Objectives: Hemodynamic monitoring is vital during liver transplant surgeries because distinct hemo­dynamic changes are expected. The continuous noninvasive arterial pressure (CNAP) monitor is a noninvasive device for continuous arterial pressure measurement by a tonometric method. This study compared continuous noninvasive arterial pressure monitoring with invasive direct arterial pressure monitoring in living-liver donors during transplant.

Materials and Methods: There were 40 patients analyzed while undergoing hepatic lobectomy for liver transplant. Invasive pressure monitoring was established at the radial artery and continuous noninvasive arterial pressure monitoring using a finger sensor was recorded simultaneously from the contralateral arm. Systolic, diastolic, and mean arterial pressures from the 2 methods were compared. Correlation between the 2 methods was calculated.

Results: A total of 5433 simultaneous measure­ments were obtained. For systolic arterial blood pressure, 55% continuous noninvasive arterial pressure measurements were within 10% direct arterial measurement; the correlation was 0.479, continuous noninvasive arterial pressure bias was -0.3 mm Hg, and limits of agreement were 32.0 mm Hg. For diastolic arterial blood pressure, 50% continuous noninvasive arterial pressure measurements were within 10% direct arterial measurement; the correlation was 0.630, continuous noninvasive arterial pressure bias was -0.4 mm Hg, and limits of agreement were 21.1 mm Hg. For mean arterial blood pressure, 60% continuous noninvasive arterial pressure measurements were within 10% direct arterial measurement; the correlation was 0.692, continuous noninvasive arterial pressure bias was +0.4 mm Hg, and limits of agreement were 20.8 mm Hg.

Conclusions: The 2 monitoring techniques did not show acceptable agreement. Our results suggest that continuous noninvasive arterial pressure monitoring is not equivalent to invasive arterial pressure monitoring in donors during living-donor liver transplant.

Key words : CNAP, Liver transplant, Living donors, Blood pressure

Introduction

During every surgical procedure irrespective of the type of anesthesia, it is mandatory to monitor the arterial blood pressure, heart rate, and oxygen saturation. Oxygen saturation and heart rate are measured continuously, but blood pressure is assessed every 3 to 10 minutes by a noninvasive intermittent oscillometric technique. However, in major surgery or high-risk patients, continuous arterial blood pressure measurement using an indwelling arterial line is preferred for close monitoring. In this respect, intra-arterial blood pressure (IABP) measurement is considered the best method.¹ However, placement of an arterial catheter is susceptible to several complications such as vascular or nerve trauma, hematoma, infection, and occlusion.^1-3

For several decades, monitors for continuous noninvasive arterial pressure (CNAP) measurement have been developed, and various studies have investigated the accuracy of these devices and compared them with direct arterial pressure measurement. However, these noninvasive monitors have not yet been used in clinical anesthesia practice. Therefore, a beat-to-beat, noninvasive, and reliable technique for tracking arterial blood pressure is desirable.⁴

The first investigations for CNAP measurements were developed by Peñáz and coworkers in the 1970s.⁵ Recently, a device (Infinity CNAP, Draeger Medical Systems, Drager Medical GmbH, Lübeck, Germany) has been developed to provide CNAP measurements. Previous studies suggested that the CNAP monitor may be superior to the intermittent oscillometric measurements because it can detect rapid changes better in different procedures. The validation of the CNAP monitor has been performed in different procedures including cardiac angio­graphy, abdominal surgery, cardiac surgery, and neurosurgery.^6,7

During anesthesia for liver donation in living-donor liver transplant surgery, arterial catheters routinely are inserted for hemodynamic monitoring in our clinic. The aim of this study was to evaluate the agreement between simultaneous CNAP monitoring and invasive direct arterial pressure monitoring in donors during living-donor liver transplant.

Materials and Methods

Subjects
This study was approved by Baskent University Institutional Review Board and Ethics Committee (project number KA12/170). Written informed consent was obtained from all patients. For the present study, 40 adult patients undergoing hepatic lobectomy for living-donor liver transplant by a single team were recruited from February 2012 to March 2014 at the Baskent University Ankara Hospital. Exclusion criteria were the presence of vascular occlusion, surgery, or disease (such as Raynaud syndrome) of the upper extremities or an anatomic deformity of the distal forearm.

