Materials and Methods

Birds—Sixteen healthy adult Hispaniolan Amazon parrots (A ventralis) obtained from the Louisiana State University School of Veterinary Medicine research flock were used in the study. Hispaniolan Amazon parrots were used in the study because they were of sufficient size to allow for arterial catheterization and were comparable in size to many other psittacines commonly kept as pets. The study was performed in accordance with regulations established by the Institutional Animal Care and Use Committee at Louisiana State University.

Experimental procedure—Birds were anesthetized with 5% isoflurane in oxygen (flow rate, 1 L/min) delivered via a face mask. Once birds were anesthetized, an uncuffed endotracheal tube (2 mm outer diameter) was placed and connected to a Bain system, and isoflurane concentration was decreased to 2%. End-tidal partial pressure of carbon dioxide was monitored, and birds were manually ventilated as needed to maintain an end-tidal partial pressure of carbon dioxide between 35 and 45 mm Hg.^a A hot-water blanket and convective air warmer^b were used to maintain body temperature while birds were anesthetized.

After sterile preparation, a 26-gauge, 19-mm catheter^c was placed percutaneously in the superficial ulnar artery. A suture was used to secure the catheter, and the wing was manually stabilized. The catheter was connected to a continuous multifunction monitor^a via a disposable pressure transducer system.^d The system was calibrated in accordance with the manufacturer's recommendations. The system was flushed with saline (0.9% NaCl) solution, and the pressure transducer was zeroed at the level of the right atrium. Direct blood pressure measurements were checked for stability and consistent waveform before proceeding.

Indirect blood pressure measurements were obtained with a Doppler ultrasonic flow detector.^e All measurements were obtained by a single individual (DG) who was blinded to direct blood pressure measurements. Indirect blood pressure measurements were obtained from the wing and the leg contralateral to the arterial catheter. For obtaining measurements from the wing, a size 1 neonatal cuff^f was placed around the humerus, and the Doppler ultrasonic flow detector was placed over the distal aspect of the superficial ulnar artery. Feathers in the area underlying the cuff were groomed so that they were not between the cuff and the humerus. In a few instances, feathers that could not be satisfactorily moved were plucked. In all instances, width of the cuff was 30% to 40% of the circumference of the wing, as recommended.⁵ The cuff was inflated until sounds of pulsating blood generated by the Doppler unit were no longer heard. The cuff was then gradually deflated until the first Doppler-generated pulsatile sounds were audible, and pressure in the cuff was recorded as systolic pressure. At the same instant, a second individual (MA) recorded systolic, mean, and diastolic arterial blood pressures. The process was repeated 3 times at 1-minute intervals and then again 3 more times after a 5-minute pause.

Attempts were also made to indirectly measure blood pressure with an oscillometric unit^g designed for animal use and a cuff that was 30% to 40% of the circumference of the proximal portion of the wing. However, all such attempts were unsuccessful.

For obtaining indirect blood pressure measurements from the leg, a size 1 neonatal cuff^f was placed around the proximal portion of the tibiotarsus, and the Doppler ultrasonic flow detector was placed over the distal aspect of the ischiatic artery. In all cases, width of the cuff was 30% to 40% of the circumference of the leg where the cuff was placed. Blood pressure measurements were obtained as described for the wing, except that only 3 measurements were obtained at 1-minute intervals.

Attempts were also made to indirectly measure blood pressure with an oscillometric unit^g and a cuff that was 30% to 40% of the circumference of the proximal portion of the leg. However, all such attempts were unsuccessful.

After all blood pressure measurements were obtained, arterial catheters were removed and the birds were allowed to recover from anesthesia. All birds were closely monitored until they were able to perch without assistance.

Statistical analysis—The Kolmogorov-Smirnoff test was used to determine whether blood pressure data were normally distributed. Because data were found to be normally distributed, values were summarized as mean, SD, and range. Repeated-measures ANOVA was used to determine whether initial blood pressure measurements (systolic, mean, and diastolic arterial pressures and indirect blood pressure) and measurements obtained 5 minutes later from the wing and initial blood pressure measurements obtained from the leg varied significantly within bird. A paired t test was then performed to determine whether initial and subsequent blood pressure measurements obtained from the wing were significantly different.

