To view this site you must be a
veterinarian surgeon or nurse.
Are you a Veterinary Surgeon or Veterinary Nurse?

No

Keeping the Finger on the Pulse -  Nuances in CV Monitoring

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.

Cardiovascular Monitoring

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

Oesophageal stethoscope

Electrocardiogram

Blood pressure monitor

Pulse oximeter

Tissue Perfusion

Mucous membrane colour

Capillary refill time (CRT) 

Blood pressure 

Urine output 

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 

Oscillometric method 

Arterial catheterisation 

Blood Volume

Pulse pressure variation

Central venous pressure

Assess blood loss

Table 1. Methods of monitoring cardiovascular status (Modified from Clarke and Trim, 2013)

Heart Rate

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:

  1. Pulse oximetry, where the pulse rate and waveform is digitally displayed alongside the oxygen saturation of haemoglobin (SpO2),
  2. 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%.

Tissue Perfusion

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:

  1. 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.
  2. 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:
    1. 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).
    2. 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.

Article by
Dr. Dan Cripwell
BSc (Hons) BVSc CertAVP (EM) PgCert (VPS) MRCVS

Veterinary Technical Advisor UK
RCVS Recognised Advanced Veterinary Practitioner

Originally published: Thursday, 26th July 2018
Last updated: Monday, 30th July 2018

References

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 practice27(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 Practice38(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.

Keep reading... More news items that may interest you.

Effect of Maropitant on Isoflurane Requirements & Postoperative Nausea & Vomiting

Despite being widely recognized in humans, postoperative nausea and vomiting (PONV), and the role of maropitant in reducing inhalational anaesthetic requirements have been poorly documented in dogs. This recent study evaluates PONV and isoflurane requirements after maropitant administration during routine ovariectomy in bitches.

Read On...

New! Alfaxan® Multidose Now Available

We are happy to announce we have enhanced our anaesthesia and analgesia portfolio with the introduction of Alfaxan®Multidose for dogs, cats and pet rabbits.

Read On...

Sevoflurane requirement in dogs premedicated with medetomidine and butorphanol

Little information is available about the effect that different doses of medetomidine and butorphanol may have when using sevoflurane for maintenance of anaesthesia in dogs. This recent study evaluates heart rate and median sevoflurane concentration required at different dose rates.

Read On...

Capnography II - What happened to the elephants? A summary of abnormal traces

In this second article of the capnography series, James provides a guide to a few of the most common traces that you will encounter during surgery. Scroll to the end of the article to download a printable capnography cheatsheet. 

Read On...

Pain, what a Pain! (Part 2) – Practical Tips On How To Perform Dental Nerve Blocks In Companion Animal Practice

In this second article of the Pain, what a Pain! series, Dan takes us through the LRA techniques associated with dental and oral surgery. In this article, you will find practical tips and pictures on common dental nerve blocks as well as safety concerns to consider.

Read On...

​Peri-anaesthetic mortality and nonfatal gastrointestinal complications in pet rabbits

This recent retrospective study looks at the cases of 185 pet rabbits admitted for sedation or general anaesthetic and evaluates the incidence and risk factors contributing to peri-anaesthetic mortality and gastrointestinal complications.

Read On...

Pain, what a Pain! How Locoregional Anaesthesia can Improve the Outcome and Welfare of Veterinary Patients (Part 1)

In this first article out of a series of two, Dan takes us through an introduction and practical tips for appropriate local anaesthesia delivery. Find out why these anaesthesia techniques, that are well recognised in human medicine, have seen an increase in popularity in veterinary medicine over the recent years

Read On...

Perspectives on Premeds – Opioids

Perspectives on Premeds is a series of articles touching on different pharmacological, physiological and clinical aspects of pre-anaesthetic medication. This second article aims to provide a refresher on opioids.

Read On...

Effects of Dexmedetomidine with Different Opioid Combinations in Dogs

Read the highlights of a recently published research paper that evaluates cardiorespiratory, sedative and antinociceptive effects of dexmedetomidine alone and in combination with morphine, methadone, meperidine, butorphanol, nalbuphine and tramadol. 

Read On...

Preoxygenation Study Highlights

This study evaluates the effectiveness of two methods of preoxygenation in healthy yet sedated dogs and the impact of these methods on time taken to reach a predetermined haemoglobin desaturation point (haemoglobin saturation (SpO2) of 90%) during an experimentally induced period of apnoea.

Read On...

Capnography – Not Just a Load of Hot Air

Capnography is the measurement of inhaled and exhaled carbon dioxide (CO2) concentration. The graphical illustration of CO2 within respired gases versus times is known as the capnogram.

Read On...

Perspectives on Premeds – Alpha-2 Agonists

Perspectives on Premeds is a series of articles touching on different pharmacological, physiological and clinical aspects of pre-anaesthetic medication. This first article aims to provide a refresher on α2 agonists.

Read On...

We are ‘injecting’ a bit of fun into BSAVA Congress!

We will be ‘injecting’ a bit of fun into BSAVA Congress on our stand (stand 702).

Read On...

Alfaxan - now licensed for use in pet rabbits

Jurox Animal Health is delighted to announce that Alfaxan is now licensed for cats, dogs and pet rabbits. This is an exciting advance and could change the way rabbits are anaesthetised in the U.K.

Read On...

Best Practice Rabbit Anaesthesia Roadshows

Jurox Announces eight country wide events on Best Practice Rabbit Anaesthesia

Read On...

Considerations in Rabbit Anaesthesia at the 2017 London Vet Show

Jurox to host talks on Considerations in Rabbit Anaesthesia at the 2017 London Vet Show.

Read On...

Vets needing more support for anaesthesia

Jurox research reveals that veterinary professionals have questions about their anaesthetic protocols

Read On...
Repeatable. Reliable. Relax.