ADVANCED PATIENT MONITORING DURING ANAESTHESIA: PART TWO

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1 Vet Times The website for the veterinary profession ADVANCED PATIENT MONITORING DURING ANAESTHESIA: PART TWO Author : CARL BRADBROOK Categories : Vets Date : October 14, 2013 CARL BRADBROOK follows on from part one on pulse oximetry and blood pressure measurement by covering the use of capnography and electrocardiography THIS two-part article (part one VT 43.40) looks at advanced monitoring equipment available for use in practice, with the benefits to patient care and information that may be gained discussed. How to approach correcting common abnormalities is also detailed. Capnography Capnography is used to assess adequacy of ventilation. A capnograph displays a capnogram (Figure 1 ) and the end-tidal carbon dioxide partial pressure (ETCO2), whereas a capnometer displays only the ETCO2. ETCO2 is used as a surrogate for the partial pressure of arterial CO2 (PaCO2), which requires arterial blood sampling to measure. Capnography is an indirect monitoring tool and provides information on CO2 production, pulmonary perfusion, respiratory rate and pattern and CO2 elimination. Human medicine risk studies have documented that capnography and pulse oximetry, when used together, could aid in the prevention of anaesthetic- related errors. Normal PaCO2 ranges be tween 35mmHg and 45mmHg and ETCO2 is also considered to have the same reference range, although it is likely to be slightly lower than PaCO2 due to ventilation and perfusion mismatching during anaesthesia. CO2 is measured in the gas sampled between the end of the patient s endotracheal tube and the breathing system (Figure 2). Two types of capnograph are available. 1 / 15

2 A mainstream device where gas is analysed at the patient end and a side stream device where gas is pumped to the monitor and analysed. Care must be taken not to damage this part of the device, as repairs are costly. The side stream device is most commonly used and has the advantage of being less bulky around the patient, although it has an associated time lag due to the tubing carrying gas from the sampling site. In practice this is not a concern. The shape of the capnogram is of benefit when assessing problems affecting ventilation, such as partial or complete airway/endotracheal tube obstruction, rebreathing of expired gases or bronchoconstriction. It is also of benefit when confirming correct placement of an endotracheal tube, alerting the clinician to disconnection of the patient from the breathing system and cardiac. arrest. Examples of different capnograms are shown in Figure 3a Rebreathing of expired gases (Figure 3c) may occur due to inadequate fresh gas flow in a nonrebreathing system, expired CO2 absorbent in a rebreathing system or a rapid respiratory rate may also be diagnosed with capnography. The trace baseline will be elevated from zero, due to the presence of CO2 in the inspired gas mixture and some monitors will also display an inspired CO2 concentration (FiCO2). Clinical use End-tidal CO2 above the reference range occurs most commonly during anaesthesia due to hypoventilation, although it may also result from a low cardiac output. Hypoventilation may result from excessive depth of anaesthesia, drugs used as part of the anaesthetic protocol and respiratory muscle weakness. Correction of hypoventilation may involve assessing and adjusting the depth of anaesthesia or instituting manual or mechanical ventilation. The inhalational anaesthetic agents are dosedependent respiratory depressants and, therefore, vaporiser setting adjustment to provide the most appropriate depth of anaesthesia and good use of multimodal anaesthetic and analgesic protocols will minimise this. ETCO2 below the reference range may result from hyperventilation due to too light a plane of anaesthesia or a patient s response to surgical stimulation. This should also alert the anaesthetist to check for a leak around the endotracheal tube, which dilutes the ETCO2. Other situations where a low ETCO2 can arise include obese patients with a reduced lung capacity and due to inadvertent endobronchial intubation. Correction of hyperventilation may involve increasing depth of anaesthesia and administering further analgesia, if appropriate. As well as a useful patient monitor during anaesthesia, capnography is of great benefit during cardiopulmonary cerebral resuscitation (CPCR). Good CPCR technique, especially chest 2 / 15

