The Vasotrac arterial blood pressure (BP) monitor (Medwave, Arden Hills, MN) is an alternative noninvasive method to measure near-continuous BP and arterial waveform. This device uses frequent gentle compression and decompression of the radial artery at the wrist and displays the arterial pressure waveform approximately every 12 to 15 heartbeats. The device may prove to be a suitable alternative to the oscillometric cuff BP and may provide some of the advantages of direct invasive BP monitoring.
In a previous report of children after congenital cardiac surgery, we demonstrated acceptable agreement between the Vasotrac measurements and those obtained from direct intraarterial BP (IABP) monitoring (1). A comparison between Vasotrac and direct IABP monitoring in this previous study was only performed once patients were in a stable condition after cardiac surgery and, thus, it was not possible to evaluate whether wide fluctuations in BP or patient position were associated with less agreement between the Vasotrac and IABP monitoring.
In this study, we evaluated the Vasotrac monitor against direct IABP measurement in adolescent children during scoliosis surgery under conditions of induced hypotension, surgical blood loss, and prone position.
After obtaining approval by the Committee on Clinical Investigation and informed consent from parents and assent from patients, we enrolled 11 children and adolescents (ASA physical status I–II) who were undergoing scoliosis surgery in this prospective study. Exclusion criteria included a history of congenital cardiac malformations, history of cardiac surgery, and history of differential brachial artery BP measurements. The elective surgery consisted of posterior spinal fusion. Routine intraoperative monitoring was used in all patients and included pulse oximetry, electrocardiography, IABP, end-tidal capnography, and end-tidal inhaled anesthetic concentration measurement (Capnomac; Datex, Tewksbury, MA). Patients were premedicated with IV midazolam, and general anesthesia was induced with fentanyl and thiopentothal and/or propofol. Anesthesia was maintained with fentanyl, nitrous oxide, oxygen, and isoflurane. The concentration of isoflurane was maintained between 0% and 0.4% during the case. All patients received intermittent doses of midazolam during the anesthetic and neuromuscular blockade (three of four twitches, train-of-four) was maintained with pancuronium. To minimize blood loss, controlled hypotension to maintain a mean arterial BP (MAP) of 55-65 mm Hg was attempted in all patients with IV labetalol (2). In addition, some patients received hydralazine as required to accomplish this goal.
After anesthetic induction, the patient’s radial artery was cannulated with a 22-gauge or 20-gauge angiocatheter that was connected to a transducer by means of a 2-foot long small-volume, noncompliant length of tubing. Before the study, the arterial catheter tubing and transducer were carefully inspected to assure that there were no air bubbles or artifacts that could cause erroneous readings. The transducer was placed at the level of the heart. The Vasotrac monitor was placed over the radial artery of the opposite wrist to the arterial catheter, using the styloid process of the radius as a fixed landmark. It was calibrated with zero at the beginning of every case, and hydrostatic differences between the level of the radial artery and the heart were adjusted for by activating the correction key and entering the distance in inches between the monitored radial artery and the midaxillary artery. The Vasotrac monitor consisted of a nondisposable hydraulically driven sensing device, a wrist holder, and a monitor containing processing, display, and control modules. The sensor, when activated over the radial artery, applies increasing pressure until enough beats past the maximum energy transfer period have been recorded. The recorded beats were used to estimate systolic BP (SBP), diastolic BP (DBP), and MAP. Approximately 12 to 15 pulse cycles were required for each BP determination, with BP being updated several times per minute. The number of heart beats that were used for analysis was approximately 3 to 4 of a 12-to-15 heartbeat period. These heart beats were recorded and using parameters including wave shape, form, and predetermined coefficients, an algorithm calculated the patient’s SBP, DBP, and MAP. A computer was interfaced with the patient operating room (OR) monitor (Philips 1165A; Boblingen, Germany) to allow simultaneous data collection of IABP and Vasotrac BP measurements to create pairs of data points.
Data were recorded continuously after the patient was positioned prone through to the time of the intraoperative wake-up test performed in each patient.
