Comparison of invasive and oscillometric blood pressure measurement techniques in anesthetized camelids

Abstract

This study assessed the accuracy of the oscillometric method for arterial blood pressure (ABP) monitoring in anesthetized camelids. Twenty camelids were anesthetized and systolic ABP (SABP), mean ABP (MABP), and diastolic ABP (DABP) were measured directly and using the oscillometric method. The mean difference between SABP measurements was −9.9 ± 21.9 mmHg with a range of −76 to 54 mmHg, and the 95% limits of agreement (LOA) were −33 to 53 mmHg. The difference between DABP measurements was −1.8 ± 15.6 mmHg with a range of −81 to 36 mmHg, and the 95% LOA were −32 to 29 mmHg. The difference between MABP measurements was −2.9 ± 17.0 mmHg with a range of −81 to 36 mmHg, and the 95% LOA were −30 to 36 mmHg. Accurate ABP monitoring in anesthetized camelids cannot be accomplished using the oscillometric method.

Introduction

General anesthesia is recommended in camelids for surgical and diagnostic procedures in which immobility, unconsciousness, and analgesia are required. The use of inhalant anesthetics for general anesthesia frequently results in hypotension, which can increase morbidity and mortality (1–3). Hypotension, defined as systolic arterial blood pressure (SABP) measuring < 80 mmHg is estimated to develop in 41% of anesthetized humans, in up to 32% of anesthetized dogs, and in up to 33% of anesthetized cats (1,4–6). To the authors’ knowledge, there are no specific studies describing normal arterial blood pressures (ABP) in conscious camelids, though 1 study reported an average SABP of 167 mmHg and an average mean ABP (MABP) of 123 mmHg in healthy conscious camelids 24 h after anesthesia (7). Maintenance of values for SABP between 95 and 125 mmHg and MABP between 70 to 100 mmHg has been recommended for anesthetized camelids (8,9).

Recommendations for monitoring camelids during anesthesia include evaluation of heart rate, respiratory rate, ABP, end-tidal carbon dioxide, pulse oximetry, and arterial blood gases (8,10–12). Invasive (direct) methods (IBP) of monitoring arterial blood pressure require skill for catheter placement. Infection, thromboembolus, hematoma formation, and tissue necrosis may occur following catheterization (12,13). Accuracy of direct pressure monitoring systems has been called into question when utilized in other species, but this is due to the frequency bandwidth requirements and natural resonance frequencies in species with high heart rates (14,15). In many species, arterial blood pressure has been measured using indirect (noninvasive) methods (NIBP) such as Doppler and oscillometric blood pressure techniques, but the accuracy of NIBP methods in various species is controversial (14,16–22). Non-invasive techniques are easily applied and have a low incidence of complications. Previously reported complications of NIBP techniques are associated with prolonged use and application of excessive pressure within the cuff (23). Non-invasive ABP monitoring has been utilized in several studies of camelid species, but the accuracy of this method in these species has not been investigated (24–27). The objective of the present study was to determine if monitoring of ABP in anesthetized camelids can be accurately accomplished using an oscillometric method. We hypothesized that accurate ABP monitoring could be accomplished using an oscillometric method.

Materials and methods

Study design

Animals used for this study were presented to the anesthesia service of The Ohio State University Veterinary Medical Center for general anesthesia to facilitate surgery for correction of a spontaneously occurring condition between June 2008 and July 2009. The study was performed in compliance with institutional guidelines for research on animals; owner consent for anesthesia was obtained.

The anesthetist determined premedication and anesthetic induction drugs. Following orotracheal intubation, each patient was connected to a standard small animal circle anesthetic circuit. Isoflurane in 100% oxygen was delivered by use of an out-of-circuit precision isoflurane vaporizer. Cardiac rate and rhythm were monitored using an ECG, with the leads placed in a base-apex configuration. Intravenous fluids were delivered via a preplaced jugular catheter. A 20-gauge 1.25 inch (3.2 cm) fluid-filled catheter (Surflo; Terumo, Elkton, Maryland, USA) for IBP was placed in an auricular or medial saphenous artery after clipping or shaving the hair and cleansing the skin. Catheters were connected to 33 inch (83.8 cm) IV tubing and a 3-way stopcock filled with sterile saline. The 3-way stopcock was connected to a pressure transducer (Edwards Lifesciences, Irvine, California, USA), which was zeroed to atmospheric pressure and placed at the level of the sternum for patients in lateral recumbency and at the level of the scapulo-humeral joint for patients in dorsal recumbency. Prior to use, the direct blood pressure monitoring system was calibrated using a mercury manometer according to the manufacturer’s specifications (28).

