Arterial blood pressure (ABP) is routinely monitored in dogs during general anesthesia, where accurate measurement is required for appropriate management of ABP alterations.1,2 Invasive blood pressure (IBP) measurement via arterial cannulation is considered the most accurate method of ABP monitoring.3 However, insertion of an intra-arterial catheter may be technically challenging and impractical for patients in which circulatory instability is not anticipated.4,5 Noninvasive blood pressure (NIBP) measurement methods, such as doppler ultrasound (DOP) and pulse oximeter plethysmography (POP), may be used as alternatives to IBP measurement.
Systolic arterial blood pressure (SAP) measurements from the forelimb and hindlimb using DOP in anesthetized dogs have been compared to IBP in several studies.6–12 DOP measured from the median caudal artery has not previously been compared to IBP in anesthetized dogs. This method of NIBP monitoring is routinely implemented for anesthetized dogs at the authors’ institution.
Pulse oximetry is a noninvasive method of measuring the percent of arterial hemoglobin saturated with oxygen (SpO2) and is routinely used to detect hypoxemia (SpO2 ≤ 90%) in anesthetized dogs.1,13 Many modern pulse oximeters display a plethysmographic waveform (POP waveform), representing cyclical changes in arterial blood volume.14,15 The POP waveform may be used to measure SAP by placement of a transmittance or reflectance probe distal to a flow-occluding cuff.6,16,17 The accuracy of POP measured from the tongue has previously been compared to IBP in anesthetized dogs and determined to be acceptably accurate.6 The Masimo Radical-7 CO-Oximeter is a portable device capable of continuously monitoring SpO2 while displaying a POP waveform. No previous studies have compared POP measured from the median caudal artery to IBP in anesthetized dogs.
The primary objective of this study was to compare the accuracy of 2 NIBP methods (DOP and POP) in the measurement of SAP from the median caudal artery to IBP measured from the dorsal pedal artery in healthy anesthetized dogs. A secondary objective was to investigate the clinical utility of the Masimo Radical-7 Pulse CO-Oximeter as a novel combined continuous SpO2 monitor and NIBP measurement device. We hypothesized DOP and POP would be clinically acceptable methods for SAP measurement in this population of anesthetized dogs.
Methods
Inclusion criteria
This prospective, randomized, blinded clinical study was conducted at the Kansas State University Veterinary Health Center from May 2022 to January 2023. Inclusion criteria were client-owned dogs > 12 months of age, > 10 kg body weight, undergoing an elective surgical procedure in dorsal recumbency, classified as American Society of Anesthesiologists Physical Status I or II, and presence of a complete tail. Chondrodystrophic and brachycephalic breeds were excluded. Informed, written owner consent was obtained before enrollment of each dog to the study. The study was approved by the IACUC at Kansas State University (protocol No. IACUC-4706). The Consolidated Standards of Reporting Trials guidelines were consulted throughout the study protocol.18
Anesthesia and monitoring
Food, but not water, was withheld for at least 8 hours before general anesthesia. Premedication, induction, maintenance of anesthesia, and postoperative management were at the discretion of the attending anesthesiologist. A 20- or 22-gauge catheter was placed in a cephalic vein. Following induction of anesthesia, patients were orotracheally intubated, connected to a circle breathing system, and administered isoflurane in 100% oxygen to effect. ABP, SpO2, end-tidal partial pressure of carbon dioxide (Petco2), electrocardiography, and esophageal temperature were monitored (Datex-Ohmeda). Mechanical ventilation was initiated if Petco2 was > 55 mm Hg and was not discontinued until recovery from anesthesia. Hypotension was defined as a mean arterial pressure < 60 mm Hg and was treated at the discretion of the attending anesthesiologist. Lactated Ringer solution was administered at 5 mL·kg−1·hour−1, IV, during the surgical procedure.
