School of Agriculture, Food Science and Veterinary Medicine, University College Dublin
Belfield, Dublin, Ireland
The Oxford English Dictionary defines monitoring as follows: 'To observe, supervise, or keep under review; to keep under observation; to measure or test at intervals, especially for the purpose of regulation or control.'
Why Bother with Monitoring?
Monitoring allows us to make decisions during anaesthesia, tailor the anaesthetic to the patient's requirements and intervene early to prevent a crisis.
A vast array of monitoring equipment is now available. Improved anaesthetic safety depends on both the correct use of equipment and the correct interpretation of information obtained. Written anaesthetic records should always be used. Despite the electronic aids available, the most important monitor still remains a dedicated anaesthetist who is constantly monitoring the patient.
What do we Monitor?
Heart rate and rhythm
Haemoglobin oxygen saturation (SpO2)
Monitoring anaesthetic depth
Heart Rate and Rhythm
Peripheral pulses--try to avoid femoral pulses all the time!
Cheap to buy and easy to use and different sizes available. Detect heart and lung sounds.
To insert, measure from tip of nose to heart base. Lubricate before insertion and place when patient asleep and intubated. Be careful not to insert too far--reflux oesophagitis can result.
Easy to use, quick to attach and has uses other than anaesthesia. They are relatively inexpensive--especially second hand.
Enables characterisation of arrhythmias:
b. AV block--'medetomidine/xylazine'
3. Identification of ventricular premature contractions
4. Identification of arrest rhythms
a. Ventricular fibrillation
However, there are some limitations:
ECG represents electrical activity and not mechanical activity
Possible to have a normal ECG and no pulse--electromechanical dissociation (EMD)
50 Hz mains electricity
Most modern monitors will filter this interference out
Measures percentage of haemoglobin saturation. Normal range under anaesthesia 95% or above and a reading of less than 90% is considered to indicate hypoxaemia.
Principles of action--a probe shines a red light through tissue and a detector picks up the transmitted light. A computer programme calculates the percentage of oxygenated haemoglobin under the probe and pulsatile blood flow.
Most pulse oximeters display heart rate and SpO2 but remember that if the heart rate displayed is incorrect, the SpO2 may not be reliable.
There are many probes available
Human ear clips
Human finger probes
Rectal probe (dentals)
In dogs and cats the probe is usually placed on the tongue. Advantages of pulse oximetry include: non- invasive technique, easy and quick to attach both under anaesthesia and in recovery, the presence of a signal indicates that the tissue under the probe is perfused and it allows early detection of hypoxaemia compared to visual assessment. The human eye cannot detect cyanosis until saturation has reached c. 70%.
However, the disadvantages include: difficulty getting readings from pigmented skin (Chows); movement and fluorescent light can cause interference; peripheral vasoconstriction may prevent an adequate signal being measured, e.g., hypothermia, medetomidine, shock.
Can be monitored visually, using spirometry, capnography or via respiratory rate detectors activated by the passage of warm gas over an electrical resistor
Side stream or main stream analyser are available
Clinical relevance of capnography:
Increased ETCO2 may indicate hypoventilation, rebreathing (base line not 0), sepsis or hyperthermia (including malignant hyperthermia)
Decreased ETCO2 may indicate hyperventilation, decreased cardiac output, hypothermia, cardiac arrest--sudden fall in reading or pulmonary embolism
Zero ETCO2 may indicate oesophageal intubation or endotracheal tube disconnection
Oxygen analysers can also be used especially in closed or semi-closed rebreathing systems and may reduce the risk of hypoxia particularly if nitrous oxide is being used.
Volatile anaesthetic agents and nitrous oxide can be measured. Particularly useful when using a circle with low fresh gas flows.
Blood Pressure Measurement
Indirect measurement of arterial blood pressure is achieved using Doppler flow detector or oscillometric techniques. Direct arterial blood pressure measurement and central venous pressure measurement are also available.
Doppler Flow Detector
Only measures systolic blood pressure. The Doppler flow detector is placed over an artery on a distal limb or tail and taped in position. A cuff attached to a manometer is then placed proximal to the Doppler. The cuff should be ≈ 40% circumference of leg. It is then inflated to prevent pulsatile sound and then gradually deflated. Pressure is read at the first return of pulsatile sound. Systolic blood pressure should be maintained at 90 mmHg or above during anaesthesia.
Cuff (40% circumference of the limb) is placed over an artery which is then inflated by the machine to a pressure above systolic pressure and then gradually deflated. Mean blood pressure is usually the most accurate and should be maintained above 70 mmHg.
