Radial vs coccygeal artery Doppler blood pressure measurement in conscious cats

Abstract

Objectives

The aims of the study were to compare the results of systolic blood pressure (SBP) measurements at the radial and coccygeal artery by Doppler ultrasonography and to assess the impact of gender, age, body weight, body condition score (BCS) and stress score on disagreements.

Methods

Eighty cats were included in this prospective, block randomised crossover trial. Blood pressure measurements were performed at home, during hospitalisation or in a local animal shelter by a single trained observer according to the latest ISFM consensus guidelines. Statistical analysis included Spearman’s correlation and ANOVA.

Results

Measurement or site order had no effect on blood pressure measurements (P = 0.157, P = 0.965). Correlation of radial and coccygeal SBP was moderate (r_sp = 0.519, P <0.001) and measurements at the tail were by mean 18.7 ± 37 mmHg higher. The differences between the measuring sites were neither correlated with age (r_sp = 0.044, P = 0.710) nor with body weight (r_sp = 0.122, P = 0.337). A significant effect of BCS (P = 0.016), but not gender (P = 0.246) or stress score (P = 0.424) was identified. Of the 25 cats with a coccygeal SBP ⩾170 mmHg, 14 (56%) were categorised as mildly hypertensive or normotensive when measurements were performed at the forelimb.

Conclusions and relevance

Choice of the measuring site, coccygeal vs radial artery, has a significant impact on Doppler ultrasonographic SBP results. In many cats, these discrepancies are huge and would certainly affect treatment decisions. Possible explanations include cone-shaped tails, especially in obese cats, leading to distal gapping or slippage of cylindrical cuffs and peripheral pulse pressure amplification. Further studies are urgently needed to clarify whether the discordances are artefacts or have a pathological background.

Introduction

Doppler-shift sphygmomanometry is well established in scientific studies on feline hypertension and is likely the most common indirect systolic blood pressure (SBP) measurement procedure used in small animal practice. The technique, although requiring some user skill, is easy to learn,¹ and the equipment relatively inexpensive. The currently recommended sites for the positioning of the inflatable cuff are the forelimbs and the tail.^2,3 Studies in anaesthetised cats comparing measurements at the forelimb with direct radiotelemtry (gold standard) showed good correlation (r = 0.928),⁴ but a small negative bias of 11.5–14 mmHg.^4,5 With a negative bias of −22.4 ± 17.1 mmHg and a correlation of 0.855, accuracy (proximity of the measurements to the true value) was clearly lower in conscious cats.⁴ Although measurements at the tail are preferred by the majority of cats, resulting in fewer failures and shorter expenditure of time,⁶ little is known about the precision (reproducibility of the measurements) and accuracy of this measurement site. Binns et al⁷ found good correlation (r = 0.91), good accuracy (mean error −4.7 mmHg), but low precision (SD ± 22.87 mmHg) when comparing tail measurements with the gold standard in 11 anaethetised cats. In 28% of the measurements, differences exceeded 20%.

The pivotal objectives of this study were to compare radial and coccygeal measurements and their impact on treatment decisions using current guidelines.³ Additionally, the impact of age, body weight, gender, anxiety score and body condition score (BCS) on SBP differences between the two sites was investigated. During the performance of this study, Whittemore et al⁸ published a similar clinical trial including 66 conscious cats. The authors found only moderate correlation (r = 0.45) between the results obtained at the different measurement sites, and SBP at the tail was a mean of 19 mmHg (15%) higher. Worryingly, on the basis of coccygeal artery results, 28 (42%) of the cats would have been categorised as hypertensive (>150 mmHg), whereas radial measurements suggested normotension in 20 of these.

Materials and methods

The prospective, crossover study was performed in April 2017 and privately owned cats, hospitalised patients of a university clinic, as well as cats from a local animal shelter were included. A stress score from 1 (fully relaxed) to 7 (terrified) as proposed by Kassler and Turner⁹ was applied. Cats with a stress score ⩾6, kittens below an age of 6 months, as well as sedated, anaesthetised and acutely injured cats were excluded.

