Oscillometric arterial blood pressure measurement is the most common noninvasive method used during surgery and in the intensive care unit (ICU) for pediatric patients. Limitations of this method include inaccuracies resulting from inappropriate cuff placement or cuff size, discomfort, and only intermittent blood pressure readings. Direct monitoring of arterial blood pressure using an indwelling arterial catheter enables beat-to-beat display of the pressure wave form and access for blood sampling, but may be associated with complications such as trauma to the arterial vessel, thrombosis, embolism, distal ischemia, and infection (1–4). Although other noninvasive continuous blood pressure monitors, such as the Colin tonometer (ScanMed Medical Instruments, Moreton-in-Marsh, UK) and the Finapres unit (Finapres Medical Systems, Amsterdam, the Netherlands), have been studied, data have been inconsistent when compared with invasive measurements of arterial blood pressure (5–14).
Vasotrac (Medwave, Arden Hills, MN) is an alternative noninvasive method to measure arterial blood pressure and arterial wave form approved for clinical use (15,16). This device uses frequent gentle compression and decompression of the radial artery at the wrist and displays the arterial pressure wave form approximately every 12 to 15 heart beats. This device may prove to be a suitable alternative to the oscillometric cuff blood pressure and provide some of the advantages of direct invasive blood pressure monitoring.
This device has been tested in adults (15,16), but data in the pediatric population are minimal. The purpose of this study was to compare the agreement of the Vasotrac method of measuring arterial blood pressure with measurements obtained invasively via an arterial catheter in children after cardiac surgery.
Data collection and chart review for purposes of this study were approved by the IRB at Children’s Hospital Boston. Written informed consent was obtained from the parents and assent from the patients during the preoperative clinic visit. The device was tested on the patients during this visit. Children between the ages of 7 to 15 years old undergoing corrective 2-ventricle cardiac surgery were recruited. Patients were excluded if they had previous systemic-to-pulmonary artery shunts, absence of bilateral radial pulses, more than 5 mm Hg differences in arterial blood pressure between the upper extremities, congenital or acquired anatomic differences between wrists, systemic vasculopathies, absence of an invasive arterial catheter for monitoring, or an improperly working radial artery catheter. Baseline demographics, surgical procedure performed, wrist circumference, and vasoactive medications were recorded.
A radial artery catheter was placed in each patient after induction of anesthesia and used for continuous monitoring during surgery. On arriving to the cardiac ICU after surgery, the arterial catheter was connected to a transducer (Argon Medical, Athens, TX). The transducer was calibrated at the level of the patient’s mid-chest, and the tubing and transducer were inspected to ensure that there were no technical issues or air bubbles that would cause erroneous recordings.
The Vasotrac unit 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. A single investigator placed all the units on the patients to insure uniformity. An appropriate size wrist guide was used according to the patient’s wrist circumference and per manufacturer guidelines. When the unit is activated, it applies increasing pressure on the artery until a satisfactory number of pulsatile events perpendicular to the transducer have passed a maximum energy transfer period (15,16). The number of heartbeats that are used for analysis are approximately 3 to 4 of a 12 to 15 heartbeat period. These heartbeats are recorded and using variables including wave shape, form, and predetermined coefficients, an algorithm calculates systolic, diastolic, and mean arterial blood pressures. A computer was interfaced with the bedside monitor (1165A; Philips, Boblingen, Germany) to allow simultaneous data collection. The Vasotrac unit was programmed to record blood pressures continuously. Recordings averaged approximately one reading every 12 to 15 heartbeats but may have been longer depending if the filters used by the Vasotrac monitor considered a reading artifact.
Simultaneous systolic, diastolic, and mean arterial blood pressures and heart rate were recorded for a maximum of 2 h in the computer database. Patients were either tracheally intubated or breathing spontaneously during this time depending on their perioperative condition and stability. Patients were supine during this period. Recordings were discontinued after 2 h or if a patient requested removal of the unit. No clinical decisions in the postoperative care of the patients were based on the Vasotrac readings.
