Vet Times The website for the veterinary profession https://www.vettimes.co.uk ADVANCED PATIENT MONITORING DURING ANAESTHESIA: PART ONE Author : CARL BRADBROOK Categories : Vets Date : October 7, 2013 CARL BRADBROOK looks at pulse oximetry and blood pressure measurement, before moving on to capnography and electrocardiography in part two of his article Summary Good monitoring during anaesthesia is essential to ensure the best patient outcome. Monitoring is best approached by looking at trends and changes in the measured variables over time. This allows the clinician to act on the information gained, making appropriate decisions on any intervention necessary. Available monitoring methods include the veterinary nurse or surgeon and electronic equipment. Direct patient contact and manual checking of measurable signs should never be omitted, but the addition of electronic monitoring methods adds to the information available to the clinician. The availability of reasonably priced multi-parameter monitors has made them more accessible to general practice and allowed their increased use. It is important, though, to be able to judge the quality of information made available and act appropriately on it. Commonly used electronic monitoring methods in practice are pulse oximetry, capnography, arterial blood pressure measurement and electrocardiography. Careful use of electronic monitoring equipment, in addition to the manual skills offered by the clinician, allows a high standard of patient care during anaesthesia. Key words anaesthesia, monitoring, morbidity, multi-parameter 1 / 30
MONITORING a patient s vital parameters during general anaesthesia is an important and necessary part of any diagnostic or surgical procedure. Electronic monitoring equipment, in addition to the manual skills offered by the veterinary nurse or surgeon, allows decisions made during anaesthesia to be based on several, rather than one, physiological variable. The availability of more reasonably priced multi-parameter monitors (Figure 1) has resulted in them being more common in general practice. The importance of good monitoring during anaesthesia cannot be underestimated. This may use a combination of manual and electronic methods, but should allow the veterinary surgeon to act on the information gained in a timely manner. When using electronic equipment it is important to be able to interpret correctly the values and waveforms displayed to aid this decision making. A common problem is not knowing the best course of action to take for the patient when an abnormality is detected. Some understanding of how the different monitoring techniques calculate their information will also be of use when troubleshooting problems. This article looks at advanced monitoring equipment available for use in practice, with the benefits to patient care and information that may be gained from each discussed. How to approach correcting common abnormalities will also be detailed. This first part considers pulse oximetry and blood pressure measurement and the second part capnography and electrocardiography. Pulse oximetry Pulse oximetry is a non-invasive method of measuring the haemoglobin (Hb) oxygen saturation (SpO2) in arterial blood. A multi-centre case control study, carried out between 2002 and 2004 looking at perianaesthetic mortality in cats, showed reduced odds of death if pulse monitoring and pulse oximetry were used as part of patient monitoring (Brodbelt et al, 2007). How does it work? The technique utilises a clip (Figure 2a) placed over a peripheral area of tissue, most commonly the tongue, although other areas such as the lip, ear, vulva and scrotum may be used. Other types of probe are available that may be wrapped around tissues (Figure 2b) or used in the rectum (Figure 2c). The clip (Figure 3) contains two light-emitting diodes (LEDs) on one side and a photo-detecting crystal on the opposite side. One LED emits red light and the other infrared (IR) light. The LEDs come on one at a time, followed by a pause with neither on this allows differentiation between oxy and deoxyhaemoglobin and compensation for any natural light. Absorption of red and IR light differs 2 / 30
between the Hb species and pulsatile blood flow is used to distinguish arterial from venous blood. The pulse oximeter using this data and a built-in algorithm calculates SpO2. More advanced units, most commonly used in medical anaesthesia, emit up to eight wavelengths of light and can differentiate between types of Hb. Clinical use SpO2 should be above 97 per cent in the anaesthetised patient breathing 100 per cent oxygen, and any persistent value below this should be investigated. The first step that often remedies most problems is to change the probe position this allows tissue beds to re-perfuse. If SpO2 is low then the heart rate (HR) measured by the monitor should be checked against a manual rate from the patient. If they do not agree, the SpO2 is likely to be inaccurate. If the value is definitely low then the patient, anaesthetic machine and breathing system should be checked. See Table 1 for some common problems encountered. Recent advances in signal