3 compressions and the development of an adequate circulation, will result in the delivery of CO2 to the lung and, therefore, an ETCO2 value will be recorded. This can be used as a measure of CPCR success and has been shown to be a useful prognostic indicator in people. Electrocardiography Electrocardiography (ECG) displays cardiac rhythm and electrical activity only and, therefore, gives no indication as to the circulation and tissue perfusion. It is a useful additional monitoring tool, but because of the above limitations, should not be relied on as the sole method of monitoring during anaesthesia. Electrical cardiac activity may continue for at least 20 to 30 minutes, postcirculatory arrest. The normal ECG (Figure 4) is a result of summation of the action potentials of each cardiac cell and is made up of three main parts: the P wave corresponding to atrial depolarisation; the QRS complex corresponding to ventricular depolarisation; and the T wave corresponding to ventricular repolarisation. Atrial repolarisation is lost within the QRS complex in most ECGs and, therefore, is not assessed. Lead attachment and setup The normal lead configuration used during anaesthesia (Figure 5a) in small animal patients is a threelead ECG, using lead II for the waveform generation. Either ECG electrodes applied to both front paws and the left hind paw, or crocodile clips applied to both axilla and the left stifle skin fold may be used for lead attachment. Oesophageal probes are also available particularly useful for orthopaedic surgery where access to limbs is restricted. ECG interpretation The ECG trace should be observed and assessed for rhythm, the presence of a P wave for every QRS complex and a QRS complex for every P wave should be determined. The R-R interval may be used to assess rhythm. The QRS complexes should be assessed for uniformity, as any difference in shape or size may suggest a ventricular arrhythmia is present. The commonly encountered ECG abnormalities will be discussed here. For further ECG interpretation, refer to the reading list. Sinus arrhythmia (Figure 6) is a normal variation in healthy patients and may become more pronounced after premedication or during anaesthesia and is associated with the respiratory cycle. 3 / 15

4 On inspiration, stretching of the thoracic wall activates stretch receptors that act via a negative feedback system to inhibit vagal tone and cause an increase in heart rate. The vagal tone increases during exhalation and heart rate decreases. No treatment is required. Atrioventricular block (AVB) occurs due to interference in conduction through the atrioventricular node. An increase in parasympathetic tone, due to drugs administered during the anaesthetic process, may lead to slowing of conduction through the node and subsequent AVB. AVB may manifest as first degree, with a prolongation of the PR interval, second degree (Figure 7), with occasional non-conducted P waves or third degree with complete dissociation of atrial and ventricular contraction. First or second degree AVB may develop during anaesthesia and is often associated with opioids, especially if given by infusion. High second or third degree AVB requires pacemaker implantation the patient should be recovered from anaesthesia pending further investigation. Ventricular premature complexes (VPCs) may be seen on the ECG, originating from an area of myocardial ischaemia due to hypoxia or infarction. They originate from the ventricle and, therefore, are not associated with a P wave and are often described as looking wide and bizarre (Figure 8). They appear earlier than a sinus beat would and, therefore, the R-R interval is not regular. If uniform and not affecting pulse quality and cardiac output, no treatment is required. If multiform, or affecting pulse quality due to runs of VPCs and consequently cardiac output, treatment with lidocaine should be commenced. An initial bolus of 1.0mg/kg to 2mg/kg IV lidocaine may be followed by an infusion if necessary. Instituting oxygen and fluid therapy, if not already underway, may help to improve cardiac tissue perfusion and oxygenation. Occasional VPCs do occur in certain patients, especially those in which alpha-2 agonists have been used. Atrial fibrillation (AF) may be auscultated prior to, or develop at, induction of anaesthesia and will be recognised by an irregular ECG. No P waves will be visible and a ventricular rate of around 40bpm in the dog may be present. Atrial fibrillation is more common in larger breed dogs, due to a larger atrial mass and in those breeds susceptible to dilated cardiomyopathy. If recognised after induction of anaesthesia, it is advisable to recover the patient and investigate further, to ensure it will not adversely affect the patient s outcome. Supraventricular tachycardia (SVT) may result from extreme sympathetic stimulation during surgery and is the classic increase in heart rate to nociception. P waves are present, although they may be difficult to see due to the rate, and QRS complexes will appear normal (Figure 9). Treatment will depend on the underlying cause, but may include administration of additional analgesia, fluid therapy and/or increasing the depth of anaesthesia. Ventricular tachycardia (VT) may result from progression of runs of VPCs, or be seen in patients with significant disease processes such as sepsis. Fast or true VT will have a rate above 160bpm, 4 / 15

