This extended patient care report highlights the nursing considerations required to safely and effectively manage a case with myasthenia gravis (MG) throughout the peri-anaesthetic period. It also focuses on the secondary conditions that may occur and outlines the nursing interventions and potential complications that can arise during the pre-anaesthetic and postanaesthetic stage. Veterinary nurse (RVN) knowledge of the condition is important in order to predict outcomes, minimise complications and maximise patient care. Autonomy of the RVN, gives further responsibility to understand and communicate potential problems with specialised diseases in order to correct, support and prevent further deterioration.
Signalment
Species: Canine
Breed: Medium crossbreed
Age: 2 years 7 months
Sex: Male (entire)
Weight: 22.6 kg
This patient was considered class III for anaesthesia according to the American Society of Anesthesiologists (ASA) Physical Status Scale (moderate risk) (Academy of Veterinary Technician Anesthetists, 2013).
Reason for admission
The patient presented to the primary care practice with diarrhoea, vomiting, regurgitation and pelvic limb weakness. The veterinary surgeon suspected neurological involvement and referred the patient to a specialist practice for further investigation. The history acknowledged a systolic heart murmur grade II/VI but no assessment from a cardiologist or echocardiogram was performed.
Patient assessment
The referral veterinary surgeon performed a full neurological examination which revealed proprioception deficits and weakness in the pelvic limbs. Combined with the history, the veterinary surgeon suspected myasthenia gravis. Temperature, pulse rate and respiration rate (TPR), blood pressure (BP) and electrocardiogram (ECG) were taken on admission and were in the normal range. A systolic heart murmur grade II/VI was confirmed on examination.
Veterinary investigation
In-house biochemistry and haematology blood work revealed no abnormalities. Blood was sent to the laboratory for acetylcholine receptor antibody titre. The patient received a general anaesthetic for full body computed tomography, thoracic radiographs and electromyography (EMG). Radiography revealed the patient had a megaoesophagus and neuromuscular deficits were detected on EMG. A neostigmine response test was performed post anaesthesia, with external blood results revealing acetylcholine receptor antibody titre levels that were consistent with acquired myasthenia gravis.
Myasthenia gravis
Myasthenia gravis is an immune-mediated neuromuscular transmission disorder that occurs spontaneously in dogs and cats causing muscle weakness (Shelton, 2016). Muscle weakness can be generalised or localised, in this case affecting the oesophagus, resulting in a secondary megaoesophagus (Branscombe and Poulton, 2015). Megaoesophagus is the abnormal dilation of the oesophagus caused by failure of oesophageal muscles; food accumulation causes regurgitation, weight loss and can result in aspiration pneumonia (Quintavalla et al, 2017). A megaoesophagus increases the risk of mortality in patients undergoing general anaesthesia so careful planning between the veterinary surgeon and nursing team needs to be in place to reduce this risk (Green and Figuieredo, 2014) (Figure 1).
Pre-anaesthesia
The aim of nursing a patient with a megaoesophagus is to prevent aspiration of saliva or food, which could lead to bacterial pneumonia, pulmonary damage and respiratory failure (Halling et al, 2013). The patient was subdued on presentation which enabled postural positioning by placing in sternal recumbency with the head elevated; thus, aiding gravitational drainage to the stomach (Khorzad et al, 2011). Products of regurgitation where the pH is <2.5 can cause significant injury to the lungs and respiratory tract if aspirated (Ambros et al, 2018). Savvas et al (2009) have explored prolonged fasting times and believe it increases the acidity of gastric contents which can cause reflux, and suggested only fasting 3 hours before induction. Other studies suggest patients with megaoesophagus requiring anaesthesia, need fasting between 8–12 hours (Green and Figueiredo, 2014). The patient had not eaten for 12 hours, which would fit with the advice given from Green and Figuieredo (2014).