Cuff too small blood pressure reading artificially high
Cuff too large blood pressure reading artificially low
Bradycardia, movement and arrhythmias affect performance
In addition, frequent cuff inflation can damage underlying tissues
Direct Blood Pressure Measurement
Aseptic placement of 22-24 gauge catheter into a peripheral artery, commonly the dorsal pedal artery, femoral artery or the auricular artery (basset hounds). The catheter is connected via saline-filled non-compressible tubing to an electrical pressure transducer and a continuous flushing device can be used to help prevent blood clotting in the catheter.
Blood pressure cannot be assessed by feeling the pulse
Blood pressure below 60 mmHg compromises blood flow to major organs
In dogs and cats the most common consequence is kidney failure
Central Venous Pressure
Assessed using a jugular catheter advanced into the cranial vena cava. A low or falling CVP indicates hypovolaemia, whereas a high or increasing CVP indicates hypervolaemia or cardiovascular insufficiency (particularly right sided). CVP measurements are commonly used in intensive care and high risk anaesthetics.
Rectal temperature does not accurately reflect core temperature in anaesthetised patients. Oesophageal temperature accurately reflects core body temperature when taken from the lower oesophagus. Tympanic membrane is closely associated with brain temperature and accurately reflects core body temperature.
Oesophageal temperature probes usually contain a thermocouple. Infrared tympanic thermometers consist of a series of thermocouples called a thermopile. This detects infrared radiation from the tympanic membrane and converts the signal to an electrical signal which reads core temperature within 3 seconds.
Clinical relevance of hypothermia:
Hypoventilation and bradycardia common
Reduced anaesthetic agent requirements overdose more likely
Prolonged recovery is very common due to reduced drug metabolism
Shivering during the recovery period increases the likelihood of hypoxia
Monitoring Anaesthetic Depth
This is a rapidly developing area in human medicine due to litigation in cases of patient awareness under anaesthesia. Awareness is reported to occur in 0.1-0.18% of human anaesthetics.
Anaesthetic depth in veterinary patients is monitored using a combination of eye position, palpebral reflex, pedal reflex, muscle tone (usually jaw tone is assessed), response to surgical stimulation and recently EEG monitoring.
Should be central at light planes of anaesthesia and during deep anaesthesia. During moderate depth of surgical anaesthesia--the eye rotates rostroventrally but during ketamine anaesthesia the eye remains central.
Gently tap the lateral canthus or stroke the eyelashes partial or complete closure of the eyelids:
Light anaesthesia--brisk palpebral reflex
Surgical anaesthesia--weak palpebral reflex
Deep anaesthesia--palpebral reflex absent
Ketamine anaesthesia--brisk palpebral reflex
Pinching the web between the toes or firm pressure on the nail bed will lead to limb withdrawal if anaesthesia is inadequate for surgery. This technique is commonly used in laboratory animals.
Change in jaw tone is often used to assess anaesthetic depth in conjunction with other parameters. An increasing jaw tone is an indicator of lightening anaesthesia whereas a decreasing jaw tone is an indicator of deepening anaesthesia.
Response to Surgical Stimulation
Responses indicating inadequate anaesthesia include movement, a rapid increase in heart rate and blood pressure (a slight increase in response to surgical stimulation is normal), increased respiratory rate / panting and apnoea.
EEG (Electro-encephalogram) Monitoring
In human anaesthesia, EEGs are used in an attempt to increase the reliability of anaesthetic depth monitoring. As yet however no method can reliably indicate depth of anaesthesia for all the combinations of anaesthetic agents that are employed especially in veterinary patients.
Bispectral index (BIS)
Machine analyses the EEG to produce a number from 0 to 100
Auditory evoked response
Analysis of the response of the EEG to repeated sounds under anaesthesia
EEG monitoring is being used in veterinary research projects but is unlikely to be useful in practice for many years.
1. Seymour C, Duke-Novakovski T. (Eds.) BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2007). BSAVA, UK.
3. Tranquilli WJ, Thurman JC, Grimm KA. (Eds). Lumb and Jones' Veterinary Anesthesia and Analgesia (4th Ed) (2007). Blackwell Publishing, USA.
4. Hall LW, Clarke KW, Trim CM. (Eds). Veterinary Anaesthesia (10th Ed) (2001). W.B.Saunders, UK.
5. Davey AJ, Diba A. (Eds). Ward's Anaesthetic Equipment (2005). Elsevier Saunders, UK.