Comparison of directly measured arterial blood pressure at various anatomic locations in anesthetized dogs OBJECTIVE To determine whether directly measured arterial blood pressure differs among anatomic locations and whether arterial blood pressure is influenced by body position.
Mark J. Acierno MBA, DVM Michelle E. Domingues BS Sara J. Ramos Amanda M. Shelby BS
ANIMALS 33 client-owned dogs undergoing anesthesia.
Anderson F. da Cunha DVM,MS Received July 3, 2014. Accepted October 9, 2014. From the Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803. Address correspondence to Dr. Acierno (Acierno@lsu. edu).
PROCEDURES Dogs undergoing anesthetic procedures had 20-gauge catheters placed in both the superficial palmar arch and the contralateral dorsal pedal artery (group 1 [n = 20]) or the superficial palmar arch and median sacral artery (group 2 [13]). Dogs were positioned in dorsal recumbency, and mean arterial blood pressure (MAP), systolic arterial blood pressure (SAP), and diastolic arterial blood pressure (DAP) were recorded for both arteries 4 times (2-minute interval between successive measurements). Dogs were positioned in right lateral recumbency, and blood pressure measurements were repeated. RESULTS Differences were detected between pressures measured at the 2 arterial sites in both groups. This was especially true for SAP measurements in group 1, in which hind limb measurements were a mean of 16.12 mm Hg higher than carpus measurements when dogs were in dorsal recumbency and 14.70 mm Hg higher than carpus measurements when dogs were in lateral recumbency. Also, there was significant dispersion about the mean for all SAP, DAP, and MAP measurements. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that arterial blood pressures may be dependent on anatomic location and body position. Because this may affect outcomes of studies conducted to validate indirect blood pressure measurement systems, care must be used when developing future studies or interpreting previous results. (Am J Vet Res 2015;76:266–271)
M
easurement of systemic arterial blood pressure has long been recognized as an important part of monitoring anesthetized patients as well as evaluating critically ill animals and their response to treatment.1,2 Because of its importance, blood pressure is quickly becoming the fourth vital sign, accompanying temperature, pulse, and respiration.3 Anesthetized cats and dogs are especially at risk for hypotension, given that many anesthetic drugs have the potential to depress cardiac output as well as decrease peripheral vascular resistance.4 Additionally, certain disease conditions5–8 and medications9 are associated with secondary hypertension in dogs. This has important clinical implications because ABBREVIATIONS DAP Diastolic arterial blood pressure LOA Limits of agreement MAP Mean arterial blood pressure SAP Systolic arterial blood pressure 266
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hypertension can damage target organs such as the eyes, kidneys, heart, and brain.7 Placement of a catheter in a suitable artery and use of a transducer to measure arterial blood pressure waves represents the criterion-referenced standard for blood pressure measurement.10,11 This is technically challenging and causes discomfort for patients; therefore, it is unsuitable for many clinical situations.12 Clinically, veterinarians primarily rely on indirect blood pressure measurements, such as those obtained with Doppler ultrasonography and oscillometric devices, to estimate actual arterial blood pressure.13 Nevertheless, studies3,14–22 designed to validate values obtained with Doppler ultrasonography and oscillometric units have yielded disappointing and sometimes conflicting results. Since 2004, several peer-reviewed studies3,14–22 have been conducted to examine the relationship between results of direct and indirect blood pressure measurements in dogs. In most of those studies,14–20
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investigators examined the relationship between results for oscillometric devices and direct blood pressure measurements, and investigators in the other 3 studies3,21,22 compared results for both oscillometric devices and Doppler ultrasonography against directly measured blood pressure. Most studies failed to confirm good agreement between the direct and indirect blood pressure measurements. The central tenet of all these validation studies has been that arterial blood pressure is a constant and the same regardless of the location where it is measured. Thus, arterial blood pressure measured on a forelimb is expected to be equivalent to arterial blood pressure measured on a hind limb. It seems plausible that the failure of these studies to confirm agreement is because of variations in blood pressure throughout the body. The purpose of the study reported here was to determine whether there would be good agreement between directly measured blood pressures obtained at 2 anatomic locations and whether body position would influence this agreement. Our hypothesis was that agreement would be poor between directly measured blood pressures obtained at 2 anatomic locations and that body position would affect this agreement.