All paired data were used for analysis unless there was documented artifact noted during the study period. All continuous variables were tested using the Kolmogorov-Smirnov goodness-of-fit statistic and followed a normal (Gaussian-shaped) distribution (18). Measurements of arterial blood pressure and heart rate for Vasotrac and the arterial line were correlated using the Pearson correlation coefficient. Bias (mean error) and precision (mean absolute value of error) were also calculated. Bland-Altman plots were constructed of systolic, diastolic, and mean arterial blood pressures and heart rate to enable visual observation of data for agreement between the two methods and to determine the 95% confidence limits (limits of agreement) (17). Because there were varying numbers of paired measurements among patients, the differences between the two methods were also determined separately for each patient and then averaged across all of the patients to avoid weighting some patients more than others. In addition, a mixed-model regression analysis was performed to verify the estimated differences between Vasotrac and the arterial line by considering within-subject and between-subject correlation and the varying number of observations between patients (19). A two-tailed value of P < 0.05 was considered statistically significant. Power analysis was performed using the nQuery Advisor software program (version 5.0; Statistical Solutions, Boston, MA). All statistical analysis was conducted with the SPSS statistical package (version 12.0; SPSS Inc., Chicago, IL).
Twenty-three patients were enrolled in the study. Seven patients were excluded; one because the scheduled surgery was cancelled, one because the patient was critically ill immediately postoperatively and was subsequently placed on extracorporeal membrane oxygen support, and five because of technical difficulties with interfacing the computer to the monitor to obtain paired measurements. This report therefore consists of 16 patients (six females and 10 males). This sample size in this prospective study was powered at a level of 90% statistical power (β = 0.10, α = 0.05) for determining agreement between the Vasotrac and arterial line methods using 95% confidence intervals for the absolute mean difference.
The mean age was 10.1 ± 2.3 yr (range, 8–15 yr), weight was 34.6 ± 11.9 kg (range, 21.4–58.4 kg), and wrist circumference was 14.4 ± 1.7 cm (range, 11.5–17.5 cm). Twelve patients underwent atrial septal defect closure, two patients had a ventricular septal defect repaired, one patient had both atrial and ventricular septal defects corrected, and one patient underwent subaortic stenosis resection. There were no complications intraoperatively. Four patients were receiving nitroprusside infusions, two patients were receiving nitroglycerin infusions, and one patient was receiving both esmolol and nitroprusside infusions postoperatively.
A total of 4,102 paired measurements were obtained (Table 1). The Pearson product-moment correlations between the Vasotrac and arterial line measurements were highly significant, indicating strong positive linear correlations between the two methods. Pearson values were r = 0.90, r = 0.80, and r = 0.91 for systolic, diastolic, and mean arterial blood pressures, respectively, and r = 0.98 for heart rate (P < 0.0001 for each). Correlations between the Vasotrac and invasive arterial monitoring for the whole group and for each of the 16 patients separately were analyzed. The within-patient correlations were similar to that for the whole group with an expected range of values. The range was r = 0.63 to 0.97, r = 0.45 to 0.98, and r = 0.37 to 0.97 for systolic, diastolic, and mean arterial blood pressures and r = 0.81 to 0.99 for heart rate. Systolic and diastolic blood pressure measurements from the Vasotrac unit were 1.3 ± 5.6 and 0.3 ± 5.8 lower, respectively, and mean blood pressure value and heart rate were 1.4 ± 4.4 mm Hg and 1.9 ± 3.8 bpm higher, respectively, than invasive arterial data. Bland-Altman plots show good agreement between the two different methods (Figs. 1–4). Paired data were tested for constant bias across the range of values by a test-of-slope. A test of the slope based on the Pearson correlation coefficient calculated by differences in measurements between the two methods (Vasotrac − arterial) correlated with the mean of the two methods showed a slight inverse bias at slower heart rates (r = −0.40, P < 0.001). This indicated that at slower heart rates, the differences between the Vasotrac and arterial methods were larger than at more rapid heart rates. This can be seen in the Bland-Altman plot (Fig. 4).
