Several studies have examined the accuracy of the uncalibrated arterial pressure–based cardiac output monitor (FloTrac™/Vigileo™ monitor; Edwards Lifesciences, Irvine, CA) in various clinical settings.1–6 These initial investigations basically examined the accuracy of this device under stable conditions over several minutes. Because of the limitations of the thermodilution method used as a reference, its performance during rapid hemodynamic changes has not been fully evaluated. Theoretically, FloTrac/Vigileo may respond to hemodynamic changes significantly faster than thermodilution because it calculates stroke volume (SV) from the arterial pressure waveform, and the coefficient that represents vascular resistance is automatically updated every 60 seconds. However, it may overestimate SV during transient hypertension and underestimate SV during transient hypotension. Additionally, because the device uses a time-averaged algorithm to estimate vascular resistance, it may also suffer from a time delay in its calculation of SV. Thus, whether this monitor can successfully track SV during sudden changes of arterial pressure is a clinically important question. In this prospective, observational study, we examined the accuracy of SV measurement by FloTrac/Vigileo against transthoracic Doppler-based SV measurement during anesthetic induction and tracheal intubation.
With IRB approval and written informed consent, 20 patients undergoing elective abdominal aortic reconstruction were enrolled in this prospective, observational study at Keio University Hospital.
After administration of 1 μg/kg fentanyl IV, the brachial artery was cannulated with a 20-gauge intraarterial catheter (Leader-Cath; Vygon, Ecouen, France), and SV (SV-FloTrac), heart rate, and arterial blood pressure were measured with a FloTrac/Vigileo monitor (version 1.10). These data were recorded every 2 seconds in the computer. SV was also measured by a USCOM transthoracic Doppler monitor (version 1.5; USCOM, Sydney, Australia) at the left ventricular outflow tract before induction.7–11 A single investigator (YK) who determined SV with the USCOM monitor in >100 anesthetized and critically ill patients made all the measurements. General anesthesia was induced with 1.5 mg/kg propofol, and the patient was subsequently paralyzed with 0.1 mg/kg vecuronium. SV was measured with the Doppler monitor (SV-Doppler) after the patient lost consciousness, during laryngoscopy, immediately after endotracheal tube placement, and 3 minutes after endotracheal tube placement. After these measurements, anesthetic management was at the discretion of the attending anesthesiologists.
Data were expressed as mean ± SD and statistically analyzed by repeated-measures analysis of variance and Bland-Altman plot. To evaluate the possible influence of blood pressure change on the accuracy of the FloTrac/ Vigileo monitor, the relationship between changes of pulse pressure and the differences between SV-FloTrac and SV-Doppler were analyzed with Pearson correlation coefficient. P < 0.05 was used for statistical significance.
The participants consisted of 17 men and 3 women, 76 ± 6 years of age (mean ± SD), 164 ± 7 cm in height, and 60 ± 9 kg in weight. The operations lasted 202 ± 54 minutes, and all patients had an uneventful intraoperative and postoperative course. The intervals between each measurement are described in Table 1. Heart rate, and systolic, diastolic, and pulse pressure, SV-FloTrac, and SV-Doppler during anesthetic induction are also summarized in Table 1. Before laryngoscopy, systolic blood pressure and pulse pressure decreased significantly from the preinduction level, but neither SV-FloTrac nor SV-Doppler demonstrated a significant change. Systolic pressure, diastolic pressure, and SV-FloTrac increased significantly immediately after endotracheal tube placement, but the SV-Doppler did not change during or after laryngoscopy (Table 1). The difference between the FloTrac measurement of SV and the Doppler measurement of SV increased significantly during laryngoscopy and endotracheal tube placement but returned to prelaryngoscopy values within 3 minutes after endotracheal tube placement (Table 2). Figure 1 demonstrates the relationship between the systolic pressure and the measurement error of SV-FloTrac. This figure collectively demonstrates that the measurement error of FloTrac increased during the transient increase of the systolic pressure. There was a significant correlation between these 2 variables (increase of measurement error [mL] = 0.41× increase of systolic pressure [mm Hg] + 3.9; r2 = 0.21, P = 0.044), suggesting that the increase of measurement error is partly explained by the systolic pressure change.