5 affect blood pressure and cardiac output and require treatment. The observed ECG will have no P waves and the QRS complexes will appear abnormal, possibly wide and bizarre (Figure 10). Treatment should consist of lidocaine as described previously, administration of additional opioid analgesia if available and good patient support. Conclusion Good patient monitoring is key to safe anaesthesia. There is no substitute for manual monitoring of a patient s vital signs, but the addition of electronic methods can add a substantial quantity and quality of information. No single piece of monitoring equipment should be relied on solely during anaesthesia and, ideally, several methods should be available to assess any trends that develop. With careful checking of the patient and sensible interpretation of information from electronic monitoring equipment, any changes in measured vital signs will be recognised promptly and acted on. References and further reading Brodbelt D C, Pfeiffer D U, Young L E et al (2007). Risk factors for anaesthetic-related deaths in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF), Br J Anaesth 99 (5): Dugdale A (2010). Monitoring animals under general anaesthesia. In Veterinary Anaesthesia: Principles to Practice, Wiley-Blackwell: Martin M (2007). Small animal ECGs: an introductory guide (2nd edn), Wiley-Blackwell. Moens Y and Coppens P (2007). Patient monitoring and monitoring equipment. In Seymour C and Duke-Novakovski T (eds), BSAVA Manual of Canine and Feline Anaesthesia and Analgesia (2nd edn), BSAVA Gloucester: Sawyer D C, Guikema A H and Siegel E M (2004). Evaluation of a new oscillometric blood pressure monitor in isoflurane-anaesthetized dogs, Vet Anaesth Analg 31 (1): / 15

6 Figure 1. Normal capnogram, respiratory rate and end-tidal CO2 value displayed by a capnograph. A: start of expiration. B: start of expiratory plateau. C: start of inspiration. D: end of inspiration. Figure 10. Ventricular tachycardia. 6 / 15

7 Figure 2. A heat and moisture exchanger showing the port for the capnograph connection. 7 / 15

8 Figure 3a. An abnormal capnogram. Note the short expiratory plateau and prolonged inspiratory phase suggestive of resistance to inspiration, such as upper airway obstruction or a mucus-blocked endotracheal tube. Figure 3b. A capnogram observed during mechanical ventilation when the patient fights the ventilator. Note the lack of uniformity to some of the waveforms. 8 / 15

9 Figure 3c. A capnogram observed during tachypnoea and hyperventilation. Note the low ETCO2 and FiCO2 above zero, due to rebreathing of expired gases as a result of the high respiratory rate. 9 / 15

10 Figure 4. A normal electrocardiogram (ECG) (three-limb lead trace obtained during anaesthesia). 10 / 15

11 Figure 5a. Threecoloured ECG leads supplied with most multiparameter monitors. Red right forelimb, yellow left forelimb, green left hindlimb. 11 / 15

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13 Figure 5b (inset). Use of commercially available disposable ECG electrodes placed on the pads of the forelimbs for connection to the ECG leads. Figure 6. A monitor showing sinus arrhythmia. 13 / 15

14 Figure 7. Second degree atrioventricular block. Figure 8. Ventricular premature complexes. 14 / 15

15 Figure 9. Supraventricular tachycardia. 15 / 15 Powered by TCPDF (