Materials and Methods ANIMALS A convenience sample of 33 dogs undergoing anesthetic procedures at the veterinary teaching hospital at Louisiana State University were enrolled in the study. Enrollment of dogs was not restricted on the basis of the type or degree of illness. The only exclusion criterion was the inability of investigators to place a 20-gauge catheter into each of 2 selected arteries. Owner consent was obtained for all dogs. The School of Veterinary Medicine Clinical Protocol Review Committee and the university institutional animal care and use committee approved the protocol for the study. PROCEDURES A prospective, randomized, single-center study was conducted. Dogs were anesthetized in accordance with teaching hospital protocols and monitored by one of the investigators (AMS). A multifunction monitor was used to continuously monitor heart rate and rhythm, pulse oximetry, expired end-tidal PCO2, and body temperature. Dogs were manually ventilated as needed to maintain end-tidal PCO2 between 35 and 45 mm Hg. A convective warmer was used to maintain body temperature, if needed. All blood pressure measurements for the study were obtained prior to the procedure. No dog received vasopressors or bolus administration of fluids before or during the study. Dogs were allocated into 2 groups on the basis of a random number table.a Catheter sites on each anesthetized dog were aseptically prepared. For dogs of group 1, a 20-gauge catheterb was placed in both the superficial palmar arch (carpus) and contralateral dorsal pedal artery (hind limb). For dogs of group 2, a
20-gauge catheterb was placed in both the superficial palmar arch (carpus) and median sacral artery (tail). Catheters were connected to a pressure transducerc via noncompliant tubing filled with saline (0.9% NaCl) solution; the saline solution–filled tubing was replaced between successive dogs.Transducers were connected to a data acquisition system.d The system was connected to a pressurized (300 mm Hg) bag of saline solution. Transducers were placed at the level of the heart base (sternum for dogs in lateral recumbency or point of the shoulder for dogs in dorsal recumbency) and calibrated (zero) to atmospheric pressure. Prior to use, each system was assessed for accuracy against a mercury manometer by use of a 4-point validation technique. Briefly, the manometer, transducer, and pressurized bag of saline solution were connected via a 4-way stopcock that allowed simultaneous pressurization of both devices. The system was pressurized, and agreement between the manometer and transducer was evaluated at 0, 50, 100, and 150 mm Hg. The dynamic response of the direct blood pressure monitoring system was tested with the fast flush test.23,24 In accordance with previously described techniques,23 the allowable damping coefficient for this system was determined to be adequate for the dogs being evaluated.The direct blood pressure monitoring system was visually inspected and periodically flushed to prevent clots and to remove air bubbles, both of which could change the damping coefficient of the system. Direct pressure measurements were assessed for stability and consistency, and the waveform was analyzed before study recordings commenced. Specifically, the anacrotic limb, flow wave, anacrotic shoulder, dicrotic notch, and dicrotic limb of the recordings were analyzed for consistency. Each dog was placed in dorsal recumbency. Direct arterial blood pressure measurements (SAP, DAP, and MAP) were recorded for both arteries of each dog 4 times, with 2 minutes between successive measurements. Dogs then were placed in right lateral recumbency, and after an equilibration period of 2 minutes, blood pressure measurements were repeated. STATISTICAL ANALYSIS Distribution of body weight and age was examined by means of the Shapiro-Wilk test of normality. Differences between the 2 groups were examined by use of the Mann-Whitney U test. Agreement between the direct blood pressure measurements was determined by use of Bland-Altman analysis in which carpal blood pressure was used as the reference. Because the sampling strategy involved a repeated-measures approach, measurements from each dog could not be considered independent.25 Therefore, mean SAP, DAP, and MAP measurements for each artery were calculated and used for analysis. Bias was defined as the mean difference between the 2 arteries, and 95% LOA were calculated as the bias ± (1.96•SD). Because of the potential for underestimating the SD of differences when
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repeated measures are used, calculated SDs were corrected as described elsewhere.26 All statistical analyses were performed with commercially available software.a Values of P < 0.05 were considered significant.
Results ANIMALS Thirty-three dogs were included in the study. Twenty dogs (9 males and 11 females) were assigned
Figure 1—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb) of 20 dogs (group 1) positioned in dorsal recumbency. The solid horizontal line represents the bias, and the dotted lines represent the 95% LOA. Blood pressure measured at the carpus was used as the reference. 268
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to group 1 and 13 (7 males and 6 females) were assigned to group 2. Dogs in group 1 required anesthesia for procedures including MRI (n = 3), ovariohysterectomy (3), removal of cutaneous masses (3), exploratory abdominal surgery (2), castration (2), rhinoscopy (2), arthroscopy (2), mandibulectomy (1), endoscopy (1), and laser ablation of an ectopic ureter (1). Procedures performed in group 2 included castration (n = 6), ovariohysterectomy (3), limb amputation (1), arthroscopy (1), removal of cutaneous masses (1), and endoscopy (1).
Figure 2—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb) of 20 dogs (group 1) positioned in lateral recumbency. See Figure 1 for key.
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Figure 3—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and median sacral artery (tail) of 13 dogs (group 2) positioned in dorsal recumbency. See Figure 1 for key.
Body weight and age were not normally distributed. Median body weight for group 1 was 30 kg (range, 8.1 to 56.0 kg), whereas median body weight for group 2 was 21 kg (range, 13.5 to 55.5 kg). Actual age was known for only 14 dogs of group 1 (median, 6.5 years; range, 0.25 to 13 years) and 5 dogs of group 2 (median, 7.0 years; range, 0.7 to 13 years). Body weight and age did not differ significantly between the 2 groups.
Figure 4—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and median sacral artery (tail) of 13 dogs (group 2) positioned in lateral recumbency. See Figure 1 for key.