Blood pressure monitoring instrumentation
Following induction of anesthesia and complete instrumentation, patients were placed in lateral recumbency. A 22-gauge 1-inch catheter (Safelet) was aseptically placed in the right or left dorsal pedal artery and connected to an electronic pressure transducer (Merit Medical) via noncompliant tubing (Merit Medical) filled with heparinized (3 U·mL−1) 0.9% saline free of air bubbles. The transducer was connected to a multiparameter monitor (Datex-Ohmeda), placed at the level of the right atrium, and zeroed to atmospheric pressure for continuous IBP measurement. The proximal one-third of the ventral tail was clipped and cleaned with chlorhexidine and alcohol. An appropriately sized flow-occluding cuff (width measuring approximately 40% of tail circumference [Mabis]) was placed at the tail base (Figure 1) and connected to a hand-held sphygmomanometer (Welch Allyn). The measured cuff width-to-tail circumference ratio (cw:tc) was calculated and recorded for each dog. The same sphygmomanometer and flow-occluding cuffs were used throughout data collection, and calibration was checked once weekly against a mercury manometer. Patients were randomly assigned to group A or B (List Randomizer, www.random.org). Two NIBP (DOP and POP) probes were placed distal to the cuff over the median caudal artery according to group assignment (A = DOP probe proximal, POP probe distal; B = POP probe proximal, DOP probe distal). The DOP probe (Parks Medical) was secured with tape and conductive gel used to enhance signal transmission. The POP probe (Masimo) was secured with tape. The CO-Oximeter was adjusted to the following settings: patient type: Adult; sensitivity mode: Adaptive Probe Off Detection; and waveform view: Plethysmograph + Signal Quality Indicator. The same DOP and POP probes and devices were used throughout data collection. The POP probe was left in place, the device was turned off, and POP data were not collected for a given dog if the CO-Oximeter was unable to generate a POP waveform due to patient-dependent factors (eg, darkly pigmented skin).
Placement of noninvasive blood pressure measurement devices on the tail of an anesthetized dog in dorsal recumbency. A—A flow-occluding cuff (cuff width approximately 40% of tail circumference) is placed on the tail base. An ultrasonic doppler probe is placed distal to the cuff over the median caudal artery. A pulse oximeter plethysmograph reflectance probe is placed distal to the ultrasonic doppler probe over the median caudal artery. B—The plethysmographic waveform (bottom of screen) and percent of arterial hemoglobin saturated with oxygen (100%) are visible on the Masimo Radical-7 Pulse CO-Oximeter touchscreen display.
Citation: American Journal of Veterinary Research 85, 6; 10.2460/ajvr.23.11.0263
IBP and NIBP measurements
Patients were moved to the operating room and positioned in dorsal recumbency. Paired IBP and NIBP measurements were recorded at 10-minute intervals for 60 minutes or until the end of the procedure, whichever occurred first. DOP and POP measurements were recorded separately by different investigators. The order in which measurements were taken at each interval was randomized based on the original group assignment (A = DOP first, POP second; B = POP first, DOP second). IBP was obtained from the multiparameter monitor and recorded by a separate investigator. Investigators recording IBP or NIBP measurements were blinded from respective measurement values to mitigate assessment bias.
In patients assigned to group A, SAP measured by IBP (IBPSAP) and DOP (DOPSAP) were simultaneously measured and recorded. The arterial catheter was flushed with 1 mL of heparinized saline and waveform assessed for over- or under-dampening using a square wave test. IBPSAP was recorded once. The DOP device was turned on with the volume set to maximum, and audible pulsatile blood flow was confirmed. The DOP probe was adjusted if audible pulsatile blood flow was not heard. DOPSAP was measured by inflation of the flow-occluding cuff to 200 mm Hg using the hand-held sphygmomanometer. The cuff was gradually deflated (3 to 5 mm Hg/s), and the pressure at which an audible pulsatile flow signal returned was recorded as DOPSAP. Three replicate DOPSAP measurements were taken at each 10-minute interval. IBPSAP and SAP measured by POP were then simultaneously recorded. The arterial catheter was flushed in the same fashion, and IBPSAP was recorded once. A POP waveform and SpO2 value of ≥ 95% were confirmed on the touchscreen display (Figure 1). The POP probe was adjusted if a POP waveform or SpO2 ≥ 95% was absent. The hand-held sphygmomanometer was used to gradually inflate (3 to 5 mm Hg/s) the flow-occluding cuff to a pressure sufficient to occlude arterial blood flow, visualized as loss of the POP waveform (POPL-SAP). Additional pressure (50 mm Hg) was added to the cuff and then gradually deflated until a visible POP waveform returned (POPR-SAP). Three replicate POPL-SAP and POPR-SAP measurements were recorded at each 10-minute interval. The order of NIBP measurement was reversed for patients in group B (ie, POP first, DOP second).
