Blood Pressure Monitoring in Exotic Animal Species

Key Points

  • Blood pressure is commonly measured to assess cardiovascular performance.
  • Mean arterial pressure is the most clinically important measurement because it represents the mean driving pressure for tissue perfusion.
  • Highly reliable and accurate direct arterial blood pressure measurements can be obtained using the central auricular artery in the rabbit and the coccygeal artery in the ferret. It can be very difficult to obtain accurate readings in other small exotic animals, particularly in birds less than 2 kg and reptiles.
  • Indirect blood pressure is most commonly measured by Doppler ultrasound or non-invasive oscillometric monitors. There are significant disadvantages associated with use of oscillometric monitors in exotic animal patients.
  • Although the accuracy of indirect blood pressure readings can be poor due to patient size or anatomy, trends in blood pressure over the course of anesthesia can provide clinically useful information.
  • Regardless of the monitor used, it is important to maintain a cuff width to limb circumference ratio of approximately 40% to reduce the margin of error.
  • Sites for indirect blood pressure measurement in small mammals include the femoral artery, medial saphenous artery, dorsal carpal branch of the radial artery, ventral coccygeal artery, and auricular artery.
  • Popular sites for Doppler probe placement in the avian patient include tibiotarsal and radial arteries. The pressure cuff is placed on the distal humerus or femur.
  • Although there can be tremendous variation seen, a Doppler blood pressure reading above 90 mm Hg is generally considered acceptable in psittacine birds and mammals.
  • Resting blood pressure in reptiles is strongly influenced by temperature, however chelonians tend to have the lowest mean values (15-30 mm Hg). Some monitor lizards have resting arterial pressures similar to mammals, ranging from 60-80 mm Hg.

Introduction

Blood pressure is an important determinant of the heart’s work and is commonly measured to assess cardiovascular performance. Arterial blood pressure is a function of heart rate, blood volume, stroke volume, and arterial compliance.23 Blood pressure alone is not the best indicator of tissue perfusion, but low pressure can lead to a reduction in blood flow and impaired oxygenation of major organs.22,24 Anesthesia, particularly inhalant anesthesia, can cause a profound decrease in blood pressure.22

This article is part of a series on anesthetic monitoring in exotic animal patients. Additional topics available include:  capnometry, electrocardiography, pulse oximetry, and monitoring vital signs.

Direct blood pressure

Direct arterial blood pressure monitoring requires specialized training and equipment and is primarily measured in academia, tertiary care referral centers, or laboratory settings.8,17,27 In the rabbit (Oryctolagus cuniculus), highly reliable and accurate direct arterial blood pressure measurements can be obtained using a 22-24-gauge over-the-needle catheter placed in the central ear artery.8,15,27 The coccygeal artery can be used in the ferret (Mustela putorius furo),25 and brachial, carotid, or metatarsal arteries can be used in the bird. 11,12 In patients less than 2 kg, catheter placement can distort pressure wave formation and the vessel is also at increased risk for thromboembolism.12

Indirect blood pressure

In domestic animals, indirect blood pressure is most commonly measured by non-invasive oscillometric or Doppler ultrasound monitors. 22 Mean arterial pressure (MAP) is the most clinically important measurement because it represents the mean driving pressure for tissue perfusion.3,2 When only systolic arterial pressure (SAP) is measured (as with the Doppler technique), the anesthetist should be aware that SAP has only a variable relationship with MAP.10 Regardless of the monitor used, it is important to maintain a cuff width to limb circumference ratio of approximately 40% to reduce the margin of error. If the blood pressure cuff selected is too loose, it will give falsely low readings. If the cuff is too tight, the readings will be high.10,17,22,26

A simple trick to remember proper cuff placement is: Low=Loose or tIGHt=HIGH.

Also make every effort to be consistent or at least take note of where the cuff is placed during each anesthetic procedure. 11 Repeated sampling that involves different cuff placements can be unreliable.11

OSCILLOMETRIC BLOOD PRESSURE MONITOR

Automated oscillometric monitors use an inflating cuff to detect oscillations produced by blood flow and these oscillations are used to determine blood pressure.7,22,27 There are significant disadvantages associated with use of oscillometric monitors in exotic animal patients. First, automated monitors take readings at a minimum of every minute. Changes can develop rapidly in critically ill exotics so the intermittent nature of the information obtained can be a problem during periods of cardiovascular instability. 8 Additionally, oscillometric monitors tend to be most unreliable when the information is needed most:  as blood pressure falls, the instrument may fail to obtain any readings at all.1,3,48,9,27 Oscillometric techniques can also fail to obtain measurements with rapid heart rates (>200 bpm), small patient size, and hypothermia.9,27

DOPPLER ULTRASOUND BLOOD PRESSURE MONITOR

To measure blood pressure with the Doppler, an ultrasonic probe is placed over the artery and a pressure cuff is placed distal to the transducer. The operator then inflates the pressure cuff to a suprasystemic pressure that extinguishes the Doppler signal. Systolic blood pressure is determined by listening for return of the pulse signal as the cuff slowly deflates (Fig 1).11,12,22,26

The required pieces for a Doppler set up: Doppler box, crystal, gel, and sphygmomanometer

Figure 1. In addition to pressure cuffs, components of a Doppler ultrasound blood pressure monitoring system include (from left to right) ultrasound gel to enhance contact between the probe and skin, the Doppler unit and the 8-MHz transducer probe, and a sphygmomanometer to measure pressure. Pediatric probes are preferred due to their higher sensitivity with small vessels. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Exotic companion mammals