GROUP 1 For dogs of group 1 in dorsal recumbency, SAP agreement analysis revealed bias of –16.12 mm Hg (LOA, –43.17 to 11.16 mm Hg; Figure 1), DAP agreement analysis revealed bias of 1.71 mm Hg (LOA, –5.93 to 9.35 mm Hg), and MAP agreement analysis revealed bias of –0.40 mm Hg (LOA, –7.49 to 6.69 mm Hg). For dogs of group 1 in lateral recumbency, SAP agreement analysis revealed bias of –14.70 mm Hg (LOA, –43.28 to 13.88 mm Hg; Figure 2), DAP agreement analysis re-
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vealed bias of 2.41 mm Hg (LOA, –4.91 to 9.73 mm Hg), and MAP agreement analysis revealed bias of –0.31 mm Hg (LOA, –7.48 to 7.82 mm Hg). GROUP 2 For dogs of group 2 in dorsal recumbency, SAP agreement analysis revealed bias of –3.08 mm Hg (LOA, –23.68 to 17.48 mm Hg; Figure 3), DAP agreement analysis revealed bias of 0.42 mm Hg (LOA, –7.66 to 8.50 mm Hg), and MAP agreement analysis revealed bias of –2.02 mm Hg (LOA, –9.96 to 5.92 mm Hg). For dogs of group 2 in lateral recumbency, SAP agreement analysis revealed bias of –4.67 mm Hg (LOA, –24.32 to 15.00 mm Hg; Figure 4), DAP agreement analysis revealed bias of –0.73 mm Hg (LOA, –7.79 to 6.33 mm Hg), and MAP agreement analysis revealed bias of –2.11 mm Hg (LOA, –9.07 to 4.85 mm Hg).
Discussion In the present study, direct blood pressure measurements obtained at 2 anatomic locations in 2 groups of dogs were compared. For group 1, arterial blood pressure was measured at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb); arterial blood pressure in group 2 was measured at the superficial palmar arch (carpus) and median sacral artery (tail). In all cases, there were pressure differences that could affect comparison studies. This was especially true for all SAP measurements but was most pronounced for SAP measurements of group 1 in which hind limb measurements were a mean of 16.12 mm Hg higher than carpus measurements when dogs were in dorsal recumbency and 14.70 mm Hg higher than carpus measurements when dogs were in lateral recumbency. In addition, there was significant dispersion about the mean insofar as hind limb SAP measurements could be as much as 43.17 mm Hg higher or 11.16 mm Hg lower than carpus SAP measurements when dogs were in dorsal recumbency and as much as 43.28 mm Hg higher or 13.88 mm Hg lower than carpus SAP measurements when dogs were in lateral recumbency. The SAP measurements for group 2 had substantially better agreement: mean SAP measurements for the tail were only 3.08 mm Hg higher than mean SAP measurements for the carpus when dogs were in dorsal recumbency and 4.67 mm Hg higher than mean SAP measurements for the carpus when dogs were in lateral recumbency. Nevertheless, measurements at the median sacral artery could still be expected to be as much as 23.68 mm Hg higher or 17.47 mm Hg lower than measurements at the carpus when dogs were in dorsal recumbency and as much as 24.32 mm Hg higher or 15.00 mm Hg lower than measurements at the carpus when dogs were in lateral recumbency. In both group 1 and 2, there were differences in DAP and MAP measurements; however, in all cases, these were < 10 mm Hg. A few small differences were detected between dorsal and lateral recumbency; however, they appeared to be relatively minor. 270
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It has long been known that arterial waveforms change as a pulse travels from the aorta to the peripheral vasculature. Specifically, the anacrotic limb becomes steeper, the peak becomes higher, the dicrotic notch becomes delayed, and the dicrotic limb dips lower.27 This phenomenon, known as distal pulse wave amplification, results in peripheral arterial waveforms with higher SAP and lower DAP than those for centrally measured arterial waveforms.28 Interestingly, MAP remains relatively unaffected.28 Distal pulse wave amplification is the result of physical characteristics (eg, impedance and resistance) of the vascular tree.27 Distal pulse wave amplification would predict that if an artery were more central than another artery, Bland-Altman analysis would yield SAP and DAP with biases on opposite sides of zero. Thus, one bias would be positive and the other negative, depending on the reference artery. In the present study, SAP and DAP biases were on opposite sides of zero; MAP had relatively little bias. However, the large negative biases for SAP measurements in dogs of group 1 suggested that the dorsal pedal artery was much more centrally located than the superficial palmar arch. This is difficult to justify anatomically. More problematic was the relatively wide LOA resulting from the extensive dispersion of pressure measurements around the bias.This implied that much of the variation was nonsystematic in nature. Therefore, results of the present study cannot be completely explained on the basis of distal pulse wave amplification. Another possible explanation for results of the study reported here was the manner in which direct blood pressure was measured. In this study, each artery was catheterized with a 20-gauge arterial catheter connected to a transducer via noncompliant saline solution–filled tubing.This generally is considered the criterion-referenced standard27 for blood pressure measurement and has been validated.29 Although electrical and fiber-optic transducers that can be inserted directly into an artery are available, they are less commonly used.30 Most importantly, to explain the lack of agreement in previous studies, we used the same techniques for direct blood pressure measurement that had been used in those studies.3,14–22 It seems plausible that relying on arterial blood pressure to move a column of water to affect a transducer may not be accurate for use in dogs; however, the present study was not designed to evaluate this. One shortcoming of the present study was the relatively few blood pressure measurements in the hypertensive range. Nevertheless, a wide range of blood pressures was examined. Because these were client-owned animals, no attempt was made to pharmacologically manipulate blood pressure. Considering that we were unable to obtain good SAP agreement for measurements in the normotensive and hypotensive range, there seemed to be little benefit to further explore agreement for measurements in the hypertensive range; nevertheless, it is possible that agreement for DAP and MAP could be worse for dogs with hypertensive conditions.