Recovery from anesthesia
The arterial catheter and NIBP probes were removed at the end of the surgical procedure. Dogs were recovered from anesthesia, monitored postoperatively, and discharged from the hospital per routine protocols.
Statistical analysis
Data normality was tested using the ‘shapiro’ and ‘skewness’ functions of RStudio (RStudio Team, 2020). After normality had been established, a linear mixed model was used to analyze the data using the ‘lmer’ function of the ‘lm4’ package of RStudio. A step-down regression was used to develop the final model that best explains the data. Initial models included the fixed effects of tensive state (hypotensive [SAP < 90 mm Hg], normotensive [SAP 90 to 130 mm Hg], and hypertensive [SAP > 130 mm Hg]), cw:tc, and NIBP probe order. The final model tested the fixed effect of DOPSAP, POPL-SAP, or POPR-SAP measurements over IBPSAP collected during the same time points. Random effects included dog, time point, and weight categories (12 to 22, 22 to 32, 32 to 42, 42 to 52, and > 52 kg) to account for the lack of independence among cases. Correlation coefficients were used to find collinearity among variables. Bland-Altman analyses were performed using the ‘blandr.statistics’ function of the ‘blandr’ package of RStudio to evaluate the NIBP measurement methods against American College of Veterinary Internal Medicine (ACVIM) evaluation criteria (bias [mean difference] ≤ ± 10 mm Hg, SD ≤ 15 mm Hg, 50% of all NIBP measurements within 10 mm Hg of the reference method, 80% of all NIBP measurements within 20 mm Hg of the reference method, and correlation ≥ 0.9).19 IBPSAP was considered the reference method. Replicate measurements at each 10-minute interval were not averaged but treated as single measurements to avoid underestimating the SD of the bias. Results were expressed as bias ± SD of IBPSAP minus DOPSAP, POPL-SAP, or POPR-SAP. Lower and upper limits of agreement and percentage error were calculated for DOPSAP, POPL-SAP, and POPR-SAP. Bias ± SD was determined for each tensive state. The significance level was set at P < .05. The true positive rate (TPR), false negative rate (FNR), and false positive rate (FPR) in the identification of hypotension (IBPSAP < 90 mm Hg) were calculated for DOPSAP, POPL-SAP, and POPR-SAP.
Results
Forty-four dogs were enrolled in the study. Two dogs were excluded due to the inability to place an arterial catheter. Two dogs were excluded due to the inability to place NIBP probes before the surgical procedure. Forty dogs, 21 females and 19 males weighing 12.2 to 54.1 kg (31.6 ± 10.1 [mean ± SD]), were included in the study. The average measured cw:tc was 0.40 ± 0.02 (mean ± SD). Twenty dogs were assigned to each group (A and B). A total of 816 DOPSAP, 678 POPL-SAP, and 678 POPR-SAP measurements were recorded. DOP measurements were recorded in all 40 dogs. POP measurements were not recorded in 7 dogs due to the inability to obtain a POP waveform.
The linear mixed model comparing DOPSAP, POPL-SAP, or POPR-SAP to IBPSAP revealed no effect from individual dog, time of measurement, or body weight. Comparison of DOP, loss of pulse oximeter plethysmograph (POPL), and return of pulse oximeter plethysmograph (POPR) to IBP resulted in a marginal R2 value of 0.59, 0.43, and 0.43, respectively. When both fixed and random factors were considered together, the conditional R2 value increased to 0.92, 0.85, and 0.87 (P < .001 for all) for DOP, POPL, and POPR versus IBP, respectively (Figure 2).
