Why monitor our patients?
All patients are exposed to the inherent risks associated with general anaesthesia, many of which are out of our immediate control. Among the known physiological changes you may expect to observe during the anaesthetic process, individual variation and responses also exist which may result in entirely unpredictable yet critical events.
The process of continuously monitoring our anaesthetised patients (before, during and after anaesthesia) is vital in order to maximise patient safety and wellbeing. Each patient should have a designated veterinarian or suitably qualified nurse/technician who remains in close and constant contact throughout the entire anaesthetic period and recovery. This not only enables assessment of adequate peri-anaesthetic depth, but also for the rapid identification of changes in a patient’s clinical state, allowing for appropriate and timely interventions to be made.
The simultaneous monitoring of several variables per body system and multiple body systems at a time is required for complete patient assessment. In this article, we will focus on the common monitoring techniques that provide information relating to the cardiovascular status of your patient.
Anaesthetic records play a pivotal role in responsible anaesthetic monitoring. Not only does this allow for visualisation of trends in the physiological variables being recorded, but provides an anaesthetic history for that patient and may also act as a legal document if required.
The heart rate, pulse rate and rhythm, tissue perfusion and blood pressure of all anaesthetised animals should be assessed at regular and frequent intervals. Examples of methods of assessing such cardiovascular parameters in our anaesthetised patients are outlined in Table 1.
Methods of Assessing Cardiovascular Function in Anaesthetised Patients
Heart Rate and Rhythm
Palpation of arterial pulse
Blood pressure monitor
Mucous membrane colour
Capillary refill time (CRT)
Bleeding at operative site
Observation of intestine colour
Capnography (See previous Anaesthesia1st articles for detailed information on this monitoring tool)
Arterial Blood Pressure
Palpation of arterial pulse
Doppler ultrasound method
Pulse pressure variation
Central venous pressure
Assess blood loss
Table 1. Methods of monitoring cardiovascular status (Modified from Clarke and Trim, 2013)
In dogs and cats, auscultation of the heart rate via a stethoscope or oesophageal stethoscope is both straightforward and inexpensive. In its simplest form, the oesophageal stethoscope consists of a traditional stethoscope earpiece attached to a blind ended plastic tube covered in a thin plastic membrane. This tube is placed into the oesophagus so that the distal tip is overlying the heart. This method only provides information relating to heart rate and rhythm, the changes in heart sound intensity correlating to systemic blood pressure, as seen in human medicine, has not ever been substantiated in the veterinary literature (Sakamoto et al. 1965; Flaherty and Musk, 2005).
More sophisticated oesophageal probes are also available for veterinary use, some of these being capable of producing an electrocardiographic trace (ECG) and measuring internal (i.e. oesophageal) temperature.
Electrocardiography: This is commonplace in anaesthetic monitoring, where heart rate and rhythm can be easily obtained via a standard three lead (i.e. leads I, II and III) configuration in small animals. It is important to appreciate that this form of monitoring only assesses the electrical activity of the heart, and therefore does not provide information on cardiac output. Changes in both cardiac rate and rhythm are common during anaesthesia, with bradycardia, tachycardia and ventricular premature complexes (VPCs) most frequently encountered (Flaherty and Musk, 2005). Although there are a variety of factors that may be responsible for changes in both heart rate and rhythm, their development under anaesthesia is usually secondary to inadequate anaesthetic depth, inadequate analgesia, hypoxaemia or hypercapnia (Flaherty and Musk, 2005). It is recommended that these four factors be ruled out first to avoid unnecessary pharmacological intervention.
Pulse Rate and Rhythm
The pulse rate and rhythm may be assessed by palpation of a peripheral arterial pulse. Pulse rate may also be assessed by:
- Pulse oximetry, where the pulse rate and waveform is digitally displayed alongside the oxygen saturation of haemoglobin (SpO2),
- Doppler, which uses an ultrasound probe to produce the audible sound of blood flow.
It is important to appreciate that pulse oximetry does not give any indication of oxygen delivery to the tissues. For example, if there was only one red blood cell in the circulation, but this was fully saturated with oxygen, then the pulse oximeter would still give an SpO2 reading of 100%.
Mucous Membrane Colour: When monitoring the appearance of the mucous membranes, pink is considered normal. A pale colouration is suggestive of hypoperfusion, anaemia or vasoconstriction, red implies localised congestion or vasodilation, brick red indicates haemoconcentration or hypercapnia and blue is suggestive of cyanosis (Mathis, 2016).