Procedure
After an 8-hour starving period, all patients were premedicated orally with midazolam (0.1 mg/kg) 1 hour before the induction of anesthesia. In the operating room, standard monitoring was applied to all patients including 5-lead electrocardiogram, noninvasive blood pressure monitor, and pulse oximetry. The anesthetic treatment was performed according to the standard procedures of our department with propofol, fentanyl, and non­depolarizing neuromuscular blockers (rocuronium). Cefazolin, methylprednisolone, and ranitidine were given at induction. After endotracheal intubation, mechanical ventilation was started using volume-controlled ventilation. Ventilation variables were adjusted (respiratory rate, 8-16/min; tidal volume, 4-7 mL/kg) to maintain pulse-oximetry saturation > 95% and end-tidal carbon dioxide pressure 32 to 38 mm Hg with sevoflurane (1-1.5 minimum alveolar concentration in 40% oxygen and 60% air mixture).

A central venous catheter was peripherally inserted into the brachial veins and a 20-gauge arterial catheter was inserted into the radial artery under general anesthesia for hemodynamic monitoring. All pressure lines were connected to a pressure transducer (Biometrix B.V., Breda, The Netherlands) and the transducer was placed and set to zero at the mid thoracic level. To eliminate errors from damping and frequency change, the natural frequency and damping coefficient for each system was determined by the flush method.^8,9

After these monitoring settings, appropriately sized cuff CNAP sensors (Infinity CNAP, Draeger) were placed on the patient’s index and middle fingers of the same extremity as the arterial catheter. A brachial noninvasive blood pressure (NIBP) cuff for calibration was placed on the contralateral arm and measurements were performed at 30-minute intervals. A single finger cuff was inflated at a time, and inflation of the cuffs was rotated between the 2 fingers every 30 minutes. After all monitoring procedures were completed, all CNAP and IABP data were manually recorded simultaneously at 2-minute intervals for systolic, diastolic, and mean pressures.

Statistical analyses
All simultaneously recorded IABP and CNAP measurements were compared. The agreement criterion for the CNAP value was set to within ± 10% IABP value. The accuracy of CNAP was assessed using analysis of bias and limits of agreement.¹⁰ Bias was the mean difference between CNAP and IABP. Differences were calculated by subtracting CNAP values from IABP values. The limits of agreement were described using the mean of the differences (mean ± SD) × 1.96. Pearson product moment correlation was used to assess the relation between CNAP and invasive IABP measurements, and Bland-Altman plots were constructed to assess the agreement between CNAP and invasive IABP measurements. Data analysis was performed using a statistical program (SPSS, Version 17.0, SPSS Inc., Chicago, IL, USA). Continuous variables were reported as mean ± SD.

Results

There were 40 patients who had donor hepatectomy for liver transplant and were enrolled in this study (Table 1). A total of 5433 concomitant paired measurement sets, with 1 set being a group of systolic, diastolic, and mean pressures by CNAP and IABP, were recorded and compared separately for systolic, diastolic, and mean arterial pressures. The average number of measurement sets per patient was 136 ± 41 in this study. There were no complications due to CNAP, and none of the patients reported any complaints about CNAP after surgery. The compatibility of direct arterial blood pressures and CNAP for systolic, diastolic, and mean pressures in terms of percentage difference, correlation, analyses of bias, and limits of agreement for all patients in combination were tabulated (Table 2). The corresponding Bland-Altman plots showed the comparison of measurements of systolic, diastolic, and mean arterial pressures with IABP and CNAP (Figure 1-2).