Linear correlations considering fixed and random factors of noninvasive blood pressure measured from the median caudal artery and invasive blood pressure (IBP) measured from the dorsal pedal artery in the measurement of systolic arterial pressure (SAP) in dorsally recumbent anesthetized dogs. Conditional R2 and P values are reported for doppler ultrasound (DOP), loss of pulse oximeter plethysmograph (POPL), and return of pulse oximeter plethysmograph (POPR) versus IBP (A, B, and C, respectively). Red lines represent a hypothetical correlation value of R2 = 1. Blue lines represent the conditional R2 value. DOPSAP = SAP measured by DOP. IBPSAP = SAP measured by IBP. POPL-SAP = SAP measured by POPL. POPR-SAP = SAP measured by POPR.
Citation: American Journal of Veterinary Research 85, 6; 10.2460/ajvr.23.11.0263
The overall bias ± SD for DOP, POPL, and POPR when compared to IBP was +7.6 ± 13.1, +3.9 ± 14.4, and +8.6 ± 15.2 mm Hg, respectively, indicating all NIBP measurement methods underestimated SAP when considering measurements together across all tensive states (Figure 3). DOP fulfilled all ACVIM evaluation criteria: number of animals ≥ 8, bias ≤ ± 10 mm Hg, SD ≤ 15 mm Hg, 50% of all NIBP measurements within 10 mm Hg of the reference method, 80% of all NIBP measurements within 20 mm Hg of the reference method, and correlation ≥ 0.9. POPL fulfilled 5 out of 6, not fulfilling a correlation ≥ 0.9. POPR fulfilled 4 out of 6, not fulfilling an SD ≤ 15 mm Hg and a correlation ≥ 0.9 (Table 1). The limits of agreement (lower, upper) for DOP, POPL, and POPR were (−18.1, +33.3), (−24.3, +32.1), and (−21.2, +38.4) mm Hg, respectively. The percentage error was 21.6%, 23.7%, and 25.0% for DOP, POPL, and POPR, respectively.
Bland-Altman analysis plots of paired systolic arterial pressure (SAP) measurements between invasive blood pressure (IBP) measured from the dorsal pedal artery and doppler ultrasound (DOP), loss of pulse oximeter plethysmograph (POPL) and return of pulse oximeter plethysmograph (POPR [A, B and C, respectively]) measured from the median caudal artery in dorsally recumbent anesthetized dogs. Black dots represent the individual bias (difference [mm Hg]) between paired IBP and noninvasive blood pressure (NIBP) measurements, calculated as IBP minus NIBP. Blue lines represent the bias (mean difference [mm Hg]) between the IBP and NIBP measurement methods. Lower and upper limits of agreement (bias ± 1.96 SD [mm Hg]) are represented by red and green lines, respectively. Average = Mean of individual paired IBP and NIBP measurements. DOPSAP = SAP measured by DOP. IBPSAP = SAP measured by IBP. PE = Percentage error. POPL-SAP = SAP measured by POPL. POPR-SAP = SAP measured by POPR.
Citation: American Journal of Veterinary Research 85, 6; 10.2460/ajvr.23.11.0263
Doppler ultrasound (DOP), loss of pulse oximeter plethysmograph (POPL), and return of pulse oximeter plethysmograph (POPR) noninvasive blood pressure (NIBP) device performance by variable in the measurement of systolic arterial pressure from the median caudal artery compared to invasive blood pressure (IBP) measured from the dorsal pedal artery in dorsally recumbent anesthetized dogs when evaluated against American College of Veterinary Internal Medicine (ACVIM) criteria.
| Variable | ACVIM criteria | DOP | POPL | POPR |
|---|---|---|---|---|
| Animals (n) | ≥ 8 | 40a | 33a | 33a |
| Bias (mm Hg) | ≤ ±10 | +7.6a | +3.9a | +8.6a |
| SD (± mm Hg) | ≤ 15 | 13.1a | 14.4a | 15.2 |
| ≤ ±10 mm Hg (%) | 50 | 65a | 70a | 66a |
| ≤ ±20 mm Hg (%) | 80 | 86a | 86a | 86a |
| Correlation (R2) | ≥ 0.9 | 0.92a | 0.85 | 0.87 |
Bias = Mean difference calculated as IBP minus NIBP. Correlation = Correlation of NIBP and IBP measurements. ≤ ±10 mm Hg = NIBP measurements within 10 mm Hg of IBP measurements. ≤ ±20 mm Hg = NIBP measurements within 20 mm Hg of IBP measurements.
