Keeping Your Finger on the Pulse; Successful Anaesthetic Monitoring
World Small Animal Veterinary Association World Congress Proceedings, 2008
Ian Self, BSc, BVSc, ARCS, CertVA, MRCVS
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.

Monitoring Equipment

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?

 Physiological parameters

 Heart rate and rhythm

 Haemoglobin oxygen saturation (SpO2)

 Respiratory rate

 Gas analysis

 CO2

 O2

 Anaesthetic agents

 Blood pressure

 Temperature

 Monitoring anaesthetic depth

Heart Rate and Rhythm

 Stethoscope

 Oesophageal stethoscope

 ECG

 Apex beat

 Peripheral pulses--try to avoid femoral pulses all the time!

Oesophageal Stethoscope

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.

ECG

Easy to use, quick to attach and has uses other than anaesthesia. They are relatively inexpensive--especially second hand.

Enables characterisation of arrhythmias:

1.  Bradycardia

a.  Sinus

b.  AV block--'medetomidine/xylazine'

2.  Tachycardia

a.  Sinus/ventricular

3.  Identification of ventricular premature contractions

4.  Identification of arrest rhythms

a.  Ventricular fibrillation

b.  Asystole

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)

 Artefacts

 Muscle activity

 Electrical interference

 50 Hz mains electricity

 Diathermy

 Most modern monitors will filter this interference out

Pulse Oximetry

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

 C probe

 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.

Respiratory Rate

Can be monitored visually, using spirometry, capnography or via respiratory rate detectors activated by the passage of warm gas over an electrical resistor

Gas Analysis

Capnography

 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.

Agent Analysis

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.

Oscillometric Method

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.

Limitations:

 Cuff too small rightwards arrow blood pressure reading artificially high

 Cuff too large rightwards arrow 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.

Clinical relevance:

 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.

Temperature Measurement

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 rightwards arrow 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.

Eye Position

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.

Palpebral Reflex

Gently tap the lateral canthus or stroke the eyelashes rightwards arrow 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

Pedal 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.

Muscle Tone

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.

References

1.  Seymour C, Duke-Novakovski T. (Eds.) BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2007). BSAVA, UK.

2.  http://dictionary.oed.com

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.

Speaker Information
(click the speaker's name to view other papers and abstracts submitted by this speaker)

Ian Self, BSc, BVSc, ARCS, CertVA, MRCVS
School of Agriculture, Food Science and Veterinary Medicine
University College, Dublin
Belfield, Dublin, Ireland


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