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In the present study, we attempted to investigate whether the failure to validate indirect blood pressure measurement devices could have been attributable to variations in arterial blood pressure at different anatomic locations.We found small differences in DAP and MAP, but there were significant differences in SAP. This was especially pronounced when comparing SAP obtained at the carpus and hind limb. Although some of the observed differences may have been caused by distal pulse wave amplification, that phenomenon cannot explain the wide dispersion of observations around the bias. Some of the observed scattering may have been attributable to the commonly used method of catheterizing an artery and connecting it to a transducer via noncompliant saline solution–filled tubing. This study was not designed to evaluate that concern; however, results of previous studies have suggested that this is a valid method for measurement of blood pressure. Until it can be determined whether the differences detected in this study were attributable to the methods used or to actual anatomically related differences in arterial blood pressure, care must be taken when designing or interpreting studies that compare directly and indirectly measured blood pressure.
9. 10.
11. 12. 13. 14. 15. 16.
17.
18.
Acknowledgments Supported in part by a Morris Animal Foundation summer research scholar stipend awarded to Ms. Domingues. Presented in abstract form at the American College of Veterinary Internal Medicine Forum, Nashville, Tenn, June 2014.
Footnotes a. b. c. d.
Prism, version 6.0 for Macintosh, GraphPad Software Inc, San Diego, Calf. Abbocath, Hospira, Lake Forest, Ill. SP 844 physiological sensor, MEMSCAP Sensor Solutions, Skoppum, Norway. VetTrends V Vital Signs monitor, System VET,Tampa, Fla.
2.
3.
4. 5. 6. 7. 8.
20. 21.
22.
References 1.
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Haskins SC. Monitoring anesthetized patients. In: Tranquilli WJ, Thurmon JC, Kurt AG, eds. Lumb & Jones’ veterinary anesthesia and analgesia. 4th ed. Carlton, VIC, Australia: Blackwell Publishing Asia, 2007;533–558. Silverstein DC, Wininger FA, Shofer FS, et al. Relationship between Doppler blood pressure and survival or response to treatment in critically ill cats: 83 cases (2003–2004). J Am Vet Med Assoc 2008;232:893–897. Garofalo NA, Teixeira Neto FJ, Alvaides RK, et al. Agreement between direct, oscillometric and Doppler ultrasound blood pressures using three different cuff positions in anesthetized dogs. Vet Anaesth Analg 2012;39:324–334. Wagner AE, Brodbelt DC. Arterial blood pressure monitoring in anesthetized animals. J Am Vet Med Assoc 1997;210:1279– 1285. Struble AL, Feldman EC, Nelson RW, et al. Systemic hypertension and proteinuria in dogs with diabetes mellitus. J Am Vet Med Assoc 1998;213:822–825. Cortadellas O, del Palacio MJ, Bayon A, et al. Systemic hypertension in dogs with leishmaniasis: prevalence and clinical consequences. J Vet Intern Med 2006;20:941–947. Bodey AR, Michell AR. Epidemiological study of blood pressure in domestic dogs. J Small Anim Pract 1996;37:116– 125. Syme HM, Barber PJ, Markwell PJ, et al. Prevalence of systolic
23. 24. 25. 26. 27.
28. 29. 30.
