Bias, sensitivity, and specificity of Doppler ultrasonic flow detector measurement of blood pressure for detecting and monitoring hypotension in anesthetized dogs

Lisa A. Bourazak Department of Clinical Sciences, Cummings School of Veterinary Medicine, Tufts University, North Grafton, MA 01536.

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Erik H. Hofmeister Department of Small Animal Medicine and Surgery, College of Veterinary Medicine, University of Georgia, Athens, GA 30602.

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Abstract

OBJECTIVE To determine the bias, sensitivity, and specificity of Doppler ultrasonic flow detector measurement of blood pressure (DBPM) to detect hypotension in dogs with various disease states and to determine whether patient characteristics could affect accuracy of DBPM in dogs.

DESIGN Prospective cross-sectional study.

ANIMALS 146 client-owned dogs undergoing general anesthesia at a veterinary teaching hospital between April 2007 and August 2010.

PROCEDURES Data collected for each dog were breed, limb conformation, sex, American Society of Anesthesiologists physical status classification, anesthetic protocol, surgical procedure, arterial catheter size and location, and DBPM location. Doppler and invasive blood pressure measurements (IBPMs; criterion standard) were simultaneously recorded every 5 minutes throughout anesthesia. Hypotension was defined as mean arterial blood pressure < 60 mm Hg or DBPM < 90 mm Hg. Repeated-measures Bland-Altman analysis was performed to determine bias between DBPMs and IBPMs. Overall sensitivity and specificity of DBPM to detect hypotension were calculated with 2 methods, and values were recalculated for specific patient groups and compared.

RESULTS Bias of DBPM was 2.8 mm Hg with wide 95% limits of agreement (−46.4 to 51.9 mm Hg). For the 2 calculation methods, sensitivity of DBPM to detect hypotension was 69.2% and 66.7% and specificity was 82.2% and 86.8%. No significant differences in sensitivity or specificity were identified regarding limb conformation, gauge of catheter (20 vs 22) used for IBPM, or side (ipsilateral or contralateral) of paired measurements.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that in dogs of the present study, DBPM was unreliable for detecting hypotension.

Abstract

OBJECTIVE To determine the bias, sensitivity, and specificity of Doppler ultrasonic flow detector measurement of blood pressure (DBPM) to detect hypotension in dogs with various disease states and to determine whether patient characteristics could affect accuracy of DBPM in dogs.

DESIGN Prospective cross-sectional study.

ANIMALS 146 client-owned dogs undergoing general anesthesia at a veterinary teaching hospital between April 2007 and August 2010.

PROCEDURES Data collected for each dog were breed, limb conformation, sex, American Society of Anesthesiologists physical status classification, anesthetic protocol, surgical procedure, arterial catheter size and location, and DBPM location. Doppler and invasive blood pressure measurements (IBPMs; criterion standard) were simultaneously recorded every 5 minutes throughout anesthesia. Hypotension was defined as mean arterial blood pressure < 60 mm Hg or DBPM < 90 mm Hg. Repeated-measures Bland-Altman analysis was performed to determine bias between DBPMs and IBPMs. Overall sensitivity and specificity of DBPM to detect hypotension were calculated with 2 methods, and values were recalculated for specific patient groups and compared.

RESULTS Bias of DBPM was 2.8 mm Hg with wide 95% limits of agreement (−46.4 to 51.9 mm Hg). For the 2 calculation methods, sensitivity of DBPM to detect hypotension was 69.2% and 66.7% and specificity was 82.2% and 86.8%. No significant differences in sensitivity or specificity were identified regarding limb conformation, gauge of catheter (20 vs 22) used for IBPM, or side (ipsilateral or contralateral) of paired measurements.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that in dogs of the present study, DBPM was unreliable for detecting hypotension.