Measurements were performed as recommended by the ISFM consensus guidelines³ and the order of measurement was block-randomised. The same Doppler ultrasonic flow detector (Model 811-B, Eickemeyer, Parks Medicals Electronics), sphygmomanometer and neonate disposable blood pressure cuffs (Philips, size one: 3.1–5.7 cm; size two: 4.3–8.0 cm; size three: 5.8–10.9 cm) were used in all cats. All measurements were performed by a single trained observer, assisted by the cat owner (privately owned cats) or a nurse (hospitalised or shelter cats), in a quiet room wearing headphones and after an acclimatisation period of at least 5 mins. Cats were allowed to settle into a position of their choosing and only exceptionally, in case of continuous attempts to escape, forced into right lateral recumbency. The latter were assigned a stress score of 5.⁹ If possible, examinations were performed in sternal recumbency on the lap of the assistant. Cuff size was selected so that the cuff width was 30–40% of the circumference of the tail or forelimb. The hair below the cuff was not removed, and all measurements were performed within 10 mins. SBP was calculated from the mean of five consecutive measurements. In contrast to the ISFM guidelines, but based on the study of Jepson et al,¹ the first of five measurements was not discarded if the variation was ⩽20% of the second measurement. BCS was determined by using a five-point scale according to Lund et al.¹⁰ Cats were grouped according to their mean SBP results, and the impact on treatment decisions (group 0: <90 mmHg; group 1: ⩾90 to <150 mmHg; group 2: ⩾150 to 159 mmHg; group 3: ⩾160 to 169 mmHg; group 4: ⩾170 mmHg).³

The study was discussed and approved by the institutional ethics and animal welfare committee in accordance with Good Scientific Practice guidelines and national legislation (ETK-11/04/2017).

Statistical analysis was performed with the laboratory software package IBM SPSS version 24. Data were tested for normality of distribution by Shapiro-Wilk test, and data are given as mean ± SD. To account for possible effects of measurement order (first vs second set of readings) or site order (coccygeal/radial vs radial/coccygeal), analysis with ANOVA for repeated measures was applied. BCS, gender, and stress score were included as covariates. For statistical analysis, cats with a stress score of 1 and 2, as well as 3 and 4 were pooled. Spearman rank correlation coefficients were calculated to assess the relationship between the various variables. For inferential statistics, the level of significance was set at P <0.05.

Results

Ninety-four cats were enrolled in the study, of which 14 cats were excluded due to intolerance of measurements (both sites: n = 7, forelimb: n = 5, tail: n = 2). The study population finally consisted of 80 cats, 68 domestic shorthairs (85%), four Persians, three British Shorthairs, two Norwegian Forest Cats, two Maine Coons and one Bengal; 45 males (six intact) and 35 females (eight intact). The cats ranged in age from 7 months to 17 years (80.8 ± 52 months) and weighed 2–7.78 kg (4.1 ± 1.3 kg). Forty-eight cats (60%) were considered to have an optimal BCS (BCS 3/5), whereas 21 (26%) were under- (BCS 1–2/5), and 11 (14%) overweight (BCS 4–5/5). Of the 80 cats that were included in the study, 31 (39%) were healthy and 49 (61%) had one or more known disease processes or conditions.

Blood pressure measurements were performed at home in 40 (50%), during hospitalisation in 20 (25%), and in a local animal shelter in 20 (25%) cats. Fifty-nine (74%), 16 (20%) and five (6%) cats were assigned a stress score of 1–2 (fully or weakly relaxed), 3–4 (weakly or very tense) and 5 (fearful, stiff, patience and restraint necessary), respectively. No clear preference for either coccygeal or radial measurement was observed in 71 (89%) of the cats. Five cats showed aversive behaviour towards coccygeal measurements, whereas four cats disliked measurements at the paws. Cuff sizes one (forelimb, n = 13; tail, n = 23) and two (forelimb, n = 64; tail, n = 57) were used in most patients, while cuff size three was needed in only three cats at the forelimb.