extraction technology have improved the accuracy and reliability of pulse oximetry. The Masimo monitor (Figure 4) is proven to cope better than others with movement and vasoconstriction. They are now available commercially. Pulse oximetry is a useful monitoring tool because it requires blood flow and, therefore, tissue perfusion to display any information. The addition of a displayed HR and, in some cases a plethysmograph (pulse waveform; Figure 5), add further information on pulse rate and quality. Errors may be introduced by factors such as movement, peripheral vasoconstriction, ambient light and inadequate or excess tissue depth. Vasoconstriction and thin tissue depth may be a particular problem in the cat, but should not discourage its use. Blood pressure measurement Arterial blood pressure is derived from the relationship between cardiac output and systemic vascular resistance. Mean pressure is physiologically the most important as it represents the mean driving pressure responsible for organ perfusion. During general anaesthesia, a mean arterial pressure greater than 60mmHg is considered the minimum to maintain autoregulation of blood flow to tissues such as the kidney, brain and liver. Without any other suitable, readily available clinical methods to measure blood flow, arterial blood pressure is used as a measurable method of assessing organ perfusion. There are two categories of blood pressure measurement invasive and non-invasive. The non-invasive method is the most commonly utilised, is easy to apply and reliable in most patients. It does not provide continuous measurement though and may be inaccurate in patients with severe hypotension and those with arrhythmias. Clinical use 3 / 30
Assessment of obtained blood pressure values may be used to aid any adjustment in depth of anaesthesia. Mean arterial blood pressure (MAP) above 60mmHg is recommended during anaesthesia and the range 60 mmhg to 90mmHg is often referred to. If a patient is hypotensive (MAP Fluid therapy should be started, if not already begun, and a 10ml/kg (5ml/kg cats) bolus of crystalloid administered over 15 to 20 minutes. MAP should then be reassessed and, if still low, a second crystalloid bolus may be administered and/or a 2ml/kg to 5ml/ kg colloid bolus. Only after these steps should drug therapy be considered. The use of inotropes and vasopressors is beyond the scope of this article. Please refer to the further reading list in part two for more information. If the patient is not at too deep a plane of anaesthesia then administration of further analgesia (such as an opioid) is likely to be required to enable the vaporiser setting to be reduced if necessary and fluid therapy instituted as above. If hypertensive (MAP > 90mmHg) then the patient may be at too light a plane of anaesthesia or insufficient analgesia is present for the current level of surgical stimulation. Depending on which may be present then either increasing the depth of anaesthesia or administering additional analgesia should be considered. Non-invasive methods Oscillometric Oscillometric blood pressure measurement is the most commonly used in small animal veterinary practice. Commercially available machines consist of an electronic display and control unit connected via hosing to an appropriate sized cuff (Figure 6). This method provides intermittent, indirect measurement of blood pressure. The measurement cuff should ideally be positioned on the metatarsus (Figure 7a), as it has been shown to perform slightly better at this site compared to the metacarpus and anterior tibia (Sawyer et al, 2004). The cuff width should be approximately 40 per cent of the limb circumference and the centre of the cuff placed over the peripheral artery. The cuff should be firmly in place, but not excessively tight. Both single and double tube cuffs are used depending on the machine. The machine inflates the cuff beyond systolic pressure (SAP) and then deflates the cuff slowly to measure the oscillations, which return at the SAP, are maximal at the mean arterial pressure (MAP) and then reduce to their minimum at the diastolic blood pressure (DAP). The single cuffs used are able to detect the oscillations created by blood flow through the artery and also measure the cuff pressure, with some monitors calculating DAP from an algorithm. A transducer converts the oscillations into an electrical signal and the machine displays the systolic, mean and diastolic blood pressure (Figure 7b). 4 / 30
