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Questionable Design to Validate the ProAQT/Pulsioflex Device

Umgelter, Andreas MD; Schmid, Roland M. MD; Huber, Wolfgang MD

doi: 10.1213/ANE.0000000000002333
Letters to the Editor: Letter to the Editor
Free

Published ahead of print July 26, 2017.

Medizinische Klinik und Poliklinik II, Klinikum rechts der Isar, Technische Universität München, Munich, Germany, Andreas.Umgelter@tum.de

Published ahead of print July 26, 2017.

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To the Editor

We read the study by Biais et al1 on the accuracy of an uncalibrated pulse-contour analysis (PCA)–derived estimate of cardiac index (CI) with interest, but we do have some concerns:

  1. Wrong title. Pulsioflex is not a device but a monitoring platform with various options to quantify CI (intermittent, continuous, transpulmonary thermodilution, calibrated and uncalibrated PCA) and the ScvO2 and the indocyanine-green-plasma-disappearance rate. The authors only investigated the uncalibrated PCA-derived estimate of CI provided by the ProAQT-subunit.
  2. Wrong setting. Uncalibrated PCA is a less invasive monitoring technology based on algorithms incorporating data on normal vascular anatomy and vascular capacitance. Consequently its domain is monitoring of relatively stable patients.

Biais et al1 use ProAQT in the setting of liver transplantation where abrupt changes of blood viscosity, abdominal pressure, afterload, blood temperature, administration of vasoconstrictors, and the relation between splanchnic and nonsplanchnic circulation are to be expected. Using uncalibrated estimates of CI in such a setting is like trying to crack a sledgehammer with a nut. Numerous studies have demonstrated the limited precision of uncalibrated estimates of CI derived from several manufacturers’ devices in this setting (Table).

Table.

Table.

  • 3. Wrong gold standard. ProAQT failed to reproduce the results of the presumptive gold standard of an automated continuous pulmonary arterial thermodilution-derived cardiac output (CCO-PAC). Derivation of CCO-PAC is based on intermittent (1/min) application of a thermal bolus by a heating filament. In case of a >10% variation of 3 consecutive measurements, the number of measurements is increased up to 8 assuming constant values of true CI. This increases the accuracy regarding constant values but necessarily impedes the detection of true changes of CI.

Critchley et al2 recognized this drawback of CCO previously:

Its accuracy is limited during periods of hypothermia…during liver transplantation.… It can take up to 12 minutes for a change in cardiac output (CO) to be fully registered by the monitor. Therefore, although continuous CO catheters can be used as trend monitors, they should not be used.2

To detect rapid changes of CO, a single thermodilution bolus would be a more appropriate gold standard. Böttiger et al3 showed that “in early phases after caval clamping and reperfusion accuracy and precision (of CCO) were not acceptable.”

  • 4. Wrong timing for trending. The limited accuracy of the estimation of true CI is a major weakness of all devices providing uncalibrated CI estimates. By contrast, the continuous, beat-by-beat-derivation of changes of PCA-based CI is considered to be their strength. Devices such as ProAQT use fast-reacting algorithms to provide continuous CI-PCA after initial calibration. The ProAQT is averaging 4 “sliding” intervals of 7.5 seconds, which results in complete recalculation within 30 seconds. Comparing a slow-reacting device requiring up to 12 minutes to estimate a changing CI with fast-reacting devices with <30 seconds latency is a flaw of the study design.
  • 5. Wrong generalization. Neither can a generalization to less challenging environments, as suggested by the headline, be based on the data presented, nor can it be inferred that changing systemic vascular resistance was the cause of the discrepancies between the methods.

Andreas Umgelter, MDRoland M. Schmid, MDWolfgang Huber, MDMedizinische Klinik und Poliklinik IIKlinikum rechts der IsarTechnische Universität MünchenMunich, GermanyAndreas.Umgelter@tum.de

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REFERENCES

1. Biais M, Mazocky E, Stecken L, et al. Impact of systemic vascular resistance on the accuracy of the pulsioflex device. Anesth Analg. 2017;124:487–493.
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