Incidence of cerebral neoplasia in dogs is 0.014% (Schubert, 2012), with breeds including the Boxer, Boston Terrier and French Bulldog identified as having an increased risk (Song et al, 2013). The most common presenting sign is seizures, of which onset may be sudden or slow. Diagnosis is typically with magnetic resonance imaging (MRI) requiring general anaesthesia of the patient (Schubert, 2012).
Anaesthesia of patients with intracranial disease can be challenging, therefore understanding relevant pathophysiology, pharmacokinetics, and effects of anaesthesia is imperative. The main aims of anaesthesia in these animals are to maintain cerebral blood flow (CBF) and prevent lifethreatening increases in intracranial pressure (ICP), which could lead to brain herniation (Raisis and Musk, 2017). Problems such as these should be anticipated and an appropriate and effective treatment plan developed before induction of anaesthesia to ensure the best outcome for the patient (Quandt, 2015).
This article details the nursing care provided to a seizing dog during the peri-anaesthetic period for advanced imaging to investigate suspected intracranial disease.
Signalment
Species: Canine
Breed: Hungarian Vizsla
Age: 10 years
Sex: Female (neutered)
Weight: 29.0 kg.
Presenting signs
The patient presented following an acute onset of seizures that morning: two generalised seizures lasting around 1 minute each and several focal seizures lasting up to 5 minutes each. The seizure type in this patient was categorised as clustered as there had been more than two seizure events lasting under 5 minutes within a 24-hour period (Platt and Olby, 2017). The patient was reported to have been normal before this acute onset of clinical signs and there was no known toxin exposure. Full biochemistry and haematology blood tests were performed at the referring practice and did not reveal an extracranial cause for the seizures. The patient received a loading dose of phenobarbital 200 mg/ml (Martindale Pharmaceuticals) intravenously at a dose of 12 mg/kg initially, followed by two 6 mg/kg doses in an attempt to control seizure activity. The patient was then referred for further investigations the same day.
At admission the patient was lying in lateral recumbency and appeared sedated because of the effects of phenobarbital administration and a post-ictal phase. She could be roused with auditory stimuli but her vision appeared poor or absent. The patient could walk when assisted to stand but was markedly ataxic in all limbs, and had a tendency to circle to the right. Oral mucous membranes were injected and capillary refill time (CRT) was 1 second (normal 1–1.75 seconds) (Aldridge and O'Dwyer, 2013). Heart rate measured 80 beats per minute (normal 60–100 beats per minute) (Aldridge and O'Dwyer, 2013), with synchronous femoral pulses, and respiratory rate was 16 breaths per minute (normal 10–12 breaths per minute) (Aldridge and O'Dwyer, 2013). Rectal temperature was 39.4°C (normal 38.3–39.2°C) (Goddard and Phillips, 2011).
Proprioception was delayed in all limbs, possibly worse on the left side. Withdrawal reflexes were normal in all limbs, the patellar reflexes were intact and the cutaneous trunci reflex was intact at the level of L5. Cranial nerve examination revealed poor menace responses in both eyes, particularly the left side. Nasal septum nociception was present but reduced bilaterally. There was no visible anisocoria, strabismus or nystagmus. The patient appeared reluctant to allow lateral neck movement to both sides and showed possible mild discomfort on direct palpation of the temporal region of the head. General clinical examination did not reveal any significant abnormalities.
Findings and history of seizures were consistent with forebrain neuroanatomic localisation, most likely on the right side. Differential diagnoses included an intracranial mass, inflammatory disease or an ischaemic event.
Veterinary investigations
The patient was admitted to the hospital and the intravenous cannula placed at the referring veterinary practice was checked and redressed immediately. A venous blood sample was taken from the left cephalic vein to test blood glucose which measured 7.3 mmol/litre.
Following examination the patient was administered butorphanol 10 mg/ml (Torbugesic, Zoetis UK Limited) at a dose of 0.2 mg/kg intravenously as a premedication, followed by co-induction of general anaesthesia with midazolam 5 mg/ml (Hypnovel, Roche Products Limited) at a dose of 0.2 mg/kg and propofol 10 mg/ml (Propoflo, Abbott Animal Health) 0.5–2 mg/kg intravenously to effect. Anaesthesia was maintained using the inhalation agent sevoflurane in conjunction with oxygen. Intravenous fluid therapy (IVFT) was instigated using lactated Ringer's solution (Vetivex 11, Dechra Veterinary Products) at a rate of 5 ml/kg/hour. Electrocardiography (ECG), capnography, blood pressure (BP) and oxygen saturation (SpO²) were all measured continuously for the duration of anaesthesia using an MRI safe multi-parameter patient machine (Figure 1) (3880 MRI Patient Monitor, IRadimed, Florida, USA).