Measurements should be taken on a maximum basis of every two minutes and ideally this should be extended to every three to five minutes, to prevent occlusion to blood flow to the distal aspect of the limb. The use of changes or trends of blood pressure over time is probably most useful when assessing a patient during anaesthesia. Doppler ultrasonic detection This technique uses the Doppler effect to detect arterial blood flow. The Doppler effect describes the change in pitch of a sound emitted from an object, depending on whether it is moving towards or away from a stationary receiver. The Doppler probe consists of an ultrasonic crystal and transducer (Figures 8a and 8b), which is placed over a peripheral artery, most commonly the dorsal metatarsal, plantar metatarsal or palmar metacarpal artery to provide an audible signal to blood flow. The concave surface of the probe should have ultrasound gel applied to it prior to placing and securing over the artery. This can be used not only to aid in blood pressure measurement but also to provide an audible pulse rate. The Doppler method provides intermittent measurement of SAP in the dog and around 15 mmhg lower than SAP in the cat. It requires placement of a cuff to occlude blood flow proximal to the Doppler crystal. Again, the cuff width should measure 40 per cent of the limb circumference and is attached to a sphygmomanometer (Figure 9), to allow measurement of pressure. The Doppler crystal is taped in place over the peripheral artery after detection of a pulse. The cuff is then inflated to around 10mmHg above the point at which the pulse is no longer audible. The cuff is then slowly deflated until the pulse is once again audible. The pressure reading at this point is recorded. This should be repeated several times and the average reading taken. This method is useful in smaller patients; particularly cats where the oscillometric method might be unreliable. The audible pulse is of benefit in patients where other methods of pulse detection may fail due to, for example, cold extremities and vasoconstriction. Care should be taken to securely attach the Doppler unit to prevent loss of signal if the patient is moved during anaesthesia. Invasive methods Direct arterial catheterisation Direct arterial catheterisation is the gold standard in blood pressure measurement and is recommended in sick and compromised patients. It requires arterial catheterisation, which in the majority of small animal patients will involve a peripheral artery. Vessels suitable for catheterisation include the dorsal metatarsal artery and plantar metacarpal or metatarsal artery (Figure 10). Some veterinary anaesthetists use the coccygeal artery in cats. Arterial catheterisation is a skilled procedure that is not without potential complications. These include significant haemorrhage, infection and tissue ischaemia distal to the catheter insertion site but these are rare. The arterial catheter is connected via saline-filled tubing to either a 5 / 30
manometer or electrical pressure transducer (Figure 11). The pressure transducer can be connected to an appropriate multi-parameter monitor, which will then display the SAP, DAP and MAP. It also displays a continuous, arterial trace waveform (Figure 12), which allows for further evaluation of the cardiovascular system. The advantages of this method are that it is a continuous, direct measurement and, therefore, significant hypotension and arrhythmias do not interfere with its readings. The arterial catheter may also be used for sample collection for blood gas and acid-base analysis. 6 / 30
Figure 1. Multi-parameter monitor with ECG, pulse oximetry, invasive and non-invasive blood pressure measurement, capnography, body temperature and gas monitoring. 7 / 30
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Figure 10. Arterial cannulation of the dorsopedal artery in a dog. A pre-flushed T-connector is attached to the arterial catheter to allow connection to the pressure transducer. 9 / 30
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Figure 11. Electronic pressure transducer and saline filled line for connection to the arterial catheter. Figure 12. Arterial pressure waveform generated by the electronic transducer with invasive blood pressure monitoring. 11 / 30
Figure 2a. Standard pulse oximeter clip placed on the tongue. Photo also shows heat and 12 / 30
moisture exchanger (HME) with port for capnograph attachment. 13 / 30
Figure 2b. Wrap around pulse oximeter probe for use on small patients. 14 / 30
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Figure 2c. Reflectance pulse oximeter probe for rectal use. 16 / 30
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Figure 3 (left). Pulse oximeter probe showing position of LEDs (white part) and photodetector (black part). 18 / 30
Figure 4 (above). Masimo pulse oximeter unit 19 / 30
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Figure 5. Portable pulse oximetry unit showing SpO2 and plethysmograph. 21 / 30
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Figure 6. Selection of cuffs for non-invasive measurement of blood pressure. 23 / 30
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Figure 7a (above). Placement of a blood pressure cuff on the metacarpal region of the dog. 25 / 30
Figure 7b (below). Cardell non-invasive blood pressure monitor showing systolic, diastolic and mean blood pressure and heart rate. 26 / 30
Figure 8a. Doppler probe for use for noninvasive blood pressure measurement. 27 / 30
Figure 8b. Speaker unit for amplification of the pulse detected by the Doppler probe. 28 / 30
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Figure 9. Sphygmomanometer for measurement of blood pressure with the Doppler probe. TABLE 1. Guide to troubleshooting low SpO2 30 / 30 Powered by TCPDF (www.tcpdf.org)