Based on all 4,102 measurements without regard to specific patients, absolute mean differences (± sd) were 4.0 ± 4.0 mm Hg, 4.4 ± 3.7 mm Hg, 3.4 ± 3.2 mm Hg, and 2.0 ± 3.5 bpm for systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, and heart rate, respectively. All 4 mean values were <5 mm Hg. When averaging the results of the mean values for the 16 patients individually, mean absolute 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 systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, and heart rate, respectively.
A repeated-measures mixed model analysis was performed to determine the mean absolute differences in arterial blood pressures between the two methods to account for the within-subject correlation (multiple measurements per patient) as well as the varying number of measurements among the patients. A compound symmetry covariance structure was used and demonstrated excellent model fit to the data. Absolute mean differences as derived from the mixed-model regression analysis (95% confidence interval) were 4.0 mm Hg (3.0–5.0 mm Hg), 4.3 mm Hg (3.1–5.5 mm Hg), 3.5 mm Hg (2.5–4.0 mm Hg), and 2.7 bpm (1.7–3.7 bpm) for systolic blood pressure, diastolic blood pressure, mean arterial blood pressure, and heart rate, respectively. For example, it can be expected that a patient’s systolic blood pressure measured by the Vasotrac unit will be within 4 mm Hg of the arterial line standard with a 95% confidence interval ranging from 3–5 mm Hg.
The Vasotrac is a noninvasive method of arterial blood pressure measurement and in this study of children after congenital cardiac surgery; we demonstrate acceptable agreement between the Vasotrac measurements and those obtained from direct intraarterial monitoring.
Invasive arterial blood pressure monitoring is the standard for direct arterial blood pressure measurement, enabling beat-to-beat monitoring and access for blood sampling. Arterial monitoring may be associated with significant known complications including vascular trauma, distal embolization, thrombosis and ischemia of distal extremities, bleeding at the site of insertion, patient discomfort, and nosocomial infection (1–4). Direct arterial catheters can be difficult to place in awake children because of pain and discomfort; therefore a continuous noninvasive monitor would be an advantage in pediatric patients. Attempts at noninvasive monitoring have included the Colin Tonometer and Finapres system, but there have been limitations. The Colin tonometer can be placed over a superficial artery, such as the radial, temporal, or dorsalis pedis artery. During measurement the wall of the artery is flattened to remove transmural pressure and using a piezoresistive crystal pressure transducer, the sensor measures the intraluminal pressure of the artery (7,14). The Finapres system, which consists of a sensor and cuff, is usually placed on a distal or end-vessel such as in a finger. By using photoplethysmography, infrared red light measures the volume of the finger, which is usually dependent on pulsatility of blood flow. The infrared signal alters the pressure of the cuff to maintain a constant volume in the finger. This counter pressure is then displayed as the measured arterial blood pressure (13). The Colin Tonometer requires frequent recalibration for fluctuations in arterial blood pressure and the Finapres system has been reported to possibly cause venous congestion and vasospasm (20). There has also been variable agreement between these two systems when compared with direct intraarterial monitoring (5–14).
The most common noninvasive measure of determining arterial blood pressure remains the oscillometric method using instruments such as the Dinamap (Johnson and Johnson Medline, Arlington, TX). By compressing the artery with a cuff and then slowly releasing pressure, pulsations from the artery are transmitted as oscillations to the cuff. After initially low amplitude of oscillations, the amplitude increases to a maximum as reductions in cuff pressure occur. The oscillations eventually disappear with further cuff pressure reduction. In general, systolic blood pressure is recorded when the amplitude of oscillations abruptly increases, mean arterial blood pressure at time of maximal amplitude of oscillations, and diastolic blood pressure when the amplitude of oscillations decreases. Despite the ease of use, arterial blood pressure measurements may be inaccurate as a result of inappropriate cuff size or improper placement. In addition, differences between oscillometric and invasive arterial blood pressure measurements have been documented in various clinical situations (21–24). The Dinamap is able to cycle in a “stat” mode that enables measurements to be obtained approximately every 30 seconds under ideal conditions; however, inflation of the cuff is associated with pain and discomfort and may disturb an otherwise settled or sleeping child. The cuff is also not suitable to be used in the “stat” mode over an extended period of time because of reports of compression injuries (25).
