The oesophageal stethoscope (Fig. 2.9) is a simple method of monitoring heart rate in dogs and cats. This monitor consists of a tube with a balloon on the end which is passed dorsal to the endotracheal tube and into the oesophagus until the tip is level with the heart. The open end of the tube is connected to an ordinary stethoscope headpiece, to a single earpiece that can be worn by the anaesthetist or surgeon, or connected to an amplifier that makes the heart sounds audible throughout the room. This monitor provides only information about heart rate and rhythm; the intensity of sound is not reliably associated with changes in blood pressure or cardiac output. Other electronic oesophageal probes are available to provide heart rate, and some also may produce an ECG and oesophageal temperature.

Heart rate and rhythm can be obtained from an ECG using standard limb leads in small animals, clip electrodes and ECG paste or gel, and selecting ECG Lead II on the oscilloscope (Fig. 2.10A). Poor contact of the electrodes to the skin from hair bunched in the clips, or close proximity to another electrical apparatus, such as the hot water circulating pad, can result in electrical interference that obscures the ECG. The leads should not be placed over the thorax as breathing will move the electrodes and result in a wandering ECG baseline (Fig. 2.10B).

A frequently used monitor lead in equine anaesthesia is the ‘base-apex’ lead. The right arm electrode is clipped on the neck in the right jugular furrow and the left arm electrode is passed between the forelimbs and clipped at the apex of the heart over the left fifth intercostal space several inches from the midline. The left leg electrode is clipped on the neck or on the shoulder. Good electrical contact is achieved with alcohol or electrode paste. Lead I is selected on the electrocardiograph and the normal configuration includes a negative R wave (see Fig. 2.10C). A bifid P wave is frequently observed in the normal equine ECG. Sinus arrhythmia in dogs and cats is abolished when atropine or glycopyrrolate has been administered. Other arrhythmias, for example second degree atrioventricular heart block and ventricular premature depolarizations (VPD) or premature ventricular complexes (PVC), may or may not require specific treatment (see Chapter 21).

There is a high incidence of sinus arrhythmia and first and second degree atrioventricular (AV) heart block in conscious unsedated horses (Robertson, 1990). In contrast, AV block during anaesthesia is uncommon except when the horse has been premedicated with detomidine, or supplemental intravenous injections of xylazine are given during anaesthesia. The appearance of this arrhythmia during anaesthesia on any other occasion is cause for concern as this rhythm may progress within a few minutes to advanced heart block (P waves only, no ventricular complexes) and cardiac arrest. Atrial fibrillation and PVCs occur rarely in horses but may require specific treatment if associated with hypotension.

Hair is first clipped from the skin on the palmar surface of the paw of dogs and cats (Fig. 2.12). A probe covered with contact gel is placed over the artery and taped in place with the electrical lead pointing down towards the toes (Fig. 2.13). Ultrasound waves emitted from one of the two piezoelectric crystals embedded in the probe pass through the skin and deeper tissues. A structure that is stationary will reflect sound back to the second crystal without any frequency change (Stegall et al., 1968). Moving objects, such as erythrocytes and the artery wall, will reflect some of the sound at a different frequency (Doppler-shift). The change in frequency can be heard through a loudspeaker as an audible swooshing sound with each pulse.

A cuff is wrapped snugly around the extremity proximal to the probe, in dogs and cats with the centre of the inflatable part of the cuff, or an arrow or dot, on the medial aspect of the limb and the air tube positioned on the distal end of the cuff. The cuff is connected to an anaeroid manometer and a bulb for manual inflation of the cuff with air (Fig. 2.14). In horses, the cuff is wrapped around the base of the tail with the cuff air bladder centred over the ventral surface of the tail. The probe is taped distal to the cuff over the coccygeal artery in the ventral midline groove. The coccygeal artery can be used for this technique in adult cattle but the results are not reliable. In foals, the probe and cuff are commonly attached to the tail. In foals and small ruminants in lateral recumbency, the probe can be taped over the metatarsal artery on the lateral surface of the hind limb or the common digital artery on the medial side of the forelimb distal to the carpus. The cuff is secured around the limb above the hock or carpus and positioned level with the sternum. In pigs, the probe is most reliable when taped over the common digital artery on the caudomedial aspect of the forelimb. The cuff should be placed between the carpus and the elbow but, because of the triangular shape of the forearm, it may be unable to occlude blood flow when inflation of the cuff causes it to slip down over the carpus. For all animals, the width of the air bladder within the cuff is important for accuracy; a bladder that is too narrow will overestimate blood pressure and one that is too wide will underestimate it. The cuff (air bladder) width should be 40% of the circumference of the extremity and the leg positioned so that the cuff is level with the manubrium. Furthermore, a cuff that is attached too loosely or slips down the extremity and becomes loose, or situated over a joint such as the carpus will result in an erroneously high value.