Monitoring of arterial BP is important during anesthesia because trends in BP changes can be tracked and provide a basis for decisions on when to treat hypotensive or hypertensive states.1,2 Hypotension during anesthesia, the most common complication associated with anesthesia in dogs,3 is important to recognize because it would indicate poor circulatory function and could affect tissue perfusion.4 Tissues particularly sensitive to poor perfusion include kidney, brain, and heart.

Blood pressure monitoring has become more common with the wide availability of NIBPM techniques.5–8 Noninvasive techniques offer less invasive ways to diagnose normo-, hyper-, and hypotension in dogs and cats, and clinicians should be aware of the accuracies of the NIBPM techniques they use to allow optimal interpretation of the results (eg, the probability of hypertension or hypotension in patients with given NIBPM results).9 Guidelines from the ACVIM7 highly recommend measurement and monitoring of BP to recognize hypertension in conscious dogs and cats, as do the American College of Veterinary Anesthesia and Analgesia10 and American Animal Hospital Association11 to ensure adequacy of circulatory function.

Blood pressure monitoring techniques can be classified as noninvasive (eg, use of a Doppler ultrasonic flow detector or oscillometric measurement device) or invasive (eg, use of an arterial catheter connected to an arterial BP transducer). In general, NIBPM monitoring techniques are technically easier, more cost-effective, and involve less risk to patients than do IBPM techniques.12 Although IBPM is the gold standard, it requires more time and technical skill, has a higher cost, and poses more risk (eg, infection, embolism, and hemorrhage) to patients.1,13–17 A DBPM in dogs and cats is usually obtained at a metatarsal or metacarpal artery but can also be obtained at the base of the tail.

Studies have compared NIBPM and IBPM in dogs1,5,6,13,16–21 and cats,2,22,23 but mainly were performed under controlled, experimental conditions with healthy animals in which hypo-, normo-, and hypertensive states were induced. Similarly, most studies1,13,17,21,22 of the oscillometric method for BP measurement were performed in controlled conditions. Such studies have been helpful in advancing our understanding of trends (increasing or decreasing BP measurements in a patient) and accuracies associated with NIBPM, compared with those associated with IBPM; however, they typically involved average-sized, healthy animals with little variation in conformation that were not undergoing clinical procedures. Those studies also looked explicitly at the agreement among BP measurement methods to establish the validity and reliability of certain methods. To the authors' knowledge, no study has been reported in which epidemiological methods were used to assess the overall bias, sensitivity, and specificity of DBPM as a diagnostic tool in dogs.

The objectives of the study reported here were to determine the bias, sensitivity, and specificity of DBPM, compared with IBPM (criterion standard), as a diagnostic tool to detect hypotension in a series of dogs with various disease states and to determine whether certain patient characteristics (eg, limb conformation) could affect the accuracy of DBPM in dogs. We anticipated that DBPM would not be as useful as IBPM in detection of hypotension, but would be useful in obtaining systolic BP measurements. We also hypothesized that measurements in chondrodystrophic dog breeds with disproportionally short and curved limbs would be less accurate than those in dogs with no such limb conformation because of cuff placement difficulties.

Materials and Methods

Animals

Dogs that underwent procedures requiring general anesthesia and arterial catheterization at the University of Georgia Veterinary Teaching Hospital between April 2007 and August 2010 were eligible for the study. Dogs were included in the study whether they underwent emergency or elective procedures. Dogs with data missing after review were excluded. The Clinical Research Committee at the University of Georgia approved the study and determined that client consent was not required because the data used for the study were already being collected and related procedures were already being performed as part of clinical management.

Data collection

Data collected for each dog were breed, limb conformation, sex, American Society of Anesthesiologists physical status classification,24 anesthetic protocol, surgical procedure, arterial catheter size and location, and DBPM location. Altered limb conformation was defined as disproportionally short and curved limbs (eg, such as typical in Dachshunds and Pembroke Welsh Corgis). Anesthetic premedication and induction protocols were determined by senior veterinary students assigned to the anesthesia service and were approved by the attending faculty anesthesiologist. Each protocol was customized for the individual patient in accordance with the planned procedure. Locoregional anesthesia was performed in appropriate circumstances and recorded.