Continuous observations are vital in the detection of deterioration in a patient with megaoesophagus (Khorzad et al, 2011). Oxygen saturation can be indirectly observed via pulse oximetry (SpO2), but blood gas analysis would give an accurate assessment on the patient's ventilation status and is acknowledged as ‘gold standard’ (Elmeshreghi et al, 2018). The veterinary surgeon did not want to run blood gases because of the short anaesthesia time, so TPR and SpO2 were recorded every 5 minutes. On admission to the anaesthesia suite, a tight fitted face mask was placed around the muzzle to deliver oxygen, and regular inspections of the oral cavity were made to remove excess fluid via suction. Suction aids, laryngoscope and endotracheal tubes (ETT) were on standby in case of regurgitation or respiratory arrest. Monitoring allowed the author to assess the current condition and trends of the patient's respiratory status.
Peri-anaesthesia
The effects of peri-operative drugs are an important consideration when selecting for patients with compromised gastrointestinal, cardiovascular and respiratory systems (Congdon, 2014). The veterinary surgeon prescribed maropitant (Cerenia 10 mg/ml, Pfizer) 1 mg/kg intravenously, an hour before anaesthetic induction to inhibit emesis and the likelihood of aspiration. It has been suggested that pre-operative drugs to increase the pH of the gastric secretions, such as cimetidine and ranitidine, could be beneficial to avoid lung damage caused by gastric acidity but research on this topic has been conflicting (Green and Figuieredo, 2014). No gastro-protectant drugs were used in this case but could be considered for future practice.
The veterinary surgeon prescribed methadone (Comfortan 10 mg/ml, Dechra) 0.2 mg/kg intravenously 15 minutes prior to induction of anaesthesia. Methadone is a mu (µ) opioid agonist which can cause depression of the cardiorespiratory system by changing the rate and rhythm of respiration (Pattinson, 2008). Methadone was given slowly over 1 minute and the patient's TPR and SpO2 were continually monitored by the author for adverse effects. An alternative to methadone could be butorphanol, a kappa agonist and µ antagonist which has a less potent effect on the cardiovascular system than methadone (Dos Santos et al, 2011). Bronchodilation properties of butorphanol can improve ventilation, however, as the patient was due to have an EMG, it was decided that methadone was a more appropriate choice for analgesia as the EMG can potentially be a nociceptive stimulant (Corletto and Joliffe, 2019). Side effects of opioids are an important consideration in patients with an already compromised respiratory function. The nursing knowledge of the selected drugs allowed the appropriate measurements of undesirable effects to be monitored.
Pre-oxygenation before anaesthesia is necessary to reduce oxygen desaturation caused by induction apnoea (Ambros et al, 2018). Pre-oxygenation enhances oxygen reserves in the pulmonary blood in order to support the patient if apnoea occurs (McNally et al, 2009). Hypoxaemia occurs as a result of respiratory and cardiovascular depression from induction agents, volatile anaesthetic gases and the underlying disease of the patient (Pang, 2016). It is argued that hyperoxaemia can occur through pre oxygenation but its risks compared with hypoxaemia have little significance (Higgs et al, 2015). Oxygen toxicity can result in oxygen free radicals causing injury to lung and myocardial tissue; this contributes to an increased risk of atelectasis, pulmonary oedema and cell death as a result of destruction of antioxidant enzyme systems (Mach et al, 2011). In human medicine, the effect of oxygen free radicals has been shown to cause myocardial infarction in the postoperative period of patients that were exposed to high inspiratory oxygen fractions (FiO2) (Petersen et al, 2018). The effectiveness of oxygenation can be decreased if the method of delivery is not tolerated (Ambros et al, 2018). Oxygen was delivered through a tight-fitted face mask around the patient's muzzle at 5 litres/minute, which was tolerated well. The patient remained saturated at 97–100% (SPO2) throughout the procedure. A circle breathing system was selected so intermittent positive pressure ventilation (IPPV) could be performed if necessary (Hughes, 2016). Fresh gas flow rates were set at 2 litres/minute of oxygen for the first 10 minutes to promote denitrogenisation and rapidly infuse the system with