A blood pressure cuff was positioned 2.5 cm distal to the carpus for NIBP, at the level of the metacarpal artery (25–27). If the patient was positioned in lateral recumbency, the cuff was placed on the non-dependent limb (25–27). The circumference of the limb at this point was measured, and a cuff was chosen that was approximately 50% of the circumference of the limb, in congruence with guidelines that the cuff width should be 40% to 60% of the circumference of the limb (20,24,29,30). If the measurement fell between 2 cuff sizes, the larger cuff was chosen. The elevation of the carpus was measured relative to the sternum for patients in lateral recumbency and relative to the level of the scapulo-humeral joint for patients in dorsal recumbency. Care was taken to ensure that the blood pressure transducer and the NIBP cuff were at the same level. The cuff was attached to the monitor (Passport; Datascope, Montvale, New Jersey, USA) by means of the manufacturer’s specified tubing. Manufacturer specified diagnostic and calibration tests including pneumatics tests and pressure calibration were performed on the NIBP system prior to use (28).

Systolic ABP, MABP, and diastolic ABP (DABP) were measured and recorded every 5 min during the surgical procedure. Readings of NIBP and IBP were virtually simultaneous, with the IBP recorded first during the acquisition of NIBP readings, and NIBP was recorded at the time of final release of pressure in the cuff. Arterial catheters and the blood pressure cuffs were removed during closure of the surgical site(s).

Statistical analysis

The data were explored by calculating descriptive statistics (mean, standard deviation, minimum, maximum) and using graphs. The difference between the 2 blood pressure measurements was also calculated (IBP-NIBP) and summarized. The 95% limits of agreement (LOA) between SABP, MABP, and DABP values obtained by NIBP and IBP were assessed using the Bland-Altman method using a repeated measures approach (31,32).

Results

The 20 camelids (17 alpacas and 3 llamas) chosen for study ranged in age from 2 wk to 15 y and weighed between 17.7 and 169 kg (38.9 and 371.8 lb) [mean 59.3 kg (130.5 lb)]. Surgical procedures were orthopedic (n = 8), dental (n = 5), exploratory laparotomy (n = 3), enucleations (n = 2), repair of a uterine tear (n = 1), and removal of a paraovarian cyst (n = 1). Eleven camelids were not premedicated, 7 camelids received a benzodiazepine and/or opioid as premedication, and 2 camelids received xylazine or xylazine, ketamine, opioid combination as premedication. Anesthesia was induced in 19 camelids using a combination of guaifenesin and ketamine; anesthesia was induced in 1 camelid using propofol.

Eighteen camelids were positioned in lateral recumbency and the location of the NIBP cuff was determined to be at the level of the sternum. Two patients were positioned in dorsal recumbency and the location of the NIBP cuff was determined to be at the level of the scapulo-humeral joint.

Duration of anesthesia was between 40 and 205 min (mean 103 min). The number of measurements per patient ranged between 5 and 35 (mean 14 measurements), depending on the time required for catheter placement and the duration of the surgical procedure. The oscillometric blood pressure device was unable to provide data for 45 min in 1 patient. The cuff was repositioned on the original forelimb and was switched to the opposite forelimb, but measurements could not be obtained. The direct ABP for these 45 min indicated a SABP between 98 and 113 mmHg, a MABP between 83 and 98 mmHg, and a DABP between 71 and 85 mmHg.

The mean of direct SABP measurements (± standard deviation) was 97 ± 20 mmHg and the mean of non-invasively measured SABP measurements was 107 ± 24 mmHg (Table 1). The mean of invasively measured DABP was 62 ± 16 mmHg and the mean of non-invasively measured DABP was 60 ± 19 mmHg. The mean of invasively measured MABP was 76 ± 17 mmHg and the mean of non-invasively measured MABP was 79 ± 21 mmHg. The average difference between the 2 SABP measurements was −9.9 ± 21.9 mmHg, with a range of −76 to 54 mmHg (Figure 1), and the 95% LOA were −33 to 53 mmHg (Figure 2). The difference between the 2 DABP measurements was −1.8 ± 15.6 mmHg, with a range of −81 to 36 mmHg (Figure 1), and the 95% LOA were −32 to 29 mmHg (Figure 3). The difference between the 2 MABP measurements was −2.9 ± 17.0 mmHg, with a range of −81 to 36 mmHg (Figure 1), and the 95% LOA were −30 to 36 mmHg (Figure 4).

Table 1.

Descriptive statistics on blood pressure measurements obtained during anesthesia using invasive and non-invasive methods in camelid patients (n = 20). Systolic, mean, and diastolic arterial blood pressures were measured and recorded every 5 minutes during anesthesia and depending on the length of the surgery, patients had between 5 to 35 measurements taken

	Systolic		Diastolic		Mean
	Invasive	Non-invasive	Invasive	Non-invasive	Invasive	Non-invasive
Mean ± SD	97 ± 20	107 ± 24	62 ± 16	60 ± 19	76 ± 17	79 ± 21
Mean differencea ± SD range	−9.9 ± 21.9		1.8 ± 15.6		−2.9 ± 17.0
	−76 to 54		−81 to 36		−81 to 36
95% B-A limits of agreementb	−33 to 53		−32 to 29		−30 to 36

^aDifference between the 2 BP measurements (calculated as invasive minus non-invasive measurement).