There was clinically significant agreement between the Vasotrac and direct arterial scatter around the zero point that was consistent with only small differences between the two methods. In addition, there was less than a 5 mm Hg difference in the absolute values for systolic, diastolic, mean arterial blood pressures, and 3 bpm for heart rate. Between the methods, <10% of the arterial blood pressure values differed by more than 10 mm Hg. This is within the standards proposed by the Association of the Advancement of Medical Instrumentation (26). In most situations, these differences would have minimal clinical significance. There was also agreement between the heart rate measurement as recorded through the direct arterial catheter and the Vasotrac, but this latter agreement was noted to have some bias at slower heart rates. No clinically significant bias was seen for the arterial blood pressure measurements. Our findings are consistent with the agreement demonstrated in two previous adult studies (15,16). Analyzing 3,955 to 17,468 paired points, the authors found R-squared values between Vasotrac and arterial blood pressure measurements of 0.89 to 0.93, 0.88 to 0.89, and 0.94 to 0.95 for systolic, diastolic, and mean arterial blood pressures, respectively. Furthermore, differences between measurements did not exceed more than 10 mm Hg for more than 90% of the paired values.
A pediatric device is currently under development; however, an adult sensor was used in this study. For an average wrist circumference of just over 14 cm, range of patients’ ages between 8 to 15 years, and weight between 21.4 to 58.4 kg, the adult sensors were reliable. Though not directly evaluated, all the patients studied noted that the Vasotrac was comfortable and the pressure applied over the radial artery did not cause discomfort. They all agreed that it was less intrusive and painful for measuring arterial blood pressure compared with the oscillometric method. The Vasotrac is a useful near-continuous monitor of arterial blood pressure that can be used in young children and may provide less stimulation in a patient who is sedated in the immediate postoperative period. However, further development of smaller sensors will be necessary to increase the application for smaller children and infants.
The Vasotrac does not provide a beat-to-beat analysis of arterial blood pressure but rather a near-continuous measurement averaging readings over 12 to 15 heartbeat cycles. The Vasotrac uses an algorithm to detect motion artifact and therefore patients are required to be relatively still during the data acquisition and monitoring. If patients move, the blood pressure measurements become less frequent than every 12 to 15 heartbeats. Vasotrac has the advantage of displaying an arterial wave form and therefore the validity of the measured results can be assessed by the quality of the signal and wave form. Although the Vasotrac does not provide continuous beat-to-beat measurements, it is able to record data more frequently than the standard oscillometric method.
Although only 16 patients were included in our study, more than 4,000 paired measurements (Vasotrac and arterial) were obtained for each variable. In addition, the patient age, size, and postoperative conditions were consistent. We did perform a power analysis in designing this study to determine the appropriate sample size requirements. In our study, 16 patients (ranging from 34 to 496 paired measurements) provided 90% statistical power to construct agreement plots based on the Bland-Altman technique with 95% confidence intervals within 5 mm Hg for pressure and within 3 bpm for heart rate. Comparison between Vasotrac and direct arterial blood pressure monitoring was only performed once patients were in a stable condition after cardiac surgery, and there were minimal adjustments to subsequent hemodynamic or ventilatory support during the course of the study. Because patients were hemodynamically stable during this period, it was not possible to determine whether wider fluctuations in arterial blood pressure would be associated with less agreement between the Vasotrac and intraarterial monitoring. Further studies will be necessary to evaluate Vasotrac during extremes of surgical and intensive care conditions in the pediatric population, particularly during hypotension or hypertension, conditions of hypothermia and peripheral edema, and with variable patient positions.
The Vasotrac monitor is currently not available for use in infants and preschoolers because of the size and bulk of the wrist sensor. From the ranges of age, weight, and wrist size in our patients, we can conclude that reliable data can be obtained in small children, but further development and investigation is needed to extend the use of this technology for smaller patients.
We conclude that arterial blood pressure measurements obtained from the Vasotrac unit agreed well with invasive arterial blood pressure monitoring in children after cardiac surgery. This unit may be an alternative for obtaining near-continuous arterial blood pressure data noninvasively in this pediatric patient population.
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