the desired concentration of volatile gas, then reduced to 200 ml/minute (which is supported by Magee and Stanway, 2016). It is recommended to use low flow oxygen rates for rebreathing systems at 5–10 ml/kg/minute for anaesthetic maintenance, which supports the oxygen requirement for the patient and also preserves oxygen supplies and reduces heat loss (Warne et al, 2018). Warne et al (2018) also advised increasing the flow rate to 50–100 ml/kg/minute (same dose as initial flow rates) when changing the volatile percentage during anaesthesia until the desired anaesthetic depth has been achieved. The author used this method while reducing anaesthetic depth at the end of the procedure. Medical air is used in human hospitals, which simulates an oxygen rich version of normal room air whereas in veterinary medicine 100% oxygen is used during anaesthesia. This will increase the risk of oxygen toxicity by comparison, and although the research in this field is limited, oxygen toxicity should be considered during anaesthetic planning. The aim with this case was to achieve normoxaemia peri-operatively to reduce risks of both hyperoxaemia and hypoxaemia by measuring the FiO2 and SPO2. The benefits of preoxygenation should be considered in all patients undergoing anaesthesia regardless of underlying disease.
Induction apnoea from propofol can be avoided if given slowly to effect over 20–40 seconds (Amengual et al, 2013). Rapid sequence induction (RSI) of anaesthesia is used to intubate patients as quickly as possible if aspiration is high risk (Sinclair and Luxton, 2005). Propofol (Propoflo Plus 10 mg/ml, Abbott) 4 mg/kg was the RSI agent of choice and was given intravenously. Alfaxalone (Alfaxan 10 mg/ml, Jurox) is favoured in cases with cardiac disease because of its minimal disruption to the cardiovascular system and could be an alternative in this case (Dehuisser et al, 2019). The patient was intubated with an 8.5 mm ETT and the cuff inflated with the head elevated throughout to avoid reflux and aspiration during this process (Figure 2). The head was then lowered onto the table with the patient remaining in sternal recumbency, once the cuff was appropriately inflated. Isoflurane (Isoflo, Zoetis) was used to maintain the patient under anaesthesia at 1.5%. RSI was necessary in this procedure thus apnoea could not be avoided but managed through preoxygenation and manual ventilation once intubated. Manual ventilation was used post intubation to reduce hypercapnia as the ETCO2 reading increased to 56 mmHg. Spontaneous respiration occurred shortly after manual ventilation started and capnograph tracing and ETCO2 values normalised.
Impaired myocardial contractility and vasodilation caused by isoflurane administration can lead to hypotension and reduced cardiac output (Hathaway et al, 2012). Oscillometry was used to monitor BP throughout anaesthesia and the mean arterial pressure remained above 70 mmHg. The minimum reading for adequate tissue perfusion is 60 mmHg and any value below could damage renal perfusion as well as other vital tissues (Congdon, 2014). Methadone has a minimum alveolar concentration (MAC) sparing effect, which allowed the author to keep the patient maintained on lower percentages of isoflurane (between 1–2%) to avoid potential hypotension (Monteiro et al, 2010). Depth of anaesthesia was monitored by checking jaw tone, palpebral reflex and eye position which allowed the author and the veterinary surgeon to adjust isoflurane controls in order to balance the plane of anaesthesia without increasing the associated side effects. Cardiac activity on ECG showed no abnormalities during the anaesthesia. An isotonic crystalloid (Aqupharm 11, Animalcare Ltd) was administered peri-operatively to counteract the effects of hypotension, maintain electrolyte balance and replace fluid losses from anaesthesia (De Morais et al, 2014). A surgical fluid rate of 5 ml/kg/hour is recommended but reduced rates are encouraged in cases where cardiac disease is present (Robinson and Borgeat, 2016). Volume overload does not always result in an increased cardiac output, it can result in pulmonary oedema and cardiogenic shock because of the effect of increased volume on a heart which has abnormal compliance, valvular morphology or contractility issues. As we are uncertain of the exact cardiac condition this patient has, conservative rates were used during this procedure and Hartmann's solution was delivered through an infusion pump at 3 ml/kg/hour.