The Doppler ultrasonic method was observed to provide both overestimated and underestimated values over a wide range of systolic pressures in conscious and anaesthetized dogs, with a tendency for overestimation when MAP <80 mmHg and underestimation at high pressures (Haberman et al., 2006; Bosiack et al., 2010). An investigation in cats comparing the Doppler ultrasonic method, using a cuff placed halfway between the carpus and the elbow, with measurements obtained from a femoral artery catheter revealed that the indirect method consistently underestimated systolic pressure by an average of 14 mmHg (Grandy et al., 1992). Caulkett et al. (1998) also determined that the Doppler method both overestimated and underestimated systolic blood pressure in anaesthetized cats but that it proved to be an accurate predictor of MAP. Using this technique of measuring blood pressure in mature horses using a cuff width 48% of the circumference of the tail (bladder width 10.4 cm) systolic pressure was underestimated and diastolic pressure overestimated by approximately 9% (Parry et al., 1982). In another investigation, measurements of systolic arterial pressure in horses anaesthetized in dorsal recumbency with halothane using a cuff 41% of the tail circumference was reasonably accurate but, in 5% of the horses, this technique had an error range of ±20 mmHg (Bailey et al., 1994).

Devices that non-invasively measure peripheral blood pressure using the oscillometric method operate by automatically inflating a cuff placed around an extremity, either above or below the carpus, above or below the hock, or the tail (Fig 2.15). The monitor measures and records the amplitude of pressure changes in the cuff caused by pulses in underlying arteries. The cuff is inflated to above systolic pressure and deflated slowly. The pressure oscillations in the cuff start small and increase to a maximum value (corresponding with MAP) and then decrease. Each monitor uses fixed or variable parameter identification points based on the MAP value to calculate electronically systolic and diastolic pressures. The algorithm determining systolic and diastolic pressures varies between models and thus measurement results are not the same for all models. There is a also small difference in the recorded MAP and MAP calculated from the systolic and diastolic pressures using the formula MAP = diastolic pressure + 0.33 (systolic − diastolic) and this value is dependent on the measurement device (Kiers et al., 2008). Values for SAP, DAP, MAP, and heart rate are digitally displayed and the monitor can be programmed to measure automatically at a specified time interval. The cuff should be situated approximately level with the heart (manubrium or sternum if in lateral recumbency) for accurate measurement. Artefactual pressure changes induced in the cuff by movement of the extremity, for example, during preparation of the surgical site, will either induce abnormal readings or prevent the monitor from obtaining a measurement (Haberman et al., 2006; Bosiack et al., 2010). Closest correlations between direct and indirect measurements have been measured using cuff widths of 40% of the circumference of the animal’s leg or tail.

By far the highest number of published articles comparing NIBP and IBP measurements have been in dogs. Not all investigations have produced similar results, in part influenced by use of different models from several manufacturers, measurement in awake versus anaesthetized animals, increased variability attributed to haemodynamic alterations imposed by disease, differences in size and gender, sites of measurement, patient body position, pharmacological methods used to alter blood pressure, and the investigators varied definitions of hypotension or hypertension. The ‘take home’ message is that NIBP does not accurately predict IBP all of the time and that no treatment decision should be made based solely on one measurement. Investigations of specific models have employed the average of three to six consecutive readings taken 30–60 seconds apart to increase accuracy. Frequently during anaesthesia, the site of surgery and accessibility dictates the site for application of the cuff. Different sites have been investigated with the result that the forelimb in dogs was determined to be more accurate when compared with lingual artery pressure and the hind limb more accurate when compared with dorsal pedal artery pressure (McMurphy et al., 2006), no difference between measurements was recorded between the fore and hind limbs (Sawyer et al., 1991), or that the tibial or coccygeal arteries provided better correlation than the metacarpal artery when compared with aortic or anterior tibial artery pressure (Sawyer et al., 2004; Haberman et al., 2006). Most oscillometric monitors provide a reasonable approximation of MAP in normotensive animals.