^bBland-Altman 95% limits of agreement.

SD — standard deviation.

Figure 1.

Mean differences between invasive and non-invasive blood pressure measurements during surgery in camelid patients. Error bars indicate standard deviations.

Figure 2.

Bland-Altman plot of agreement between direct systolic arterial blood pressure measurements and indirect arterial blood pressure measurements.

Figure 3.

Bland-Altman plot of agreement between direct diastolic arterial blood pressure measurements and indirect arterial blood pressure measurements.

Figure 4.

Bland-Altman plot of agreement between mean direct arterial blood pressure measurements and indirect arterial blood pressure measurements.

Discussion

The results indicate that accurate ABP monitoring cannot be accomplished using the oscillometric method and the non-invasive oscillometric method for measurement of ABP does not serve as a predictor of directly measured ABP. In addition, there is significant variability in measurements of SABP, MABP, and DABP within individual camelids and within the means of the pooled data. Measured differences are occasionally positive and at other times negative, and the large LOAs indicate an inability to conclude that the oscillometric method results in consistently high or low differences between the blood pressure measurement techniques, regardless of the blood pressure status of the patient (hypotensive, hypertensive, or normotensive).

The mean differences in SABP indicate that invasive SABP averaged 9.9 mmHg lower than non-invasive SABP. While this mean difference may seem clinically insignificant, the range in the differences between invasive SABP and non-invasive SABP indicates that invasive SABP could be as much as 76 mmHg lower than non-invasive SABP or as much as 54 mmHg higher. The mean differences and ranges in differences between invasive and non-invasive MABPs and DABPs are consistent with the results from the SABPs. Further, our results indicate that the relationship is not consistent from measurement to measurement, limiting the ability of NIBP in predicting trends over time.

These results are consistent with those in other species. Acierno et al (14) determined that oscillometric blood pressure monitoring could not be recommended for ABP monitoring in anesthetized cats because there was poor agreement between NIBP and IBP. In a recent study using conscious dogs that were categorized as hypotensive, normotensive, or hypertensive, NIBP did not meet the standards for validation in human medicine (16). To the authors’ knowledge, there is only 1 report comparing IBP monitoring to NIBP monitoring in a food animal species (19). Indirect values were compared with directly measured femoral arterial values in 1 anesthetized sheep. In this sheep the mean difference between IBP and NIBP was 8 mmHg (19).

Oscillometric methods of NIBP monitoring utilize the pulsation in an artery to determine ABP. A cuff is placed on an extremity and the cuff is inflated until blood flow is occluded and the pulsation ceases. The pressure (air) is slowly released from the cuff and the artery begins to pulse as blood flow returns. The monitor used in this study utilizes the point of maximum oscillations of the pulse to determine MABP, then uses a proprietary algorithm to calculate SABP and DABP (14,23,33). If the pulsation in the artery is weak (as with hypotension), if the cuff is too large, or if the cuff is above the heart, ABP determined by the oscillometric method will be falsely low. Alternatively, if the cuff is too small or the cuff is at a level below the heart, the oscillometric method will falsely overestimate ABP. Care was taken to maintain consistent positioning of the cuff on the leg, the position of the leg with regard to the rest of the body, and the position of the cuff and pressure transducer at the same level during the period of data collection, minimizing these effects.

Potential limitations of this study include a lack of standardization of the number of measurements per patient, the variety of procedures, variability of the site of catheter placement, and the use of a non-veterinary specific NIBP monitor. One animal only had 5 measurements, while another had 35 measurements, with an average of 14 measurements per animal. The study utilized clinical patients anesthetized for a specific condition in which blood pressure monitoring would normally occur. The type of procedure did necessitate the use of different arteries for catheterization, which may have lead to differences in arterial blood pressure measurements. The 5 dental procedures necessitated placement of the arterial catheter in the medial saphenous artery, and the auricular artery was catheterized in the other 15 camelids. Differences in arterial catheter site could affect SABP and DABP, but would be unlikely to significantly affect MABP (34,35). Transducer and cuff placement in relation to the heart was standardized, catheter length and gauge were the same regardless of catheterization site, and the length and rigidity of saline-filled tubing remained the same thus reducing the effect of the site of catheterization. Oscillometric blood pressure monitors utilize the same working principles regardless of species, thus it is unlikely that this resulted in inconsistencies between invasive and oscillometric blood pressure measurements in camelids.

The American College of Veterinary Anesthesiologists (ACVA) has recommended that arterial blood pressure should be monitored in anesthetized veterinary patients (36). The ACVA has also stated that in horses, arterial blood pressure monitoring is “strongly recommended whenever inhalation anesthesia is used” (37). Given the results of the study reported here ABP monitoring in anesthetized camelids should be performed whenever possible using the invasive method. The use of the oscillometric method of blood pressure monitoring in this population of anesthetized camelids was inconsistent with invasively measured arterial blood pressure.