Measurement order (first vs second set of measurements) or site order (coccygeal/radial vs radial/coccygeal) had no effect on blood pressure measurement (P = 0.157, P = 0.965). SBP measurements at the radial artery (141 ± 32, range 50–280 mmHg) were 18.7 ± 37 mmHg (P <0.001) lower than at the coccygeal artery (160 ± 45, range 51–280 mmHg), but discordances occurred in both directions (see Figure 1). The correlation was moderate (r_sp = 0.519, P <0.001). The differences between the measuring sites were neither correlated with age (r_sp = 0.044, P = 0.710) nor with body weight (r_sp = 0.122, P = 0.337). A significant effect of BCS (P = 0.016, ANOVA, see Figure 2), but not gender (P = 0.246) or anxiety score (P = 0.424) was identified. The discordances were higher in overweight (BCS ⩾4) compared with normal (BCS 3, P = 0.021) or underweight (BCS ⩽2, P = 0.036) cats.

Figure 1

Bland–Altman plot of agreement between forelimb and tail systolic blood pressure (SBP) measured with Doppler ultrasonography. The straight black line indicates the mean bias. The dotted black lines indicate the limits of agreement (± 2 SD). The broken line indicates zero bias

Figure 2

Box and whisker plots for systolic blood pressure (SBP) differences between forelimb and tail measurements, grouped by body condition score (BCS) categories. The lines within the boxes represent the median and the upper and lower boundaries of the boxes, the 25th and 75th percentiles. The whiskers include 25% of the cases, and outliers are depicted as dots. A significant effect of BCS (P = 0.016, ANOVA) was identified. The discordances were higher in overweight (BCS 4 and 5) compared with normal (BCS 3, P = 0.021) or underweight (BCS 1 and 2, P = 0.036) cats

In 48 (60%) cats, the choice of measurement site would not have impacted the treatment decision. This was not the case in 32 (40%) cats where tail measurements were higher (24, 30%) or lower (eight, 10%) than results obtained at the forelimb. Of the 25 cats with an SBP ⩾170 mmHg (group 4) measured at the tail, four (16%), six (24%) and eight (32%) cats were assigned to groups 3 (⩾160 mmHg), 2 (⩾150 mmHg) and 1 (⩾90 to <150 mmHg) when measurements were performed at the forelimb, respectively. One cat was categorised as hypotensive based on coccygeal (51 mmHg) and radial (50 mmHg) SBP measurements. Follow-up blood pressure measurements were performed in some of the 20 hospitalised cats, but unfortunately the site of measurement was not recorded. Cardiological examinations were performed in two of the four cats with severe hypertension measured at the tail (⩾ 170mmHg), and mild hypertension when measured at the forelimb. Concentric left ventricular hypertrophy was diagnosed in one of these patients.

Discussion

The results of this study corroborate the results of Binns et al⁷ in anaesthetised and Whittemore et al⁸ in conscious cats, which showed that the choice of the measuring site (forelimb or tail) massively impacts SBP results obtained with Doppler-shift sphygmomamometry. As in the latter study, mean SBP measurements at the coccygeal and radial artery were only moderately correlated (r = 0.519 vs 0.45), and readings at the tail were on average 18.7 mmHg (vs 19 mmHg) higher. Worryingly, of 25 cats with an SBP ⩾170 mmHg at the tail, 14 (56%) were categorised as mildly hypertensive or normotensive, when measurements were performed at the forelimb. Accordingly, the choice of measurement site would have greatly impacted treatment decisions.

Possible explanations for these discrepancies include, on the one hand, artefacts caused by body positioning, stress/discomfort or cone-shaped tails, especially in overweight animals, and on the other hand true differences caused by peripheral pulse pressure amplification.

Based on the study of Bodey and Michell¹¹ in dogs, where significantly lower coccygeal SBP were measured in a standing position, as compared with lateral recumbence, Whittemore et al⁸ proposed that perhaps lateral positioning caused the disagreement. However, as most of the cats in the current study were allowed to settle in a sternal position, this hypothesis can be discarded. It is also unlikely that stress or discomfort was causative, as no effect of the stress score was observed.