hypertension in cats with chronic renal failure at initial evaluation. J Am Vet Med Assoc 2002;220:1799–1804. Cowgill LD, James KM, Levy JK, et al. Use of recombinant human erythropoietin for management of anemia in dogs and cats with renal failure. J Am Vet Med Assoc 1998;212:521–528. Binns SH, Sisson DD, Buoscio DA, et al. Doppler ultrasonographic, oscillometric sphygmomanometric, and photoplethysmographic techniques for noninvasive blood pressure measurement in anesthetized cats. J Vet Intern Med 1995;9:405–414. Brown SA, Henik RA. Diagnosis and treatment of systemic hypertension. Vet Clin North Am Small Anim Pract 1998;28:1481–1494. Acierno MJ, Labato MA. Hypertension in renal disease: diagnosis and treatment. Clin Tech Small Anim Pract 2005;20:23–30. Bartges JW,Willis AM, Polzin DJ. Hypertension and renal disease. Vet Clin North Am Small Anim Pract 1996;26:1331–1345. Sawyer DC, Guikema AH, Siegel EM. Evaluation of a new oscillometric blood pressure monitor in isoflurane-anesthetized dogs. Vet Anaesth Analg 2004;31:27–39. McMurphy RM, Stoll MR, McCubrey R. Accuracy of an oscillometric blood pressure monitor during phenylephrine-induced hypertension in dogs. Am J Vet Res 2006;67:1541–1545. Wernick M, Doherr M, Howard J, et al. Evaluation of high-definition and conventional oscillometric blood pressure measurement in anaesthetised dogs using ACVIM guidelines. J Small Anim Pract 2010;51:318–324. Shih A, Robertson S, Vigani A, et al. Evaluation of an indirect oscillometric blood pressure monitor in normotensive and hypotensive anesthetized dogs. J Vet Emerg Crit Care (San Antonio) 2010;20:313–318. Acierno MJ, Fauth E, Mitchell MA, et al. Measuring the level of agreement between directly measured blood pressure and pressure readings obtained with a veterinary-specific oscillometric unit in anesthetized dogs. J Vet Emerg Crit Care (San Antonio) 2013;23:37–40. Drynan EA, Raisis AL. Comparison of invasive versus noninvasive blood pressure measurements before and after hemorrhage in anesthetized Greyhounds using the Surgivet V9203. J Vet Emerg Crit Care (San Antonio) 2013;23:523–531. MacFarlane PD, Grint N, Dugdale A. Comparison of invasive and non-invasive blood pressure monitoring during clinical anaesthesia in dogs. Vet Res Commun 2010;34:217–227. Bosiack AP, Mann FA, Dodam JR, et al. Comparison of ultrasonic Doppler flow monitor, oscillometric, and direct arterial blood pressure measurements in ill dogs. J Vet Emerg Crit Care (San Antonio) 2010;20:207–215. Rysnik MK, Cripps P, Iff I.A clinical comparison between a noninvasive blood pressure monitor using high definition oscillometry (Memodiagnostic MD 15/90 Pro) and invasive arterial blood pressure measurement in anaesthetized dogs. Vet Anaesth Analg 2013;40:503–511. Kleinman B, Powell S, Kumar P, et al. The fast flush test measures the dynamic response of the entire blood pressure monitoring system. Anesthesiology 1992;77:1215–1220. Gardner RM. Direct blood pressure measurement—dynamic response requirements. Anesthesiology 1981;54:227–236. Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986;1:307–310. Bland JM, Altman DG. Measuring agreement in method comparison studies. Stat Methods Med Res 1999;8:135–160. Mark JB, Slaughter TF, Reves JG. Cardiovascular monitoring. In: Miller RD, ed. Anesthesia. 5th ed. London: Churchill Livingstone, 2000. Available at: web.squ.edu.om/med-Lib/ MED_CD/E_CDs/anesthesia/site. Accessed Jul 3, 2014. Papaioannou TG, Protogerou AD, Stefanadis C. What to anticipate from pulse pressure amplification. J Am Coll Cardiol 2010;55:1038–1040. Shinozaki T, Deane RS, Mazuzan JE. The dynamic responses of liquid-filled catheter systems for direct measurements of blood pressure. Anesthesiology 1980;53:498–504. Ward M, Langton JA. Blood pressure measurement. Contin Educ Anaesth Crit Care Pain 2007;7:122–126.
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Mark J. Acierno MBA, DVM Michelle E. Domingues BS Sara J. Ramos Amanda M. Shelby BS
ANIMALS 33 client-owned dogs undergoing anesthesia.
Anderson F. da Cunha DVM,MS Received July 3, 2014. Accepted October 9, 2014. From the Department of Veterinary Clinical Sciences, School of Veterinary Medicine, Louisiana State University, Baton Rouge, LA 70803. Address correspondence to Dr. Acierno (Acierno@lsu. edu).
PROCEDURES Dogs undergoing anesthetic procedures had 20-gauge catheters placed in both the superficial palmar arch and the contralateral dorsal pedal artery (group 1 [n = 20]) or the superficial palmar arch and median sacral artery (group 2 [13]). Dogs were positioned in dorsal recumbency, and mean arterial blood pressure (MAP), systolic arterial blood pressure (SAP), and diastolic arterial blood pressure (DAP) were recorded for both arteries 4 times (2-minute interval between successive measurements). Dogs were positioned in right lateral recumbency, and blood pressure measurements were repeated. RESULTS Differences were detected between pressures measured at the 2 arterial sites in both groups. This was especially true for SAP measurements in group 1, in which hind limb measurements were a mean of 16.12 mm Hg higher than carpus measurements when dogs were in dorsal recumbency and 14.70 mm Hg higher than carpus measurements when dogs were in lateral recumbency. Also, there was significant dispersion about the mean for all SAP, DAP, and MAP measurements. CONCLUSIONS AND CLINICAL RELEVANCE Results suggested that arterial blood pressures may be dependent on anatomic location and body position. Because this may affect outcomes of studies conducted to validate indirect blood pressure measurement systems, care must be used when developing future studies or interpreting previous results. (Am J Vet Res 2015;76:266–271)
M
easurement of systemic arterial blood pressure has long been recognized as an important part of monitoring anesthetized patients as well as evaluating critically ill animals and their response to treatment.1,2 Because of its importance, blood pressure is quickly becoming the fourth vital sign, accompanying temperature, pulse, and respiration.3 Anesthetized cats and dogs are especially at risk for hypotension, given that many anesthetic drugs have the potential to depress cardiac output as well as decrease peripheral vascular resistance.4 Additionally, certain disease conditions5–8 and medications9 are associated with secondary hypertension in dogs. This has important clinical implications because ABBREVIATIONS DAP Diastolic arterial blood pressure LOA Limits of agreement MAP Mean arterial blood pressure SAP Systolic arterial blood pressure 266