Fulfills ACVIM criteria.
The bias ± SD during normotension for DOP, POPL, and POPR when compared to IBP was +4.7 ± 8.9, –0.82 ± 7.4, and +3.5 ± 7.2 mm Hg, respectively. All NIBP methods overestimated SAP during hypotension (–5.2 ± 8.9, –18.3 ± 5.2, and –13.8 ± 5.3 mm Hg [bias ± SD]) and underestimated SAP during hypertension (+19.8 ± 16.9, +21.8 ± 16.6, and +27.6 ± 18.0 mm Hg [bias ± SD]) when compared to IBP for DOP, POPL, and POPR, respectively (Table 2). The TPR in the identification of hypotension for DOP, POPL, and POPR was 52%, 17%, and 17%, while the FNR was 48%, 83%, and 83%, respectively. The FPR was 3.2%, 0.0%, and 1.3% for DOP, POPL, and POPR, respectively (Table 3).
Bias (mean difference) and SD of doppler ultrasound (DOP), loss of pulse oximeter plethysmograph (POPL), and return of pulse oximeter plethysmograph (POPR) across different tensive states in the measurement of systolic arterial pressure (SAP) from the median caudal artery compared to invasive blood pressure (IBP) measured from the dorsal pedal artery in dorsally recumbent anesthetized dogs.
| NIBP method | Tensive state | N | Bias (mm Hg) | SD (± mm Hg) |
|---|---|---|---|---|
| DOP | Hypo | 33 | −5.2a | 8.9 |
| DOP | Normo | 603 | +4.7b | 8.9 |
| DOP | Hyper | 180 | +19.8c | 16.9 |
| POPL | Hypo | 18 | −18.3a | 5.2 |
| POPL | Normo | 504 | -0.82b | 7.4 |
| POPL | Hyper | 156 | +21.8c | 16.6 |
| POPR | Hypo | 18 | −13.8a | 5.3 |
| POPR | Normo | 504 | +3.5b | 7.2 |
| POPR | Hyper | 156 | +27.6c | 18.0 |
Hyper = SAP > 130 mm Hg. Hypo = SAP < 90 mm Hg. n = Number of measurements. Normo = SAP 90 to 130 mm Hg.
Bias differs significantly (P < .05) across tensive state when superscripts differ within a noninvasive blood pressure (NIBP) method.
True positive rate (TPR), false negative rate (FNR), and false positive rate (FPR) for doppler ultrasound (DOP), loss of pulse oximeter plethysmograph (POPL), and return of pulse oximeter plethysmograph (POPR) measured from the median caudal artery in the identification of hypotension (systolic arterial pressure < 90 mm Hg) determined by invasive blood pressure measured from the dorsal pedal artery in dorsally recumbent anesthetized dogs.
| NIBP method | TPR (%) | FNR (%) | FPR (%) |
|---|---|---|---|
| DOP | 52 | 48 | 3.2 |
| POPL | 17 | 83 | 0.0 |
| POPR | 17 | 83 | 1.3 |
NIBP = Noninvasive blood pressure.
Discussion
DOP measurement of SAP from the median caudal artery has not previously been compared to IBP in anesthetized dogs. In this study, DOP overall underestimated SAP when compared to IBP. Similar studies evaluating DOP at different anatomic locations have reported varying results with DOP either underestimating7,9 or overestimating8,10,11 SAP compared to IBP. Similarly, POP overall underestimated SAP when compared to IBP with POPL underestimating to a lesser magnitude than POPR. A previous study applying POP to the tongue in anesthetized dogs found POP to underestimate SAP when compared to IBP.6 In the present study, DOP and POP performance was compared to ACVIM criteria for NIBP device evaluation.19 DOP fulfilled all criteria and exceeded performance in previous studies7,9,10,12 comparing DOP at different anatomic sites with IBP. POP partially fulfilled ACVIM criteria. No previous studies have evaluated POP against ACVIM criteria in anesthetized dogs.