The greatest concern during most anaesthetic episodes is the accurate identification of hypotension to avoid cardiac arrest or postoperative organ malfunction. The Surgivet V60046, with the cuff placed distal to the hock, provided accurate information for MAP in normotensive and hypotensive dogs, with 90.6% of the dogs with MAP <70 mmHg being accurately diagnosed as hypotensive (Deflandre & Hellebrekers, 2007). The monitor was less accurate at high pressures. The Cardell 9401, DINAMAP (Device for Indirect Noninvasive Automatic Mean Arterial Pressure) 8100, DINAMAP 8300 and PetMAP overestimated pressures during hypotension (Sawyer et al., 1991; Meurs et al., 1996; Bosiack et al., 2010; Shih et al., 2010), and did not reliably predict the severity of hypotension in animals with acute haemorrhage or in critically-ill patients in the ICU. Wernick et al. (2010) compared measurements in anaesthetized dogs between pressures recorded from the dorsal pedal artery and from a cuff around the antebrachium using the DINAMAP 8300. The DINAMAP slightly underestimated SAP and DAP and overestimated MAP; comparisons that fell just short of the validation requirement proposed by the American College of Veterinary Internal Medicine (Brown et al., 2007). In contrast, the DINAMAP 8300 underestimated all BP parameters in awake experimental Beagles, and to a greater extent at high pressures generated by administration of phenylephrine (Haberman et al., 2006). Comparisons with the Cardiocap II in clinical canine patients using premedication to alter blood pressure (acepromazine or medetomidine) generated a smaller bias than previously reported (Sawyer et al., 2004; McMurphy et al., 2006) but greater limits of agreement (variability) that was attributed to variation in clinical population (MacFarlane et al., 2010).

In clinical practice, it may be necessary to identify hypertension before anaesthesia and to measure accurately high pressures during anaesthesia in patients at risk, such as phaeochromocytoma, renal or endocrine disease. Oscillometric methods tend to underestimate systolic blood pressures during hypertension (Haberman et al., 2006; McMurphy et al., 2006; Deflandre & Hellebrekers, 2007; Bosiack et al., 2010), consistently underestimate MAP or DAP (Haberman et al., 2006), overestimated MAP or DAP (Bosiack et al., 2010), or underestimated SAP and DAP with close agreement with MAP (McMurphy et al., 2006).

The Cardell 9301V with 2.5 cm-wide neonatal pressure cuff placed around the forelimb above the carpus, with the monitor set at ‘small cuff’ mode has been evaluated in cats (Pedersen et al., 2002). Low pressure MAP <60 mmHg was achieved by increasing depth of anaesthesia with isoflurane and high pressures MAP >140 mmHg were created by administration of medetomidine. Mean differences between NIBP and IBP were <5 mmHg for MAP and DAP in all pressure ranges; SAP differences increased as pressure increased from hypotension to hypertension resulting in underestimation of SAP. The DINAMAP had good correlation with IBP systolic pressure in anaesthetized cats (Caulkett et al., 1998), however, the Datascope Passport did not accurately estimate direct blood pressure in cats (Branson et al., 1997).

In horses, the oscillometric method of blood pressure measurement is used when direct measurement of arterial pressure is not possible, for total intravenous anaesthesia, and for awake animals. The cuff is wrapped around the tail of mature horses and foals or around the hind limb near the metatarsal artery in foals. The cuff should not be wrapped tightly. Some investigators have recommended that the tail cuff be placed close to the base of the tail but, in this author’s opinion, more accurate readings are obtained with the cuff applied approximately 10 cm from the base of the tail, where the tail diameter is constant for the length of the cuff. In standing horses, cuff placement may be higher than the atrium, resulting in a measurement that is lower than the actual pressure. Early investigations of the DINAMAP confirmed accurate and clinically useful values for arterial pressure using a cuff width 24% of the tail circumference in ponies (Geddes et al., 1977) and 25–35% in horses (Latshaw et al., 1979; Muir et al., 1983). However, measurements were inaccurate at heart rates of less than 25 beats/minute. Our experience using a Model 8300 DINAMAP and a cuff width 35–40% of the tail circumference ratio (child or small adult cuff for a mature horse depending on tail thickness and the amount of hair) has been that MAP value obtained from this monitor is usually the same as that obtained by direct blood pressure measurement. Occasionally, the DINAMAP recorded pressures 10–20 mmHg higher than the true MAP. An evaluation of the Cardell Model 9402 and the DINAMAP Pro 100 in anaesthetized foals of less than 7 days of age using cuff sizes recommended by the manufacturers revealed that both monitors had similar performance (Giguère et al., 2005b). Analysis of MAP values confirmed acceptable accuracy with the cuffs on the tail or metatarsus for conditions of normotension and hypotension, although the Cardell was most accurate over the coccygeal artery and the DINAMAP over the metatarsal artery. Cuffs placed over the median artery produced less accurate values and wider variability. This study also confirmed that blood pressure did not correlate well with cardiac output, i.e. a good MAP does not guarantee that cardiac output is also acceptable. In summary, indirect method of measurement of blood pressure provides useful information in most horses, but may produce erroneous values in a small number. Consequently, blood pressure should be measured by direct means whenever possible in horses at risk of developing low blood pressure, for example, during inhalation anaesthesia.