All paired readings except artifactual readings obtained during data collection were analyzed statistically. Artifactual readings were defined as electrocautery interference, flushing of the arterial catheter, blood sampling from the arterial catheter, and oscillometric cuff reading interference. All continuous data were tested for normality using the Kolmogorov-Smirnov test and were found to follow a normal (Gaussian-shaped) distribution closely (3). Thus, data are expressed in terms of the mean difference and sd between the BP measured by the Vasotrac and the IABP. Measurements of BP and heart rate for the Vasotrac device and the IABP were correlated using the Pearson product-moment correlation coefficient (r). Bias (mean error) and precision (mean absolute value of error) were also calculated. Histograms were made to summarize the percentage of observations that differed between Vasotrac and IABP based on each patient and then combined to produce an average distribution for BP and heart rate. Plots were constructed using the method of Bland and Altman for SBP, DBP, and MAP, as well as heart rate, to allow visual inspection of data for agreement between the 2 methods of measurement and to determine the 95% confidence intervals (limits of agreement) (4). This was also performed in a subgroup analysis on patients having mean BP by IABP of <60 mm Hg. Paired data were evaluated to assess whether the bias was constant across the range of values by evaluating whether the Pearson correlation between the difference the two methods (Vasotrac – IABP) and the mean of the two methods differed from zero (test of slope). In addition, a mixed-model regression analysis was performed to verify the estimated differences between Vasotrac and the IABP by accounting for the varying numbers of measurements among patients (5,6). A power analysis was performed and indicated that the sample size of 11 patients and 5607 paired measurements provided 80% statistical power (β = 0.2, α = 0.05) for determining average agreement between Vasotrac and IABP methods using 95% confidence limits for the mean difference in BP and heart rate (version 5.0, nQuery Advisor; Statistical Solutions, Boston, MA). For all comparisons, a two-tailed value of P < 0.05 was considered statistically significant. Statistical analysis was conducted using the SPSS statistical package (version 12.0; SPSS Inc., Chicago, IL).
Eleven patients (10 female) were enrolled, median age 14.6 yr (interquartile range, 13.3-14.8 yr), and none were excluded from analysis. A total of 5607 pairs of Vasotrac and IABP measurements were recorded during this study in patients in the prone position. The mean estimated blood loss for these patients was 1598 ± 626 and the mean fluid replacement was 4084 ± 1654 mL of crystalloid solutions and 984 ± 627 mL of colloid solutions and packed red blood cells. The mean esophageal temperature during the anesthesia was 35.5°C ± 0.5°C. There were no adverse reactions reported as a result of the Vasotrac device.
Based on all measurements from the 11 patients, Pearson correlations between the Vasotrac device and the IABP measurements were highly significant: r = 0.82 and r = 0.83 for SBP and DBP, r = 0.90 for MAP, and r = 0.95 for heart rate (all P < 0.001). Compared with the IABP, measurements (bias, mean ± sd) from the Vasotrac unit were 1.7 ± 8.8 mm Hg higher for SBP with a precision (absolute mean ± sd) value of 6.5 ± 6.1 mm Hg, 0.5 ± 4.9 higher for MAP with a precision of 3.3 ± 3.7, and 2.2 ± 5.2 bpm higher for heart rate. On average, DBP was 1.7 ± 5.5 mm Hg lower for the Vasotrac device than for the IABP, with a precision value of 4.1 ± 4.0 mm Hg.
When considering the individual patients separately and then averaging the mean differences (± sd) for the group of 11 patients together, we observed the differences between Vasotrac and IABP methods as shown in Table 1. Histograms of the difference between the Vasotrac and IABP are shown in Figure 1, a-d. For each of the 11 patients, the distribution was computed as a percentage of the total number of measurements and then all of these distributions were combined to derive an average distribution of the difference between the Vasotrac and IABP methods (shown as mean ± sd). In Figure 1, a-c, the bar centered over 0 mm Hg represents a difference of 0 ± 2.5 mm Hg and in Figure 1d, 0 bpm ± 2.5 bpm. Calculated values of obtained by Vasotrac differed from those of IABP by <10 mm Hg for 92.5% of the mean BP measurements.