BP measurements

After induction of anesthesia, an arterial catheter for IBPM was placed into a femoral, radial, dorsal pedal, or coccygeal artery. Prior to placement, the hair over the catheter site was clipped, and the skin was aseptically prepared. A 22-, 20-, or 18-gauge catheter was placed percutaneously into the artery, and a transducer was connected to the arterial catheter with a 48-inch noncompliant pressure tubing line filled with saline (0.9% NaCl) solution. The transducer was positioned as close to the level of the right atrium as possible. Afterward, a fast-flush test was performed: the transducer was zero calibrated to atmospheric pressure, the catheter was flushed with 1 to 2 mL of heparinized saline solution, and the arterial pressure waveform was observed for 1 complete oscillation. Blood pressure measurements (by IBPM [SAP, diastolic arterial BP, and MAP] and NIBPM [SAP by DBPM]) were obtained every 5 minutes along with heart rate, respiratory rate, partial pressures of carbon dioxide, and end-tidal partial pressure of carbon dioxide.

For NIBPM, a median artery or plantar artery was used. Hair at the site (palmar or plantar aspect, respectively) of the selected foot was clipped. Coupling gel was applied to the probe of the Doppler ultrasonic flow detector, then the probe was placed on the skin over the respective artery and secured in place with adhesive tape. An inflatable cuff of appropriate size (cuff width, 40% of the circumference of the limb) was placed around the limb midway between the carpus and elbow when the DPB was to be monitored from the median artery or midway between the tibiotarsal joint and stifle joint when the DPB was to be monitored from the plantar artery.8,25 The sphygmomanometer was then attached to the cuff and used to inflate the cuff until a pulse was no longer audible through the Doppler ultrasonic flow detector probe. The pressure in the cuff was then slowly released, and the pressure at which the dog's pulse became audible again was recorded as its SAP.

Statistical analysis

Agreement between IBPMs and DBPMs of SAP was determined with the Bland-Altman plot technique for repeated measures. Bias was defined as the mean difference in values between SAP obtained by IBPM and peripheral systolic pressure obtained by DBPM, and the 95% LOA were calculated as the bias ± 1.96 SD. To be included in this analysis, dogs were required to have received at least the median number of paired measurements (IBPM and DBPM) for all dogs in the study.

The sensitivity and specificity of DBPM to detect hypotension, defined as MAP < 60 mm Hg on IBPM (criterion standard) and SAP < 90 mm Hg on DBPM, was determined for each patient by 2 methods. Method 1 involved random selection with a random number generator programa of 1 time point/dog for which paired BP recordings (IBPM and DBPM) were obtained during anesthesia and used in the calculations. Method 2 involved use of mode values for IBPM and DBPM that had been recorded for each dog. An unpaired t test was used to compare sensitivity and specificity between patients grouped by the size of catheter used for IBPM (20 vs 22 gauge), breed type (those with disproportionately short limb conformation vs proportional limb length conformation), and whether IBPMs and DBPM involved ipsilateral or contralateral limbs (excluding those paired with coccygeal arterial DBPM). Significance was set at P < 0.05. Statistical analyses were performed with available software.b–d