MRI of the brain was performed, revealing a mass in the region of the right frontal lobe, causing compression of the adjacent parenchyma and peri-lesional oedema. The imaging characteristics were most consistent with a meningioma. The patient was recovered and kept in the hospital for monitoring and medications overnight.
Discussion
In this case monitoring end-tidal CO2 (ETCO2), BP, temperature, and signs of increased ICP were the key considerations and will be discussed in further detail.
Blood pressure
The function of the brain is dependent on maintaining cerebral circulation. Intracranial disease can interfere with this, causing ischaemic damage. Perfusion is directly dependent on mean arterial pressure (MAP), so BP monitoring in these cases is essential (Leece, 2011). In this patient oscillometric readings were deemed most appropriate. Doppler and invasive techniques, although possibly more accurate, were not used as the Doppler contains metal, and is therefore not MRI safe, and placement and set up for an arterial line was deemed time consuming, invasive and costly (Bosiack et al, 2010).
In this case a MAP <60 mmHg was identified as an intervention point on the anaesthetic checklist. BP readings were taken every 3 minutes and MAP never went below 64 mmHg. Some literature recommends maintaining MAP between 70–80 mmHg in patients with intracranial pathology as optimal for sustaining adequate perfusion (Leece, 2011; Raisis and Musk, 2017). Higher target MAP will be suggested for future cases, and interventions such as boluses of IVFT discussed to form part of the plan.
Hypertension can indicate increased ICP as the patient attempts to maintain cerebral perfusion. Commonly seen in conjunction with bradycardia and respiratory disturbances, these three symptoms form what is known as the Cushing's triad, suggesting vital medullary centres may be compromised. It is therefore vital that multi-parameter monitoring is conducted consistently to detect changes in these parameters (Leece, 2011). These parameters were monitored continuously throughout induction, maintenance and recovery. The patient was never bradycardic, with a heart rate of between 80–100 beats per minute, her MAP stayed between 64–110 mmHg, and mechanical ventilation was used to promote normocapnia. Intervention with administration of mannitol 10% at a dose of 0.5 g/kg intravenously was recommended if MAP >160 mmHg.
On reflection BP was monitored to a good standard in this case, and the most suitable MRI safe technique adopted. Invasive monitoring of BP may be possible in future cases if tubing was extended sufficiently to allow all metal parts, such as the transducer, to be outside of the scan room. Measurement intervals were appropriate, however, in the future higher target MAP will be suggested.
Intracranial pressure and end-tidal CO2
Increased ICP occurs when pathological increases in intracranial volume exceed compensatory decreases in other intracranial tissues. This causes reduced cerebral perfusion pressure (CPP), ischaemia, dysfunction and neuron death. Many factors affecting these variables can be manipulated by the anaesthetist, and decreases in ICP can be achieved with informed drug selection, controlled induction, appropriate monitoring, patient positioning, and promoting stress free recovery (Raisis and Musk, 2017).
Drug selection
Drug selection for premedication will vary depending on the presenting mental state of the patient. Opioids can be useful for analgesia, however, low doses should be used to avoid respiratory depression and consequential increases in ICP. In this case butorphanol (Torbugesic, Zoetis UK Limited) was selected; butorphanol is a synthetic opioid partial agonist analgesic. This was because the patient was sedated at presentation and was not overtly painful. Butorphanol does not alter CBF, or increase ICP, and has minimal respiratory and cardiovascular depressive effects when used at low doses, such as in this case 0.2 mg/kg intravenously (Raisis and Musk, 2017). Previous drug administration and post-ictal changes made it challenging to assess pain accurately in this case.
When full agonist opioids are required morphine should be avoided as it commonly induces vomiting causing increased ICP (Leece, 2011). Low dose fentanyl (1–5 μg/kg) could be considered via intravenous or intramuscular injection. Fentanyl not only causes minimal respiratory and cardiovascular depression at this dose, but is short acting and can be administered as a continuous rate infusion, enabling easy titration in unstable patients (Raisis and Musk, 2017). This will be suggested for future cases requiring additional analgesia.
Ketamine, a dissociative anaesthetic agent is associated with increases in ICP and higher incidence of seizures, so should be avoided in these cases. Sedative drugs such as dexmedetomidine, an alpha-2 agonist, can be associated with marked sedation, cardiopulmonary dysfunction, increased MAP and bradycardia, simulating the Cushing reflex, so may not always be a suitable option (Raisis and Musk, 2017). In contrast to this, the increase in MAP may help maintain CPP, therefore it should be considered for use on an individual basis (Arulvelan et al, 2016). On reflection pre-medication selection in this case was appropriate.