We also performed a repeated-measures mixed-model analysis to determine, using a regression modeling approach, the predicted difference (Vasotrac – IABP) based on all measurements, with the individual patient accounted for and the varying number of measurements among patients incorporated into the analysis. We did this using a compound symmetry covariance structure, which provided good fit to the data based on the Akaike Information Criterion (7). The results were comparable to that of the means in Table 1. The predicted SBP was expected to be 2.3 mm Hg higher for Vasotrac than the IABP and anywhere from 0.7 mm Hg lower to 5.2 mm Hg higher (95% confidence interval for difference between methods, -0.7 to 5.2 mm Hg), DBP was predicted to be 1.6 mm Hg lower for Vasotrac (95% CI, -3.4 to 0.3 mm Hg), MAP was predicted to be 0.8 mm Hg higher for Vasotrac (95% CI, -0.8 to 2.4 mm Hg), and heart rate was predicted to be 2.0 bpm higher for Vasotrac compared with the IABP method (95% CI, 1.0 to 3.0 bpm).
Bland-Altman plots revealed good agreement between the Vasotrac and IABP measurements (Figs. 2a-d). A slope test indicated that the difference between the two methods was approximately constant throughout the range of values for SBP, DBP, and MAP. There was a significant negative correlation, indicating that larger differences between Vasotrac and IABP were observed when the heart rate was lower (r = −0.27, P < 0.01).
A subgroup analysis was performed on patients with hypotension. Considering first measurements in which MAP was <50 mm Hg, there were only 270 Vasotrac–IABP paired measurements involving 4 of 11 patients. Compared with the IABP, measurements (mean ± sd) from the Vasotrac unit were 3.6 ± 5.8 mm Hg higher for SBP, 0.1 ± 5.0 mm Hg higher for DBP, 0.4 ± 3.7 higher for MAP, and 5.8 ± 7.6 bpm higher for heart rate. SBP and DBP were significantly higher using the Vasotrac device compared with the IABP (P < 0.01). Pearson correlations indicated low positive correlations between Vaostrac and IABP measurements for SBP and DBP (both r = 0.36, P < 0.01), moderate correlation for MAP (r = 0.65, P < 0.001), and a high correlation with respect to heart rate (r = 0.95, P < 0.001). Calculated values of obtained by Vasotrac differed from those of IABP by <10 mm Hg for 97.4% of the mean BP measurements.
Because of the small number of paired measurements at a mean BP <50 mm Hg, we next considered measurements in which the MAP was <60 mm Hg. This pertained to 1906 paired measurements and all 11 patients had measurements in which MAP < 60 mm Hg. Improved Pearson correlations were observed between the Vasotrac device and IABP measurements for all four variables: SBP (r = 0.49, P < 0.01), DBP (r = 0.50, P < 0.01), MAP (r = 0.71, P < 0.001), and heart rate (r = 0.96, P < 0.001). Calculated values obtained by Vasotrac in this group differed from those of IABP by <10 mm Hg for 95.9% of the mean BP measurements.
Based on all 1906 paired measurements with MAP <60 mm Hg, without considering individual patients separately, the Vasotrac measurements (mean ± sd) were 2.4 ± 8.1 mm Hg higher for SBP, 0.7 ± 4.2 higher for MAP, and 3.6 ± 6.7 bpm higher for heart rate. On average, DBP was 1.3 ± 5.1 mm Hg lower for the Vasotrac device than for IABP.
When the individual patients were considered separately, and averaging the mean differences (± sd) for the group of 11 patients together, the mean differences between Vasotrac and IABP methods are shown in Table 2. The absolute mean differences between Vasotrac and IABP are shown in Table 3.
Bland-Altman plots revealed fairly good agreement between Vasotrac and IABP measurements under the conditions of hypotension (MAP <60 mm Hg) (Fig. 3a-d). A slope test indicated that the difference between the two methods was approximately constant throughout the range of values for SBP. However, differences between the two methods with respect to DBP were larger when DBP was higher (r = 0.26, P < 0.01). Similarly, differences between Vasotrac and IABP values were larger for MAP when it was higher (r = 0.44, P < 0.001). For heart rate, there was a significant negative correlation, indicating that larger differences between Vasotrac and IABP were observed when heart rate was lower (r = -0.42, P < 0.001).
In this study comparing the Vasotac monitor with IABP measurement during anesthesia in the prone position and with controlled hypotension, we found significant correlation and agreement for SBP, MAP, and DBP, and heart rate. The agreement remains within the minimum performance standard set by the Association for the Advancement of Medical Instrumentation with the recommendation that noninvasive BP devices be accurate within 5 mm Hg and have a precision within 8 mm Hg (8).