Results

Animals

One hundred forty-seven dogs were reviewed for inclusion; however, 1 dog was excluded because some IBPMs and DBPMs had not been recorded simultaneously. Therefore, 146 dogs were included in the study and were classified (by breed as reported by the owners) as Labrador Retriever (n = 33 [22.6%]); mixed-breed dog (12 [8.2%]); German Shepherd Dog (9 [6.2%]); Dachshund (including 2 miniature Dachshund) and Golden Retriever (8 [5.5%] each); Beagle (6 [4.1%]); Boxer, Shih Tzu (including 1 miniature Shih Tzu), and Jack Russell Terrier (5 [3.4%] each); Yorkshire Terrier [4 [2.7%]); Australian Shepherd, Cocker Spaniel, Pekingese, and Staffordshire Bull Terrier (3 [2.1%] each); Bullmastiff, corgi-type dog, Doberman, English Bulldog, Great Pyrenees, Greyhound, Maltese, and Pug (2 [1.4%] each); and 23 other breeds (1 [0.7%] each). Altered limb conformation was noted in 18 of the 146 (12.3%) dogs (8 Dachshunds, 3 Pekingese, 2 English Bulldogs, 2 corgi-type dogs, 1 Cairn Terrier, 1 Bassett Hound, and 1 Shih Tzu). Fiftyone of the 146 (34.9%) dogs were neutered males, 48 (32.9%) were spayed females, 28 (19.2%) were sexually intact males, and 19 (13.0%) were sexually intact females. Mean age was 6.7 years (range, 2 months to 15.9 years).

Anesthesia

Reported physical status of dogs according to the American Society of Anesthesiologists scale (1 = healthy to 6 = brain-dead) ranged from 1 to 5. All premedications included an opioid, and inductions were performed with various drugs, including propofol, ketamine, diazepam, thiopental, or etomidate. Anesthesia was maintained with inhalant anesthetic (isoflurane, n = 127/146 [87.0%]; sevoflurane, 17 [11.6%]; or desflurane, 2 [1.4%]) delivered in oxygen. Mean ± SD duration of anesthesia was 229.2 ± 96.4 minutes (range, 52 to 560 minutes). Three dogs were euthanized during their procedure.

BP measurements

Arterial catheters were placed (dorsal pedal artery, n = 112/146 [76.7%]; coccygeal artery, 26 [17.8%]; femoral artery, 4 [2.7%]; or radial artery, 4 [2.7%]) for IBPM monitoring. The catheter sizes used were 22 gauge (n = 106/146 [72.6%]), 20 gauge (37 [25.3%]), and 18 gauge (3 [2.1%]). Location combinations for paired IBPMs and DBPMs, respectively, were dorsal pedal and median arteries of 48 contralateral and 45 ipsilateral limbs (n = 93/146 [63.7%]), coccygeal and median arteries (20 [13.7%]), dorsal pedal and plantar arteries of contralateral limbs (19 [13.0%]), coccygeal and plantar arteries (6 [4.1%]), radial and median arteries of contralateral limbs (4 [2.7%]), and femoral and median arteries of contralateral limbs (4 [2.7%]).

Statistical analysis

Median number of paired measurements per dog was 24, and 85 of the 146 (58.2%) dogs met this criterion for inclusion in the Bland-Altman analysis. Mean bias for agreement in SAP values between the 2 techniques for these 85 dogs was 2.8 mm Hg (95% LOA, −46.4 to 51.9 mm Hg; Figure 1).

Figure 1—
Figure 1—

Repeated-measures Bland-Altman plot of agreement of paired DBPMs and IBPMs of SAP for 85 client-owned dogs that underwent general anesthesia for emergency or elective procedures at a veterinary teaching hospital and had arterial catheters placed. The solid line indicates the mean difference (bias, 2.8 mm Hg) in SAP values obtained by DBPM minus those obtained by IBPM for ≥ 24 paired measurements/dog. The dashed lines indicate the 95% LOA (−46.4 to 51.9 mm Hg). Each circle represents a single patient.

Citation: Journal of the American Veterinary Medical Association 253, 11; 10.2460/javma.253.11.1433

Sensitivity and specificity of DBPM to detect hypotension (on the basis of data from all 146 dogs) were 69.2% and 82.2%, respectively, for method 1 and 66.7% and 86.8%, respectively, for method 2. No differences in sensitivity or specificity were found on the basis of breed type, catheter size (20 gauge vs 22 gauge), or whether paired measurements involved ipsilateral or contralateral limbs.