The Pearson correlation method was used to determine whether body weight was significantly correlated with blood pressure measurements. Agreement between the direct and indirect blood pressure measurements was determined by use of the Bland-Altman method. Because the sampling strategy used a repeatedmeasures approach, the mean for each of the pressure measurements was calculated and used for comparison.⁷ Bias was defined as the mean difference between the 2 methods, and limits of agreement were calculated as the bias ± 1.96 SD. Because of the potential for underestimating the SD of the differences with a repeated-measures approach, SDs were corrected by use of a previously described technique.⁷ The α was set at P ≤ 0.05. All statistical analyses were performed with commercially available statistical programs.^h,i

Results

Arterial catheters were successfully placed in all 16 birds included in the study. An additional 5 birds were eliminated from the study because an arterial catheter could not be placed. Mean ± SD body weight of the 16 birds included in the study was 294.3 ± 22.3 g (0.65 ± 0.05 lb; range, 254 to 333 g [0.56 to 0.73 lb]). Indirect blood pressure measurements were obtained from the wing in all 16 birds but were obtained from the leg in only 12 birds.

Initial blood pressure measurements and blood pressure measurements obtained 5 minutes later from the wing did not vary significantly within bird (for the initial blood pressure measurements, P = 0.2, 0.6, 0.4, and 0.4, respectively, for systolic, mean, and diastolic arterial blood pressures and indirect blood pressure; for the subsequent blood pressure measurements, P = 0.1, 0.1, 0.3, and 0.3, respectively, for systolic, mean, and diastolic arterial blood pressures and indirect blood pressure). Similarly, blood pressure measurements obtained from the leg did not vary significantly within bird (P = 0.2, 0.4, 0.3, and 0.3, respectively, for systolic, mean, and diastolic arterial blood pressures and indirect blood pressure). Initial blood pressure measurements obtained from the wing were not significantly different from subsequent measurements (P = 0.1, 0.2, 0.2, and 0.4, respectively, for systolic, mean, and diastolic arterial blood pressures and indirect blood pressure). Because significant differences were not identified within birds, mean values were calculated for the blood pressure measurements (Table 1). Body weight was not significantly (P > 0.1) correlated with any of the blood pressure measurements.

Derived and corrected SDs for comparisons between indirect blood pressure measurements obtained with the Doppler detector and direct systolic pressures were 27 and 31 mm Hg, respectively. The SD for indirect blood pressure measurements obtained from the leg was 10 mm Hg; the corrected value was 14 mm Hg.

Analysis of agreement between systolic arterial blood pressure and indirect blood pressure measurements obtained from the wing revealed substantial disagreement (bias, 24 mm Hg; limits of agreement, −37 to 85 mm Hg; Figure 1). Similarly, analysis of agreement between systolic arterial blood pressure and indirect blood pressure measurements obtained from the leg revealed substantial disagreement (bias, 14 mm Hg; limits of agreement, −14 to 42 mm Hg; Figure 2).

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Bland-Altman plot of agreement between direct systolic arterial blood pressure measurements and indirect blood pressure measurements obtained from the wing in 16 Hispaniolan Amazon parrots anesthetized with isoflurane.

Citation: Journal of the American Veterinary Medical Association 233, 10; 10.2460/javma.233.10.1587

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Bland-Altman plot of agreement between direct systolic arterial blood pressure measurements and indirect blood pressure measurements obtained from the leg in 12 Hispaniolan Amazon parrots anesthetized with isoflurane.

Citation: Journal of the American Veterinary Medical Association 233, 10; 10.2460/javma.233.10.1587

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Discussion

Results of the present study suggested that there was substantial disagreement between indirect blood pressure measurements obtained with a Doppler ultrasonic flow detector in anesthetized Hispaniolan Amazon parrots and systolic arterial blood pressures measured directly. Attempts to obtain indirect blood pressure measurements from these birds with an oscillometric unit were consistently unsuccessful.

One possible explanation for the lack of agreement in blood pressure measurements in the present study was that the cuff that was used was an inappropriate size for these birds. Improper cuff size has been shown to adversely impact blood pressure measurement in other species.^1,8 However, errors associated with improper cuff selection in other species have been shown to be consistent.^1,7 That is, use of cuffs that are too wide results in pressure measurements that are consistently too low, whereas use of cuffs that are too narrow results in pressure measurements that are consistently too high. In the present study, however, indirect blood pressure measurements that were obtained were not consistently higher or lower than direct systolic blood pressure values. Thus, improper cuff size was not a likely cause of the disagreement in the present study.

Having a single individual performing all indirect blood pressure measurements could be considered a weakness of the present study. However, the individual performing indirect blood pressure measurements was familiar with this technique, and we believe that use of a single experienced individual reduced variability.

The most likely explanation for the poor agreement between indirect and direct blood pressure measurements is that the Doppler ultrasonic flow detector was not sensitive enough for use in these birds. Therefore, until further studies can be performed, use of indirect blood pressure measurements in Hispaniolan Amazon parrots and other similar-sized birds cannot be recommended.