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hypertension can damage target organs such as the eyes, kidneys, heart, and brain.7 Placement of a catheter in a suitable artery and use of a transducer to measure arterial blood pressure waves represents the criterion-referenced standard for blood pressure measurement.10,11 This is technically challenging and causes discomfort for patients; therefore, it is unsuitable for many clinical situations.12 Clinically, veterinarians primarily rely on indirect blood pressure measurements, such as those obtained with Doppler ultrasonography and oscillometric devices, to estimate actual arterial blood pressure.13 Nevertheless, studies3,14–22 designed to validate values obtained with Doppler ultrasonography and oscillometric units have yielded disappointing and sometimes conflicting results. Since 2004, several peer-reviewed studies3,14–22 have been conducted to examine the relationship between results of direct and indirect blood pressure measurements in dogs. In most of those studies,14–20
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investigators examined the relationship between results for oscillometric devices and direct blood pressure measurements, and investigators in the other 3 studies3,21,22 compared results for both oscillometric devices and Doppler ultrasonography against directly measured blood pressure. Most studies failed to confirm good agreement between the direct and indirect blood pressure measurements. The central tenet of all these validation studies has been that arterial blood pressure is a constant and the same regardless of the location where it is measured. Thus, arterial blood pressure measured on a forelimb is expected to be equivalent to arterial blood pressure measured on a hind limb. It seems plausible that the failure of these studies to confirm agreement is because of variations in blood pressure throughout the body. The purpose of the study reported here was to determine whether there would be good agreement between directly measured blood pressures obtained at 2 anatomic locations and whether body position would influence this agreement. Our hypothesis was that agreement would be poor between directly measured blood pressures obtained at 2 anatomic locations and that body position would affect this agreement.
Materials and Methods ANIMALS A convenience sample of 33 dogs undergoing anesthetic procedures at the veterinary teaching hospital at Louisiana State University were enrolled in the study. Enrollment of dogs was not restricted on the basis of the type or degree of illness. The only exclusion criterion was the inability of investigators to place a 20-gauge catheter into each of 2 selected arteries. Owner consent was obtained for all dogs. The School of Veterinary Medicine Clinical Protocol Review Committee and the university institutional animal care and use committee approved the protocol for the study. PROCEDURES A prospective, randomized, single-center study was conducted. Dogs were anesthetized in accordance with teaching hospital protocols and monitored by one of the investigators (AMS). A multifunction monitor was used to continuously monitor heart rate and rhythm, pulse oximetry, expired end-tidal PCO2, and body temperature. Dogs were manually ventilated as needed to maintain end-tidal PCO2 between 35 and 45 mm Hg. A convective warmer was used to maintain body temperature, if needed. All blood pressure measurements for the study were obtained prior to the procedure. No dog received vasopressors or bolus administration of fluids before or during the study. Dogs were allocated into 2 groups on the basis of a random number table.a Catheter sites on each anesthetized dog were aseptically prepared. For dogs of group 1, a 20-gauge catheterb was placed in both the superficial palmar arch (carpus) and contralateral dorsal pedal artery (hind limb). For dogs of group 2, a
20-gauge catheterb was placed in both the superficial palmar arch (carpus) and median sacral artery (tail). Catheters were connected to a pressure transducerc via noncompliant tubing filled with saline (0.9% NaCl) solution; the saline solution–filled tubing was replaced between successive dogs.Transducers were connected to a data acquisition system.d The system was connected to a pressurized (300 mm Hg) bag of saline solution. Transducers were placed at the level of the heart base (sternum for dogs in lateral recumbency or point of the shoulder for dogs in dorsal recumbency) and calibrated (zero) to atmospheric pressure. Prior to use, each system was assessed for accuracy against a mercury manometer by use of a 4-point validation technique. Briefly, the manometer, transducer, and pressurized bag of saline solution were connected via a 4-way stopcock that allowed simultaneous pressurization of both devices. The system was pressurized, and agreement between the manometer and transducer was evaluated at 0, 50, 100, and 150 mm Hg. The dynamic response of the direct blood pressure monitoring system was tested with the fast flush test.23,24 In accordance with previously described techniques,23 the allowable damping coefficient for this system was determined to be adequate for the dogs being evaluated.The direct blood pressure monitoring system was visually inspected and periodically flushed to prevent clots and to remove air bubbles, both of which could change the damping coefficient of the system. Direct pressure measurements were assessed for stability and consistency, and the waveform was analyzed before study recordings commenced. Specifically, the anacrotic limb, flow wave, anacrotic shoulder, dicrotic notch, and dicrotic limb of the recordings were analyzed for consistency. Each dog was placed in dorsal recumbency. Direct arterial blood pressure measurements (SAP, DAP, and MAP) were recorded for both arteries of each dog 4 times, with 2 minutes between successive measurements. Dogs then were placed in right lateral recumbency, and after an equilibration period of 2 minutes, blood pressure measurements were repeated. STATISTICAL ANALYSIS Distribution of body weight and age was examined by means of the Shapiro-Wilk test of normality. Differences between the 2 groups were examined by use of the Mann-Whitney U test. Agreement between the direct blood pressure measurements was determined by use of Bland-Altman analysis in which carpal blood pressure was used as the reference. Because the sampling strategy involved a repeated-measures approach, measurements from each dog could not be considered independent.25 Therefore, mean SAP, DAP, and MAP measurements for each artery were calculated and used for analysis. Bias was defined as the mean difference between the 2 arteries, and 95% LOA were calculated as the bias ± (1.96•SD). Because of the potential for underestimating the SD of differences when
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repeated measures are used, calculated SDs were corrected as described elsewhere.26 All statistical analyses were performed with commercially available software.a Values of P < 0.05 were considered significant.