As myasthenia gravis was not yet diagnosed, it was uncertain if the muscles to the respiratory system were affected, so in order to best prepare for respiratory arrest or inadequate ventilation, the ventilator was set up on standby. Capnography (ETCO2) and SPO2 were closely monitored to observe for respiratory abnormalities. The capnograph waveform was monitored for a prolonged expiratory upstroke and hypercapnia (values >45 mmHg), which could indicate an obstruction (Schauvliege, 2016). This obstruction could be aspiration of gastric reflux from the megaoesophagus. The patient ventilated well without intervention during the rest of the procedure and the ETCO2 remained between 35–45 mmHg.
Post-anaesthesia
According to Higgs et al (2015), hypoxaemia can occur following extubation, and therefore oxygen therapy is recommended during the recovery phase. Isoflurane was reduced to 0% and the patient continued on 100% oxygen until the gag reflex had returned. As supported by Greene and Grubb (2014), extubation occurred and oxygen was delivered via face mask until SpO2 was maintained at 95% for at least 5 minutes. During the recovery period, the author continued to monitor TPR, BP and SpO2 while the patient remained in sternal recumbency and the chest was auscultated for lung sounds associated with aspiration pneumonia. The patient's anaesthesia was stable and no regurgitation occurred. The patient was positioned in kennels with the head elevated and remained this way throughout hospitalisation.
Conclusion and future considerations
Fasting times need to be explored further as 12-hour fasting can decrease pH of gastric contents and cause reflux, but is advised in cases with a megaoesophagus. This guidance appears to be contradicting as the risk of regurgitation is increased with extended fasting times but recommended by some authors. Blood gas analysis is best to assess the outcome of the care delivered through oxygen therapy and to assess oxygen perfusion and ventilation status, so its use should be encouraged. Although this patient had no side effects of the selected drugs, the author would recommend an open discussion with the veterinary surgeon, offering alternative ideas which may be best suited to the patient's current disease such as the use of alfaxalone over propofol to support the undiagnosed cardiac issue. The inclusion of gastro-protectant drugs could be a central point for debate as their use could potentially support the patient in this situation. Preoxygenation plays a significant role in patients with respiratory disease and can prepare, in advance, for times of low oxygen delivery such as RSI by increasing oxygen saturation. The nursing interventions in a case with myasthenia gravis undergoing anaesthesia are vital in the detection of impending arrest. Early signs of patient deterioration may be identified via trends with vigilant monitoring. Elevation of the head to reduce the risk of aspiration pneumonia is a simple yet important nursing intervention which could change the outcome of a patient with myasthenia gravis. Nurses should be able to associate these interventions with the disease. Without the knowledge of the condition and the required nursing care, the mortality risks of a patient with myasthenia gravis undergoing general anaesthesia would be significantly increased.
KEY POINTS
- Myasthenia gravis is an immune-mediated disease that can cause muscles of the oesophagus to fail resulting in a megaoesophagus.
- Fasting times should be discussed with the veterinary surgeon as this could have an effect on the pH of gastric contents which can cause injury to the respiratory tract if aspirated. The use of gastro-protectant drugs is also something to consider but evidence for their use varies from author to author.
- Rapid sequence induction (RSI) is recommended in cases where aspiration of gastric contents is a high risk.
- Patients should be placed in sternal recumbency with the head elevated until the endo-tracheal tube is cuffed in order to prevent aspiration and aid drainage of gastric fluid down into the stomach.
- Monitoring SpO2 and capnography will help monitor respiratory function during anaesthesia but the use of blood gas analysis is encouraged to get a full understanding of the ventilation status of a patient.
- Oxygen therapy is advised pre-induction to support patients during RSI and postoperatively to avoid hypoxaemia in the extubation period.