Discussion

The use of living-donor liver transplant became increasingly popular because the number of deceased donors for solid-organ transplants was limited. Despite the benefits to the recipients, there is a risk of major complications, including mortality risk, for solid-organ donors.¹¹ Therefore, donor safety and comfort is the major ethical issue. Reduction of invasive procedures contributes to increased patient safety. However, reducing these interventions can be accomplished only by replacing them with reliable and validated methods.3 In this report, we investigated the compatibility between 2 hemo­dynamic monitoring techniques including direct IABP and CNAP. The main result of our prospective, observational clinical trial was that CNAP monitoring was not equivalent to direct IABP monitoring in donors for living-donor liver transplant.

Several previous reports emphasized that > 20% hypotensive episodes during surgery were missed and another 20% were detected with a delay by NIBP.¹² These findings indicate that closer monitoring should be done, especially in critically ill or high risk patients.^3,12 Existence of important complications of arterial catheters has led to a search for devices that can make noninvasive and continuous measurement of blood pressure possible.^1,3 The basic principle of these devices is the vascular unloading technique introduced first by Penáz and coworkers in 1973.^13,14 In recent years, several devices have been developed for measuring blood pressure noninvasively and continuously. The CNAP device tested in this study uses volume unloading technology. This device monitors blood flow into the finger. By encircling finger cuffs, it senses the blood flow oscillations and converts them to waveforms and numeric values.

There are several studies that have compared CNAP and IABP, the best method, in different patient populations and have shown reliability of CNAP. Schramm and colleagues reported that CNAP could be safely used in older and high risk patients.⁷ Similar results were reported in other studies for CNAP use in the prone position, children, and intensive care patients who received inotropic support.^3,15 However, Gayat and associates showed that CNAP rapidly reflected changes in blood pressure but failed to be in concordance with arterial pressure measurements.¹⁶ Ilies and associates showed in another study that measurements obtained by CNAP at different stages of anesthesia were unreliable.¹²

In our study, CNAP was used in living donors during liver transplant for liver transplant. After anesthesia induction and patient stabilization, CNAP was properly placed and measurements were obtained. Invasive IABP and CNAP measurements obtained from the same monitor every 2 minutes were recorded manually. All patients were either young or middle-aged patients in ASA class 1 or 2. A total 5433 data sets for 40 patients were recorded. The comparison of CNAP and direct IABP measurements showed that the ratios of systolic, diastolic, and mean blood pressure measurements in 10% safety limits were 55%, 50%, and 60%. An analysis of data showed that CNAP measured blood pressure with limits of agreement ± 32.0, ± 21.1, and ± 20.8 mm Hg for systolic, diastolic, and mean arterial pressures. These limits were too wide to substitute CNAP for direct IABP values. The reason for this finding might be the hemodynamic changes during surgery, with an intraoperative change in mean arterial pressure of > 20% to 30% compared with preoperative values. Similar to our results, Findlay and coworkers reported that a noninvasive arterial blood pressure monitor (Vasotrac, Biopac Systems, Goleta, CA, USA) was not an accurate substitute for direct IABP monitoring during liver transplant surgery¹⁷ The authors explained that the differences observed in their results, in contrast with other previously published studies, were because of the cardio­vascular pathophysiology of their patients under­going liver transplant; their patients had high cardiac output, low peripheral resistance, and alterations in arterial wall compliance that may affect the accuracy of the noninvasive (Vasotrac) measurements.¹⁷

During the use of CNAP, finger sensor cuffs are placed on the fingers in turn to decrease pressure in the fingers. We set this interval of rotation at 30 minutes. Ilies and associates revealed no findings regarding the adverse events of CNAP, but long-term use may be associated with vascular occlusion, ischemia, and pain.¹² We did not observe such complications in our patients. After CNAP was placed on the patient’s arm, when movement of the sensors occurred, we observed that intrasensor pressure changes were directly reflected on the pressure traces on the monitor. The values obtained during these changes were omitted from the records. However, the frequency and causes of interruptions of the waves were not recorded.

Limitations of the present study include the unavailability of automatic recording of the data, the small number of measurements per patient, and not taking hypotensive attacks and arrhythmias into consideration.

In conclusion, we observed that CNAP and invasive IABP monitoring did not show acceptable agreement. The CNAP monitoring was not equivalent to invasive IABP monitoring in living liver donors during transplant.

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