Results ANIMALS Thirty-three dogs were included in the study. Twenty dogs (9 males and 11 females) were assigned
Figure 1—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb) of 20 dogs (group 1) positioned in dorsal recumbency. The solid horizontal line represents the bias, and the dotted lines represent the 95% LOA. Blood pressure measured at the carpus was used as the reference. 268
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to group 1 and 13 (7 males and 6 females) were assigned to group 2. Dogs in group 1 required anesthesia for procedures including MRI (n = 3), ovariohysterectomy (3), removal of cutaneous masses (3), exploratory abdominal surgery (2), castration (2), rhinoscopy (2), arthroscopy (2), mandibulectomy (1), endoscopy (1), and laser ablation of an ectopic ureter (1). Procedures performed in group 2 included castration (n = 6), ovariohysterectomy (3), limb amputation (1), arthroscopy (1), removal of cutaneous masses (1), and endoscopy (1).
Figure 2—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb) of 20 dogs (group 1) positioned in lateral recumbency. See Figure 1 for key.
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Figure 3—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and median sacral artery (tail) of 13 dogs (group 2) positioned in dorsal recumbency. See Figure 1 for key.
Body weight and age were not normally distributed. Median body weight for group 1 was 30 kg (range, 8.1 to 56.0 kg), whereas median body weight for group 2 was 21 kg (range, 13.5 to 55.5 kg). Actual age was known for only 14 dogs of group 1 (median, 6.5 years; range, 0.25 to 13 years) and 5 dogs of group 2 (median, 7.0 years; range, 0.7 to 13 years). Body weight and age did not differ significantly between the 2 groups.
Figure 4—Bland-Altman plots for analysis of agreement between directly measured SAP (A), DAP (B), and MAP (C) obtained at the superficial palmar arch (carpus) and median sacral artery (tail) of 13 dogs (group 2) positioned in lateral recumbency. See Figure 1 for key.
GROUP 1 For dogs of group 1 in dorsal recumbency, SAP agreement analysis revealed bias of –16.12 mm Hg (LOA, –43.17 to 11.16 mm Hg; Figure 1), DAP agreement analysis revealed bias of 1.71 mm Hg (LOA, –5.93 to 9.35 mm Hg), and MAP agreement analysis revealed bias of –0.40 mm Hg (LOA, –7.49 to 6.69 mm Hg). For dogs of group 1 in lateral recumbency, SAP agreement analysis revealed bias of –14.70 mm Hg (LOA, –43.28 to 13.88 mm Hg; Figure 2), DAP agreement analysis re-
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vealed bias of 2.41 mm Hg (LOA, –4.91 to 9.73 mm Hg), and MAP agreement analysis revealed bias of –0.31 mm Hg (LOA, –7.48 to 7.82 mm Hg). GROUP 2 For dogs of group 2 in dorsal recumbency, SAP agreement analysis revealed bias of –3.08 mm Hg (LOA, –23.68 to 17.48 mm Hg; Figure 3), DAP agreement analysis revealed bias of 0.42 mm Hg (LOA, –7.66 to 8.50 mm Hg), and MAP agreement analysis revealed bias of –2.02 mm Hg (LOA, –9.96 to 5.92 mm Hg). For dogs of group 2 in lateral recumbency, SAP agreement analysis revealed bias of –4.67 mm Hg (LOA, –24.32 to 15.00 mm Hg; Figure 4), DAP agreement analysis revealed bias of –0.73 mm Hg (LOA, –7.79 to 6.33 mm Hg), and MAP agreement analysis revealed bias of –2.11 mm Hg (LOA, –9.07 to 4.85 mm Hg).