We found that the FloTrac/Vigileo overestimated SV during laryngoscopy and endotracheal tube placement. The error coincided with a significant increase of blood pressure. Because the FloTrac/Vigileo monitor uses pulse contour analysis of the arterial pressure waveform to measure SV, its accuracy depends on an accurate estimate of arterial compliance.12 The FloTrac/Vigileo monitor uses an autocalibration factor to estimate arterial compliance that is derived from both biometric and shape variables of the arterial pressure waveform obtained from a large database. First-generation software updates the factor every 10 minutes, but second- and third-generation software updates it every minute.13 Sudden changes of arterial compliance may increase measurement error before the device successfully recalculates arterial compliance. However, this possibility has not been investigated. In our study, SV, as measured by a Doppler device, remained relatively stable during anesthetic induction and during tracheal intubation despite a significant increase in arterial blood pressure. This finding indicates that hypertension during laryngoscopy is predominantly caused by an increase in vascular resistance. Thus, it is reasonable to assume that the FloTrac/Vigileo monitor overestimates SV because the increase in blood pressure and the FloTrac/Vigileo measurement of SV occurred before the estimate of arterial compliance was updated. In our study, hypertension and the FloTrac/Vigileo error were transient and disappeared 3 minutes after tracheal intubation. It is not known whether the FloTrac/Vigileo correctly compensated for the change in arterial blood pressure or whether the return of blood pressure to the preinduction level alone caused the FloTrac/Vigileo error to disappear.
We used the USCOM monitor as our reference method. It is based on continuous-wave Doppler measurement of blood flow at the left ventricular outflow tract or main pulmonary artery, providing real-time, pressure-independent estimates of SV. We also assumed that the error in the Doppler measurement of SV was constant throughout the study period and thus it can be used as a reference device.
Although the participants may have had systemic atherosclerotic changes and these pathological changes may have affected the performance of the FloTrac/Vigileo monitor, we believe our findings can be extrapolated to a more diverse population because the hemodynamic changes found in this study are similar to the changes typically seen during anesthetic induction.14,15 It remains to be seen whether the FloTrac can accurately track SV during acute blood loss in which hypotension and increased vascular resistance concomitantly occur.
In our study, we used second-generation software (version 1.10). A more sophisticated algorithm is now available for the FloTrac/Vigileo monitor. Although the third-generation software has significantly improved the performance of the FloTrac/Vigileo monitor under hyperdynamic conditions caused by sepsis,13 the processes of reevaluating arterial compliance including the interval of recalculation seem to be similar between the second-generation and the third-generation software. It remains to be seen whether the ability of FloTrac/Vigileo to track rapid hemodynamic changes in SV is improved with the software update.
In conclusion, the measurement bias of the FloTrac/ Vigileo system significantly increased during the transient increase of blood pressure triggered by laryngoscopy and endotracheal tube insertion. However, this phenomenon is transient in nature, and the performance returned to the previous level after 3 minutes.
Name: Yoshifumi Kotake, MD, PhD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Yoshifumi Kotake has seen the original study data, reviewed the analysis of the data, approved the final manuscript, and is the author responsible for archiving the study files.
Conflicts of Interest: Yoshifumi Kotake consulted for Edwards Lifescience.
Name: Takashige Yamada, MD.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Takashige Yamada has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Takashige Yamada reported no conflicts of interest.
Name: Hiromasa Nagata, MD.
Contribution: This author helped conduct the study and analyze the data.
Attestation: Hiromasa Nagata has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Hiromasa Nagata reported no conflicts of interest.
Name: Junzo Takeda, MD, PhD.
Contribution: This author helped design the study and write the manuscript.
Attestation: Junzo Takeda has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Junzo Takeda consulted for Edwards Lifescience.
Name: Hideyuki Shimizu, MD, PhD.
Contribution: This author helped design the study and subject enrollment.
Attestation: Hideyuki Shimizu has seen the original study data, reviewed the analysis of the data, and approved the final manuscript.
Conflicts of Interest: Hideyuki Shimizu reported no conflicts of interest.
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