Discussion

Studies15,17,26–28 have compared BP values obtained by DBPM with those obtained by the criterion standard of IBPM by arterial catheterization, and most findings indicate that DBPM generally underestimates IBPMs of SAP. Two studies26,27 in dogs have shown DBPMs to closely reflect IBPMs of SAP, and DBPM reportedly has good sensitivity and specificity in rabbits.28 A more recent comparison of BP values obtained in dogs with an oscillometric device method, DBPM, and IBPM showed that oscillometric measurements had a better agreement with IBPMs than DBPMs.25

In a recent survey,29 diplomates of the American College of Veterinary Anesthesia and Analgesia and the European College of Veterinary Anesthesia and Analgesia defined hypotension as SAP < 87 mm Hg for dogs undergoing surgical procedures and MAP < 62 mm Hg for dogs undergoing diagnostic procedures. For survey respondents, the lowest patient BP that prompted treatment intervention during surgical procedures was SAP < 85 mm Hg and MAP < 61 mm Hg, and the lowest during diagnostic procedures was SAP < 84 mm Hg and MAP < 63 mm Hg. These BP values closely resembled the values chosen in the present study to define hypotension.

Mean bias of DBPM as calculated in the present study was low, indicating that, on a population basis, DBPM was an accurate method for SAP monitoring. However, the LOA were fairly wide, indicating a low precision of this bias estimate and, therefore, wide variability in agreement between the 2 techniques (or accuracy of DBPM) among individual dogs. Although DBPMs may not have accurately reflected true SAP values (per IBPM) in individual dogs and accurate BP measurement may not have been clinically important in some circumstances, such wide departures from true values could result in errors in diagnosing hypotension on the basis of a certain cutoff value in clinical settings. Similar findings of wide LOA were reported by Vachon et al,25 who found that DBPM bias ± SD was −4.1 ± 24.7 mm Hg. In addition, Weiser et al30 also reported a wide range of BP values but indicated that values were not significantly different between IBPM and DBPM methods.

The low sensitivity of DBPM to detect hypotension in dogs of the present study indicated that DBPM was not reliable for screening these dogs for hypotension. However, specificity was relatively high, indicating that DBPM could be useful for confirming hypotension. These findings are clinically useful because they suggested that when DBPM is used as a screening test for dogs, cases of hypotension are likely being missed, whereas when DBPM is used for hypotensive dogs, the resulting measurements or trends in those measurements can likely be trusted.

In a previous study,6 DBPM overestimated SAP in 20 dogs and failed to meet the ACVIM validation criteria7 for measuring BP. Although the methods used in that study are not comparable to those of the present study, the finding coincided in that DBPM was unreliable.6

The BP cuffs used in veterinary medicine are adapted from use in humans, and the lack of validation for NIBPM devices in veterinary medicine may be in part because limbs in dogs and cats are more conical than in people. However, no differences were found in the present study for sensitivity or specificity of DBPM on the basis of limb conformation. A recent study31 compared BP measurements obtained from a traditionally used BP cuff (cylindrical) with measurements obtained from a specially made conical BP cuff for hound dogs. The specially made BP cuff met the ACVIM consensus guidelines for BP measurement; however, use of the conical cuff resulted in no better agreement between NIBPMs and IBPMs than use of the cylindrical cuff. Results of the present study were consistent with these findings and suggested that dog limb conformation may not have had a notable effect on the validity of NIBPM methods.

Results of the present study indicated no difference in results obtained when DBPM and IBPM were performed ipsilaterally versus contralaterally. Obtaining NIBPMs and IBPMs on the same side of the body did not affect measurements, likely because of the physical separation cranially and caudally. In addition, oscillometric BP measurements in dogs are reportedly unaffected by whether hair over the site of cuff application has been clipped.22 Given these findings, other variables (eg, cuff size) may be more important in DBPM accuracy than limb conformation, body size, or hair presence.