As especially obese animals have cone-shaped tails, cylindrical cuffs are used. The consequences are distal gapping, slippage during inflation and, finally, the need for higher cuff pressures to occlude the arteries. In accordance with this hypothesis, overweight cats had significantly higher paw-tail differences than well proportioned or underweight cats in this study. So called ‘radial’ conical cuffs are now commercially available for human patients.¹² These special cuffs are the consequence of studies that demonstrated that up to 15% of obese people are falsely classified as hypertensive, if traditional cylindrical cuffs are used.^13,14, Although the average error was only 2.0 ± 0.4 mmHg in the latter study, differences of up to 9.7 mmHg were found in individuals with large arms.¹⁴ In a recent study including 17 anaesthetised dogs, the use of specially-fabricated conical cuffs at the limb did not show any benefit.¹⁵ Unfortunately, all of the dogs under investigation were normotensive, the blood pressure range was very narrow, the study was not randomised, and BCS was not given.

Another possible cause for SBP differences between measurement sites is peripheral pulse pressure (PP) amplification. It is a clear misconception to believe that blood pressure is constant, regardless of the location where it is measured. In short, the pulse waves generated by the heart travel to the peripheral arteries where they are reflected. These retrograde waves encounter the forward pressure waves, which are summated. The consequence is an increase in the whole amplitude of the PP as it travels distally. The degree of this amplification, which can reach >30 mmHg as the wave moves from the aorta to the brachial artery in humans, depends on the distance to the reflection site, the arterial properties (eg, elasticity/stiffness, vasomotor tone) and heart rate. As a consequence, PP amplification has a high inter- and intra-individual variability.^16–18

In line with this concept, direct measurements at the median sacral artery or dorsal pedal artery in dogs were on average 3.08/16.12 mmHg (dorsal recumbence) or 4.67/14.7 mmHg (lateral recumbence) higher than direct measurements at the superficial palmar arch.¹⁹ In humans, blood pressure differences between the arms of 10 mmHg or more are associated with peripheral vascular disease with a high specificity. As differences of more than 15 mmHg are associated with an increased all-cause and cardiovascular mortality, bilateral brachial measurement has been recommended as a screening test for peripheral vascular disease.²⁰

Interestingly, BCS influenced the difference between the measurements at different body sites in this study. Obesity is clearly associated with cardiovascular disease in humans²¹ and, in combination with the metabolic syndrome and hypertension, increases PP amplification.¹⁸ The best way to verify different SBPs in the radial and coccygeal arteries of affected cats would be to use direct techniques such as radiotelemetry. The main obstacle is the very small size of the feline coccygeal artery.

Given the discordant SBP results between forelimb and tail measurements in this and a recent study,⁸ the question arises which site to use in future feline studies and in clinical practice. In our opinion, the recommendation of Whittemore et al⁸ to favour the tail is not compelling. Although we agree that measurements at the tail are very well tolerated, and according to current literature the accuracy is high (low bias), the precision is low (high standard deviation). In the only study comparing Doppler ultrasonographic measurements at the tail with direct radiotelemetry,⁷ correlation was 0.91 at the tail and 0.96 at the hindlimb. Twenty-six percent and 21.5% of the readings had a difference >20 mmHg when measured at the tail and the hindlimb, respectively. In the same study, the mean error was −4.7 ± 22.87 mmHg at the tail and 9.4 ± 14.86 mmHg at the hindlimb. In another study including a radiotelemetry system, measurements at the forelimbs performed better than at the hindlimbs.⁴ In summary, published data do not allow preference of one or the other site. In our opinion, it is no disadvantage that forelimb measurements underestimate SBP, as the diagnostic precision is high^4,5,22 and most studies to determine reference intervals or treatment targets were performed using this measurement site. In contrast to an earlier study using high definition oscillometry,⁶ we found no clear preference for either coccygeal or radial measurement.

Conclusions

Given the findings of the study reported here and the study of Whittemore et al,⁸ further studies including cats with a high BCS and the investigation of target organ damage are urgently needed to clarify whether the conspicuous discordances of the SBP between the measurement sites are an artefact or have an anatomical or pathological background. Until then, the site of measurement should be documented and the results have to be interpreted with great caution.

Footnotes

Conflict of interest

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Accepted: 9 October 2017

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