVG was not sensitive to a standard fluid challenge early after cardiac surgery. Unfortunately, they misunderstood the concept of IDVG, and they did not simultaneously measure cardiac output (CO). They used IDVG as a surrogate measure of circulating blood volume or plasma volume (PV), but IDVG is a measure of the central extracellular fluid volume, which is PV plus the interstitial volume of highly perfused tissues. As such, IDVG has a better correlation with CO after major surgery than PV (3).
Their intravascular fluid administration did not yield an increase in IDVG. Interestingly, mean arterial blood pressure (MAP) also remained unchanged despite an increase in central venous pressure (CVP), suggesting that a considerable part of administered fluid volume distributed in the peripheral compartment rather than the central compartment. We observed a decrease in IDVG after redistribution of fluid volume to the peripheral compartment even when PV remained unchanged (4). Moreover, we have recently reported that intravascular fluid administration during hemodynamically unstable states early after esophagectomy consistently yielded an increase in IDVG, systolic arterial blood pressure, CVP, CO, and intrathoracic blood volume (5). In contrast to CVP and intrathoracic blood volume, both CO and IDVG were not consistently decreased when hypotension developed. However, the relationship between IDVG and CO was consistent.
van Tulder et al. (1) also reported weak reproducibility of IDVG. However, it seems inappropriate to test the reproducibility of IDVG early after cardiac surgery because patients might have internal bleeding, temperature change, alterations in vasomotor tone, or fluid shifts between compartments during the repeated measurements of IDVG (1) even though routine clinical monitoring supported stable hemodynamic states. Junghans et al. (6) demonstrated that clinical judgments about the patient's intravascular volume based on conventional monitoring even by experienced physicians are absolutely contrary to those based on measurement of intrathoracic blood volume after upper gastrointestinal tract surgery. We have already reported the repeatability of IDVG in hemodynamically stable patients, including cardiac surgical patients (7). Patients whose admittance to the intensive care unit was <12 h before glucose challenge were excluded from the study. The bias of our IDVG measurements was 0.08 ± 0.32 (sd) L at a 30-min interval.
In conclusion, we believe that the central extracellular fluid volume rather than PV has a major impact on a considerable variability of IDVG in individual patients and that the variability is not attributable to methodological flaws of IDVG determination.
References
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