When evaluating a NIBP measurement method, it is important to determine how the device, method or both may perform in different clinical contexts (eg, different tensive states). In this study, DOP and POP overestimated SAP during hypotension when compared to IBP, with DOP overestimating to a lesser magnitude than POP. The performance of DOP in comparison to ACVIM evaluation criteria and absolute bias across tensive states in this study may support its use for the measurement of SAP from the median caudal artery in a similar population of anesthetized dogs. DOP had an absolute bias of ≤ 10 mm Hg during normo- and hypotension. However, considering a sensitivity to detect hypotension of 52% and wide limits of agreement, DOP may fail to identify hypotension in a portion of SAP measurements. POP partially fulfilled ACVIM evaluation criteria and had an absolute bias of ≤ 10 mm Hg during normotension but a greater magnitude of bias during hypo- and hypertension. The sensitivity to detect hypotension for both POPL and POPR was 17%, indicating POP may fail to identify the majority of hypotensive SAP measurements. POP similarly had wide limits of agreement and of a greater magnitude than DOP. The findings of this study do not support the use of POP for the measurement of SAP from the median caudal artery in a similar population where identifying hypotension is of clinical interest. These results partially support our hypothesis finding DOP to be clinically acceptable, while POP was an unacceptable method for SAP measurement in this study population.
DOP measurements were recorded in all dogs included in this study, indicating reliability in obtaining a DOP flow signal in this patient population. The Masimo Radical-7 CO-Oximeter has not previously been evaluated for use as a NIBP device in anesthetized dogs. POP measurements were unable to be recorded in some dogs due to darkly pigmented skin preventing SpO2 detection and generation of a POP waveform. This highlights a potential limitation of this CO-Oximeter for use as a combined continuous SpO2 monitor and NIBP measurement device and warrants further investigation.
There were limitations to this study. First, individual IBP transducers were not calibrated prior to use in each dog. This was based on the reported limits of accuracy for the transducer used and the need to implement additional steps in the daily clinical anesthesia schedule. The transducers are reported to perform within the Association for the Advancement of Medical Instrumentation accuracy limits (≤ 2% of the true pressure or ± 1 mm Hg, whichever is greater) over the range of pressures –30 to 300 mm Hg.20 Despite this information, the lack of calibration of each transducer may introduce variability and limit the interpretation of the data. Second, SAP measurement comparisons were made between 2 differing anatomic sites (dorsal pedal artery for IBP and median caudal artery for DOP and POP). Measured SAP has been shown to vary based on anatomic location in anesthetized dogs and may be a source of variability and lack of agreement between IBP and NIBP measurements observed in this study.21 However, the dorsal pedal artery is a commonly used anatomic site to measure IBP and comparison of SAP in relation to other anatomic sites may be of clinical relevance. Additionally, cannulation of the same anatomic site (median caudal artery) as placement of the NIBP probes was not possible in this study. Finally, this study utilized clinical patients where anesthetic complications were promptly treated, limiting the number of NIBP measurements recorded during hypotension. A comparatively fewer number of SAP measurements occurred during hypotension versus normo- and hypertension. Bias, SD, TPR, and FNR values for DOP and POP reported in this population may not reflect true performance during hypotension. Future studies evaluating DOP and POP measured from the median caudal artery in a hypotensive population are warranted to further determine accuracy in this SAP range.
In conclusion, DOP and POP measured from the median caudal artery overall underestimated SAP when compared to IBP measured from the dorsal pedal artery in the dorsally recumbent, apparently healthy anesthetized dogs weighing > 10 kg included in this study. DOP performance was determined to be acceptable for clinical use in a similar population but may fail to identify some hypotensive dogs. POP performance was found to be unacceptable for clinical use in a similar population due to greater bias and FNR during hypotension. The Masimo Radical-7 CO-Oximeter placed over the median caudal artery may have clinical utility as a combined SpO2 and NIBP measurement device, although further studies are needed to evaluate its performance.
Acknowledgments
None reported.
Disclosures
Dr. Kapaldo is a member of the AJVR Scientific Review Board, but was not involved in the editorial evaluation of or decision to accept this article for publication.
No AI-assisted technologies were used in the generation of this manuscript.
Funding
The research was funded by the authors’ departments.
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