Sites for indirect blood pressure measurement in small mammals include the hindlimbs (femoral artery or medial saphenous artery, dorsal pedal artery), forelimbs (dorsal carpal branch of the radial artery), coccygeal artery (on the ventral surface of the tail base), and the auricular artery (Fig 2).9,15,17,27 Doppler blood pressure measurements have been shown to correlate well with direct arterial pressure in rabbits under isoflurane anesthesia. 9 Oscillometric readings from a forelimb (but not hindlimb) cuff were also accurate in rabbits, but only at low or normal ranges. 27 High definition oscillometric readings were validated in sedated ferrets and correlated well with direct arterial readings. Readings obtained from the tail and forelimb of ferrets were considered more accurate than measurements from the hind limb.25

Doppler crystal placed over the dorsal pedal artery in a rabbit

Figure 2. A Doppler transducer placed over the dorsal pedal artery in a rabbit (Oryctolagus cuniculus). Placement of the probe is often more medial in the rabbit when compared to other mammals. Rabbits lack true foot pads so take care to clip the carpal or tarsal area and not the feet. Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Hypotension is usually defined as a MAP <60 mm Hg in anesthetized dogs and cats. 9,22,24 Twenty conscious guinea pigs (Cavia porcellus) “restrained comfortably in a holder without training” had systolic, mean and diastolic blood pressures of 94.0 +/- 2.2, 63.6 +/- 1.7 and 48.4 +/- 1.6 mm Hg, respectively.17 A forelimb Doppler arterial blood pressure reading beneath 80 mm Hg was considered hypotensive in rabbits under isoflurane anesthesia.9 A high definition oscillometric reading less than 110 mg Hg was considered hypotensive in heavily sedated ferrets.25

Birds

Birds have relatively large hearts and thick vessels. The reduced elasticity of the arterial walls means SAP is usually much higher in birds than in mammals. 12 Normal pressures range between 90-180 mm Hg in conscious psittacine birds.12,18 Normal SAP for psittacine birds under isoflurane or sevoflurane anesthesia is 90-150 mm Hg. Systolic blood pressure readings < 90 or > 145 mm Hg should be evaluated for causes of possible hypotension and hypertension respectively in psittacine birds.18,19 There is great diversity in the bird world, and 90 mm Hg can represent hypotension in some avian species. For instance, mean systolic blood pressure measured 140 +/- 7.2 mm Hg in Canada goose (Branta canadensis) anesthetized at least 15 minutes under isoflurane anesthesia. Diastolic pressure measured 75+/- 5.0 mm Hg and mean arterial pressure measured 101+/- 2.0 mm Hg. 13

Unfortunately, indirect blood pressure readings in birds less than 2 kg do not correlate to direct arterial pressure measurements. 19 Oscillometric devices have demonstrated a high rate of error and are not recommended in the avian patient.1,11,22,28 Doppler blood pressure measurements have also been shown to be unreliable in some birds, like psittacine birds and ducks, however this monitoring technique can still be used to evaluate trends during anesthesia.1,11,14,18 The pressure cuff is placed on the distal humerus or femur (Fig 3) and the Doppler probe is placed on the tibiotarsal or radial artery.12,18,19

A non-invasive blood pressure cuff placed on the wing of a whooping crane

Figure 3. Blood pressure cuff on the distal humerus of a whooping crane (Grus americana). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Reptiles

Blood pressure in reptiles is controlled by mechanisms similar to those described in mammals, however resting blood pressure is also strongly influenced by temperature.16,21There is a distinct paucity of information on typical blood pressure parameters in reptile species, however the range can be lower than in mammals. Blood pressure can also vary significantly among species making it difficult to describe “normal” reptile blood pressure values.21

  • Chelonians tend to have the lowest MAP of 15-30 mm Hg.21
  • Resting MAP in the green iguana (Iguana iguana) ranges from 40-50 mm Hg.21
  • Some varanid lizards have resting arterial pressures similar to mammals, ranging from 60-80 mm Hg.21

Indirect blood pressure measurements correlate poorly with direct arterial blood pressure in reptiles.5,6,23 Commercially available oscillometric devices applied to the snake’s tail or the lizard’s femoral triangle seem to have limited value but can provide information on trends (Fig 4).2,5,6,16

Blood pressure cuff on an iguana

Figure 4. Pressure cuff on the forelimb of an iguana (Iguana iguana). Photo credit: Katrina Lafferty, CVT, VTS. Click image to enlarge.

Conclusion

Arterial blood pressure is a function of heart rate, blood volume, stroke volume, and arterial compliance. Measurement of direct arterial blood pressure requires specialized training and equipment. Indirect blood pressure is most commonly measured by Doppler ultrasound monitors or by non-invasive oscillometric techniques in some larger exotic animal patients, like the rabbit. Although the accuracy of indirect BP readings can be poor in exotic animal patients, trends in blood pressure over the course of anesthesia can provide clinically useful information. In exotic companion mammals, sites for indirect blood pressure measurement include the hindlimbs (femoral artery or medial saphenous artery), forelimbs (dorsal carpal branch of the radial artery), ventral surface of the tail base, and auricular arteries. In birds, the pressure cuff is placed on the distal humerus or femur. Popular sites for Doppler probe placement include tibiotarsal and radial arteries. Sites for indirect blood pressure measurement in the reptile include the snake’s tail or the femoral triangle in the lizard. Reptile blood pressure readings tend to be lower than in mammals and is strongly influenced by temperature. Turtles and tortoises tend to have the lowest mean arterial pressure, while some varanid lizards have resting arterial pressures similar to mammals, ranging from 60-80 mm Hg.

References