Discussion In the present study, direct blood pressure measurements obtained at 2 anatomic locations in 2 groups of dogs were compared. For group 1, arterial blood pressure was measured at the superficial palmar arch (carpus) and dorsal pedal artery (hind limb); arterial blood pressure in group 2 was measured at the superficial palmar arch (carpus) and median sacral artery (tail). In all cases, there were pressure differences that could affect comparison studies. This was especially true for all SAP measurements but was most pronounced for SAP measurements of group 1 in which hind limb measurements were a mean of 16.12 mm Hg higher than carpus measurements when dogs were in dorsal recumbency and 14.70 mm Hg higher than carpus measurements when dogs were in lateral recumbency. In addition, there was significant dispersion about the mean insofar as hind limb SAP measurements could be as much as 43.17 mm Hg higher or 11.16 mm Hg lower than carpus SAP measurements when dogs were in dorsal recumbency and as much as 43.28 mm Hg higher or 13.88 mm Hg lower than carpus SAP measurements when dogs were in lateral recumbency. The SAP measurements for group 2 had substantially better agreement: mean SAP measurements for the tail were only 3.08 mm Hg higher than mean SAP measurements for the carpus when dogs were in dorsal recumbency and 4.67 mm Hg higher than mean SAP measurements for the carpus when dogs were in lateral recumbency. Nevertheless, measurements at the median sacral artery could still be expected to be as much as 23.68 mm Hg higher or 17.47 mm Hg lower than measurements at the carpus when dogs were in dorsal recumbency and as much as 24.32 mm Hg higher or 15.00 mm Hg lower than measurements at the carpus when dogs were in lateral recumbency. In both group 1 and 2, there were differences in DAP and MAP measurements; however, in all cases, these were < 10 mm Hg. A few small differences were detected between dorsal and lateral recumbency; however, they appeared to be relatively minor. 270
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It has long been known that arterial waveforms change as a pulse travels from the aorta to the peripheral vasculature. Specifically, the anacrotic limb becomes steeper, the peak becomes higher, the dicrotic notch becomes delayed, and the dicrotic limb dips lower.27 This phenomenon, known as distal pulse wave amplification, results in peripheral arterial waveforms with higher SAP and lower DAP than those for centrally measured arterial waveforms.28 Interestingly, MAP remains relatively unaffected.28 Distal pulse wave amplification is the result of physical characteristics (eg, impedance and resistance) of the vascular tree.27 Distal pulse wave amplification would predict that if an artery were more central than another artery, Bland-Altman analysis would yield SAP and DAP with biases on opposite sides of zero. Thus, one bias would be positive and the other negative, depending on the reference artery. In the present study, SAP and DAP biases were on opposite sides of zero; MAP had relatively little bias. However, the large negative biases for SAP measurements in dogs of group 1 suggested that the dorsal pedal artery was much more centrally located than the superficial palmar arch. This is difficult to justify anatomically. More problematic was the relatively wide LOA resulting from the extensive dispersion of pressure measurements around the bias.This implied that much of the variation was nonsystematic in nature. Therefore, results of the present study cannot be completely explained on the basis of distal pulse wave amplification. Another possible explanation for results of the study reported here was the manner in which direct blood pressure was measured. In this study, each artery was catheterized with a 20-gauge arterial catheter connected to a transducer via noncompliant saline solution–filled tubing.This generally is considered the criterion-referenced standard27 for blood pressure measurement and has been validated.29 Although electrical and fiber-optic transducers that can be inserted directly into an artery are available, they are less commonly used.30 Most importantly, to explain the lack of agreement in previous studies, we used the same techniques for direct blood pressure measurement that had been used in those studies.3,14–22 It seems plausible that relying on arterial blood pressure to move a column of water to affect a transducer may not be accurate for use in dogs; however, the present study was not designed to evaluate this. One shortcoming of the present study was the relatively few blood pressure measurements in the hypertensive range. Nevertheless, a wide range of blood pressures was examined. Because these were client-owned animals, no attempt was made to pharmacologically manipulate blood pressure. Considering that we were unable to obtain good SAP agreement for measurements in the normotensive and hypotensive range, there seemed to be little benefit to further explore agreement for measurements in the hypertensive range; nevertheless, it is possible that agreement for DAP and MAP could be worse for dogs with hypertensive conditions.
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In the present study, we attempted to investigate whether the failure to validate indirect blood pressure measurement devices could have been attributable to variations in arterial blood pressure at different anatomic locations.We found small differences in DAP and MAP, but there were significant differences in SAP. This was especially pronounced when comparing SAP obtained at the carpus and hind limb. Although some of the observed differences may have been caused by distal pulse wave amplification, that phenomenon cannot explain the wide dispersion of observations around the bias. Some of the observed scattering may have been attributable to the commonly used method of catheterizing an artery and connecting it to a transducer via noncompliant saline solution–filled tubing. This study was not designed to evaluate that concern; however, results of previous studies have suggested that this is a valid method for measurement of blood pressure. Until it can be determined whether the differences detected in this study were attributable to the methods used or to actual anatomically related differences in arterial blood pressure, care must be taken when designing or interpreting studies that compare directly and indirectly measured blood pressure.
9. 10.
11. 12. 13. 14. 15. 16.
17.
18.
Acknowledgments Supported in part by a Morris Animal Foundation summer research scholar stipend awarded to Ms. Domingues. Presented in abstract form at the American College of Veterinary Internal Medicine Forum, Nashville, Tenn, June 2014.
Footnotes a. b. c. d.
Prism, version 6.0 for Macintosh, GraphPad Software Inc, San Diego, Calf. Abbocath, Hospira, Lake Forest, Ill. SP 844 physiological sensor, MEMSCAP Sensor Solutions, Skoppum, Norway. VetTrends V Vital Signs monitor, System VET,Tampa, Fla.
2.
3.
4. 5. 6. 7. 8.
20. 21.
22.
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