Standards have been set for automated sphygmomanometers in human medicine by several organizations, including the British Hypertension Society, the Association for the Advancement of Medical Instrumentation (AAMI) and the American National Standards Institute, Inc. (ANSI). Devices validated for use in humans must have a correlation of ≥0.9 across a range of measurements with specific % of readings to be within 5, 10, or 15 mmHg of the standard measurement. Requirements for validation were tightened in 2011. These standards have been met for MAP at low and normal pressures in dogs with the Surgivet V60046 (Deflandre & Hellebrekers, 2007), for MAP and DAP in cats with the Cardell 9301V (Pedersen et al., 2002), and for MAP in foals using the Cardell 9402 and DINAMAP Pro 100 (Giguère et al., 2005b). For the most part, oscillometric blood pressure monitors provide useful information for management of the clinical patient. Trends in pressures can be used to direct management of anaesthesia and, although hypotension may not be accurately diagnosed, a decrease in measurements to low values should be a warning that action should be taken. A single abnormal measurement should be confirmed by taking two more readings. Failure of the monitor to achieve a reading (‘timed out’) when it had just previously been recording may indicate a change in peripheral blood flow. It must be remembered that although on average there is reasonable correlation between direct and indirect pressure measurements, in individual patients the difference in readings can be up to 20 mmHg and a patient may not be identified as having a MAP outside an accepted range. Therefore, NIBP readings must be assessed together with other observations of cardiovascular performance and depth of anaesthesia.

In contrast to previously described oscillometric techniques, high-definition oscillometric (HDO) technology discriminates between pressure waveform changes that are characteristic for systolic, mean, and diastolic pressures. The technology allows much more rapid assessments that minimize the impact of outside factors, and is able to accommodate fast heart rates and weak pulse signals. The expectation is for increased accuracy at high and low blood pressures and increased reliability for use in ill patients. In anaesthetized dogs, comparison between IBP measured at the dorsal pedal artery and a high-definition oscillometric device, Memodiagnostic MD_15/90 Pro, revealed that values for SAP, MAP, and DAP were all overestimated by HDO and the overestimation increased for SAP and DAP with increasing pressures (Wernick et al., 2010). The precision exceeded the 15 mmHg limit required for validation and only 50–60% of readings were within 10 mmHg. In a group of conscious experimental Beagles with implanted aortic devices for telemetry of blood pressures, measurement of pressures from a cuff around the base of the tail using HDO revealed that MAP most closely paralleled the telemetered pressures (Meyer et al., 2010). The standards set by AAMI were not met in this study. The authors noted that the mean standard deviations were substantially decreased by use of six averaged measurements at any time point compared with a single measurement.

Measurement of arterial blood pressure directly is accomplished by insertion of a 20 gauge, 22 gauge or, in very small animals, a 24 gauge catheter aseptically into a peripheral artery; the dorsal pedal, anterior tibial, femoral, coccygeal, metacarpal, auricular or lingual artery in dogs (Fig. 2.16), the dorsal pedal, femoral or coccygeal artery in cats, the lateral nasal, facial, transverse facial, or metatarsal artery in horses (see Fig. 2.7), or an auricular artery in ruminants, pigs and rabbits (Fig. 2.17). Placement of a catheter in an artery in most animals is not difficult with experience. Complications are rare provided there is strict attention to asepsis during insertion, wrapping to maintain cleanliness, use of sterile solutions for flushing, and appropriate pressure on the artery when the catheter is removed to avoid a haematoma. The cap or extension must be secured firmly to the arterial catheter to prevent accidental disconnection and blood loss. The catheter is connected by saline-filled tubing to either an anaeroid manometer (Fig. 2.18) or an electrical pressure transducer (Fig. 2.19) for measurement of arterial pressure. Either an air gap or a connector containing a latex diaphragm should be maintained next to the anaeroid manometer to prevent saline entering the manometer and to maintain sterility. An optional addition to the electrical transducer is a continuous flushing device (Fig. 2.20) that can be inserted between the transducer and the artery. This device is connected to a bag of saline that has been pressurized to 200 mmHg and will deliver 2–4 mL saline/hour to help prevent clotting of blood in the catheter. Extension tubing sold specifically for arterial pressure measurement is non-distensible to preserve an accurate waveform.

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