We have previously studied the agreement between these two methods of BP measurement in children after cardiac surgery. Under controlled conditions, with the children being normotensive, supine, and not receiving volume or additional vasoactive support, based on 4102 paired measurements the mean differences between methods were 4.1 ± 2.1 mm Hg, 4.3 ± 2.2 mm Hg, 3.6 ± 2.1 mm Hg, and 2.7 ± 1.9 bpm for SBP, DBP, MAP, and heart rate, respectively. Because patients were sedated, supine, and hemodynamically stable during this study, we could not determine whether wider fluctuations in BP would be associated with less agreement between the Vasotrac and IABP monitoring. In this current study under the conditions of general anesthesia, prone position, blood loss related to scoliosis surgery, and controlled hypotension, we demonstrated slightly less variability in the mean differences between the Vasotrac and IABP, i.e., + 2.2 ± 4.2 mm Hg for SBP, - 1.5 ± 2.7 mm Hg for DBP, + 0.8 ± 2.3 mm Hg for MAP, and + 2.0 ± 1.5 bpm for heart rate.
The agreement and correlation between the Vasotrac and IABP measurements in a study of 80 surgical and critically ill adult patients positioned in the supine position were reported by Belani et al. (9) They demonstrated a bias (mean ± sd) and precision (absolute mean ± sd) of 0.0 ± 5.4 and 3.9 ± 3.7 for SBP, -0.2 ± 3.0 and 2.1 ± 2.2 for MAP, and -0.4 ± 3.9 and 2.7 ± 2.8 for DBP. Under the different surgical conditions in our study, we found a slightly higher bias and precision of 1.7 ± 8.8 and 6.5 ± 6.1 for SBP, 0.5 ± 4.9 and 3.3 ± 3.7 for MAP, and -1.7 ± 5.5 and 4.1 ± 4.0 for DBP for patients in the prone position. We were able to demonstrate a correlation between the two methods of measuring BP, which was also comparable to that reported in adults by Belani et al. In Belani et al.’s study, the differences between measurements did not exceed more than 10 mm Hg for more than 90% of the paired values, and in our study more than 92% of mean Vasotrac measurements differed by <10 mm Hg and more than 98% of these measurements varied by <15 mm Hg.
Controlled hypotension was induced with labetalol in each patient according to our usual practice to minimize blood loss during scoliosis surgery at our institution. When we initially subdivided our data to separately examine the BP measurements <50 mm Hg, there were only 270 paired measurements which made interpretation problematic. However, when we further analyzed the data to include all measurements at a MAP <60 mm Hg (1906 paired measurements), we were able to demonstrate agreement once again between the Vasotrac and IABP methods. Belani et al. also found negligible bias in measurements in adult volunteers undergoing rapid BP changes secondary to the administration of vasoactive drugs (10).
Although monitoring arterial BP with an invasive arterial catheter is probably advisable in pediatric cases, where one anticipates moderate to extensive blood loss and subsequent need for blood transfusions, arterial catheterizations are not without risk in the pediatric population. Radial artery catheterization has been associated with infection, ischemic injury, arterial-venous fistula, and pseudoaneurysm and even with limb shortening in children (11–17). These complications are rarer in children than adults probably because the incidence of peripheral vascular disease is less frequent in children. The Vasotrac device allows monitoring of the BP 3 to 4 times per minute without the risks that invasive arterial monitoring entails. Although the oscillometric noninvasive BP devices can be used in a "stat" mode resulting in BP measurements approximately 2-3 times per minute and have been found to be accurate during this mode, prolonged usage has resulted in complications secondary to compression injuries (18–22).
Our data suggest that the Vasotrac system correlates closely with IABP monitoring in anesthetized children under normotensive conditions in the prone position and during pharmacologic-induced hypotension. Based on these results and the small variability in the precision and bias between the Vasotrac and IABP methods of BP measurement reported in studies, the Vasotrac is a reliable alternative to cuff and direct arterial BP measurement during routine surgical cases. In pediatric cases involving the use of induced hypotension, the Vasotrac device may not be a suitable alternative to invasive arterial BP measurements.
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© 2005 International Anesthesia Research Society
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