One limitation of the present study was that DBPMs and IBPMs for each dog were obtained mainly by fourth-year veterinary students. The students may have been less experienced in performing DBPM or in troubleshooting DBPMs or IBPMs if problems arose. For instance, in a previous study,32 a student, a technician, a third-year cardiology student, and a board-certified veterinary cardiologist were compared on their ability to obtain DBPMs. All 4 operators successfully obtained systolic BP measurements in all their attempts; however, the highest variability in measurements was observed for the least experienced operator.32 Nevertheless, that investigation was designed to mimic the different training levels of individuals monitoring anesthesia and performing DBPM in any setting. Factors influencing DBPM accuracy include cuff fit (including size and tightness) and position in that a falsely high measurement could be obtained when a cuff is positioned below the level of the heart and a falsely low measurement could be obtained when a cuff is too large, too tight, or positioned above the level of the heart.33 Factors influencing IBPM accuracy include inaccurate zero calibration of the transducer, placement of the transducer at a level different from the level of the right atrium, length of the connection tubing, and air bubbles within the line.

Another limitation to the present study was that the fast-flush test was performed by hand with a syringe versus a bag of saline solution pressurized to 300 mm Hg, as described elsewhere.34 Performance by hand could have resulted in less accurate IBPMs; however, such practice is common in clinical settings.

Different sites were used for IBPM in the present study, depending on the individual circumstances (eg, dog body size and site accessibility). The differences in vessel sizes with the various sites could have resulted in small differences in the pulse-pressure waveform, which reflected stroke volume. Therefore, a small vessel could have collapsed and contributed to subsequent errors in IBPM, whereas this was less likely with a larger vessel. A study35 that compared various anatomic sites for arterial BP measurement showed that significant BP differences existed among sites and that differences were more pronounced with SAP measurement than with MAP measurement. Measurements obtained at the dorsal pedal artery were significantly higher than those obtained at the carpus.35 Considering this and that most of the catheters for IBPM in the present study were placed in a dorsal pedal artery, our IBPMs of SAP could have been falsely high because of the site of arterial catheterization. Nonetheless, this mimicked clinical settings because many clinicians at the time placed catheters for IBPM in a dorsal pedal artery. On a related note, different arterial catheter sizes were used in the present study, and the size chosen depended mainly on the vessel size of the patient. Ideally, a large arterial catheter should be used.36 This is relative, however, because a 22-gauge catheter could be considered large for a small dog, compared with a 20-gauge catheter that could be considered appropriate for a large dog.

Results indicated that for dogs requiring general anesthesia and arterial catheter placement in the present study, DBPM was less accurate for detecting hypotension and monitoring BP than was IBPM, the criterion standard. Therefore, we believe that decisions made regarding BP management in anesthetized dogs, particularly those regarding hypotension, should be made with IBPM instead of DBPM, despite the greater technical skill required to perform IBPM.

Acknowledgments

The authors declare that there were no conflicts of interest.

Presented in abstract form at the International Veterinary Emergency and Critical Care Symposium, Washington, DC, September 2015.

ABBREVIATIONS

ACVIM

American College of Veterinary Internal Medicine

BP

Blood pressure

DBPM

Doppler ultrasonic flow detector measurement of blood pressure

IBPM

Invasive blood pressure measurement

LOA

Limits of agreement

MAP

Mean arterial blood pressure

NIBPM

Noninvasive blood pressure measurement

SAP

Systolic arterial blood pressure

Footnotes

a.

True random number generator, Randomness and Integrity Services Ltd, Dublin, Ireland.

b.

GraphPad Prism, version 5, GraphPad Software Inc, La Jolla, Calif.

c.

MedCalc, version 16.8.4, MedCalc Software, Ostend, Belgium.

d.

Excel, version 14.6.0, Microsoft Corp, Redmond, Wash.

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