Capillary Refill Time (CRT): CRT gives a subjective but still valuable indication into a patient’s peripheral perfusion. A CRT which is equal to or slightly less than 2 seconds is considered normal where as a delayed CRT (i.e. greater than 2 seconds) is suggestive of peripheral hypoperfusion or vasoconstriction.
Urine Output: This is dependent on renal blood flow and as such provides insights into kidney perfusion. Measuring urine output is of added clinical benefit in patients with known renal pathology. A urine output of 1 to 2ml/kg/hour is considered normal (Flaherty and Musk, 2005), and this value should increase with increasing intravenous fluid therapy. Due to increased anti-diuretic hormone (ADH) secretion in anaesthetised patients, urine output may be reduced and in such circumstances an output of 0.7ml/kg/hr is considered acceptable (Mathis, 2016). It is important that the bladder is fully drained of urine at the start of the procedure if any accurate deductions are to be made by measuring urine output.
Blood Pressure Assessment
Arterial blood pressure (ABP):This is the product of cardiac output (CO) and total peripheral resistance (TPR), such that, ABP = CO x TPR. As a result, ABP is used to provide information relating to cardiac output, and therefore insight into tissue perfusion in anaesthetised patients.
During general anaesthesia, consensus is that systolic arterial pressure (SAP) should be maintained above 80-90mmHg and the mean arterial pressure (MAP) above 60-70mmHg (Clarke and Trim, 2013; Flaherty and Musk, 2005; Mathis, 2016).
Providing MAP falls within 50-150mmHg then the autoregulation of blood flow to major organs ensures a relatively constant blood supply (Mathis, 2016). However, should MAP increase or decrease outside of this range then damage to these organs may occur. The diastolic arterial pressure (DAP) should also remain above 40mmHg otherwise coronary perfusion may be compromised.
MAP may appear normal in cases of reduced CO but increased TPR. In such circumstances tissue perfusion may be impaired despite normal ABP values.
ABP may be measured by either non-invasive or invasive techniques:
- Invasive Blood Pressure (IBP): This technique involves cannulation of a peripheral artery and as a result is an option both more invasive and associated with greater complications (i.e. haemorrhage, thrombosis, infection). Despite this, results obtained via this technique are continuous, more accurate and more reliable than non-invasive methods.
- Non-Invasive Blood Pressure (NIBP): Less reliable results are obtained via this method when compared to IBP measurement, however this technique will still indicate trends in a changing blood pressure and is generally simpler to perform. NIBP methods involve the placement of an appropriately sized cuff around the limb or tail of the patient. This cuff is then inflated to occlude blood flow. The deflation of the cuff then allows reperfusion of the region distal to cuff placement which can be used to give an indication of blood pressure. Two methods of detection of this reperfusion are used:
- Doppler: An advantage of this method is that it can be used in patients of any size. This technique has shown to have good correlation with IBP methods in dogs (Weiser et al. 1997; Haberman et al. 2006) but in anaesthetised cats this method tends to underestimate true values, with a correction value of approximately 14mmHg having to be added to the observed reading (Grandy et al. 1992).
- Oscillometry: This method involves connection of the cuff to an electric monitor which gives regular readings. This technique provides a MAP which is considered the most reliable result (Flaherty and Musk, 2005).
NIBP does not accurately predict IBP in every instance, as a result no treatment decisions should be exclusively based on a single measurement.
Originally published: Thursday, 26th July 2018
Last updated: Monday, 30th July 2018
Clarke, K.W., Trim, C.M. 2013. Veterinary Anaesthesia E-Book. Elsevier Health Sciences.
Flaherty, D., Musk, G. 2005. Anaesthetic monitoring equipment for small animals. In practice. 27(10): 512-521.
Haberman, C.E., Kang, C.W., Morgan, J.D. and Brown, S.A. 2006. Evaluation of oscillometric and Doppler ultrasonic methods of indirect blood pressure estimation in conscious dogs. Canadian journal of veterinary research. 70(3): 211.
Mathis, A. 2016. Practical guide to monitoring anaesthetised small animal patients. In Practice. 38(8): 363-372.
Sakamoto, T., Kusukawa, R., Maccanon, D.M., Luisada, A.A. and Harvey, I. 1965. Hemodynamic determinants of the amplitude of the first heart sound. Circulation research. 16(1): 45-57.
Weiser, M.G., Spangler, W.L. and Gribble, D.H. 1977. Blood pressure measurement in the dog. Journal of the American Veterinary Medical Association. 171(4): 364-368.
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