Patient positioning
The patient was positioned with her head elevated on a bean-bag at an angle of approximately 30 degrees (Figure 2). Care was taken when handling and positioning not to occlude the jugulars, preventing increased ICP and promoting venous drainage (Raisis and Musk, 2017). Bean bags have limitations in providing stable and consistent positioning. Angles may be inaccurate and changeable, easily resulting in over or under extension of the head and occlusion of the jugular veins. Consequential increases in ICP may cause brain dysfunction and cerebral ischaemia, followed by poor cerebral perfusion and brain herniation (Sturges et al, 2019). One study described the use of a craniotomy head stand to avoid jugular vein compression, which although ideal for accurate head elevation during surgery, if made of metal would not be appropriate for use during MRI (Seki et al, 2019). Non-magnetic head stands will be investigated for future use as they would enable much more accurate, consistent positioning, preventing jugular compression and increases in ICP.
Intracranial pressure measurement
Direct ICP measurements are the most accurate way to assess ICP and can be obtained via a transducer placed in the cerebral hemisphere. This technique however, is extremely invasive and costly, usually placed during craniectomy so would not be appropriate in this case. If this patient was to undergo surgery this technique could be considered to enable accurate monitoring of ICP and faster response times (Seki et al, 2019).
Patient induction
Induction of anaesthesia was instigated with an initial dose of 0.5 mg/kg propofol 10 mg/ml (Propoflo, Abbott Animal Health, followed by 0.2 mg/kg midazolam 5 mg/ml (Hypnovel, Roche Products Limited). Further propofol was then administered slowly to effect until palpebral response was sluggish and the patient did not respond to stimulus. This sequence of drug administration during co-induction has been shown to produce less excitatory phenomena and is drug sparing when compared with giving each drug separately (Sánchez et al, 2013).
Appropriate anaesthetic depth must be reached before any attempt to intubate as coughing or gagging will increase ICP. Co-induction helps reduce the amount of induction agent required, associated side effects, and with midazolam specifically can prevent coughing and reduce seizure activity (Liao et al, 2017), a factor especially important in this case. Use of lidocaine or fentanyl intravenously as co-induction agents has been proven to reduce cough response significantly, so should could be considered for use in the future (Thompson and Rioja, 2016; Bravo et al, 2020).
End-tidal CO2
After induction the patient's ETCO2 increased from 35 mmHg to 43 mmHg, at this point mechanical ventilation (Penlon, Nuffield 200, Oxford, UK) was instigated to maintain an ETCO2 lower than 40 mmHg as per the intervention plan (Figure 3). It is generally recommended to maintain ETCO2 between 30 mmHg and 35 mmHg (McMillan, 2015) in anaesthetised patients. Prolonged hypocapnia below 30 mmHg can cause vasoconstriction and reduce CBF. The opposite is true for increases in carbon dioxide, which can lead to increased ICP. ETCO2 remained between 33–37mmHg for the duration of anaesthesia so no other interventions were required. This intervention was vital and appropriate in this case, and prevented further increases in ICP and associated consequences (McMillan, 2015).
Patient temperature
Patient temperature could not be monitored during the scan as an MRI safe thermometer was not available. This was concerning as the patient had a history of seizures and presented hyperthermic. The MRI scan room is kept cool to facilitate running of the machine and so temperature loss is typically prevented with blankets and bubble wrap. The patient's temperature at induction was 39.4°C, so it was decided not to use warming aids in this case, however any changes would not be noted until the patient returned to the recovery area. This prevented timely interventions and has previously resulted in patients recovering from MRI both hyper and hypothermic.
At extubation the patient's temperature was 37.5°C and she was returned to a heated kennel and covered with a blanket. Any panting or shivering in an attempt to thermoregulate can result in alterations to ICP and have severe consequences, so normothermia should be promoted swiftly (McMillan, 2015). In the future it will be suggested that either patients be taken out of the scanner and a rectal temperature taken regularly, or the practice invests in a fibre-optic temperature sensor which is unaffected by electromagnets. Temperature management could have been better in this case, luckily there were no detrimental consequences but in the future improvements must be made.
Extubation and recovery
Before extubation it should be ensured that the patient can ventilate spontaneously, adequately, and maintain normocapnia. This is especially important when mechanical ventilation has been used, as it may take longer for normal respiratory function to return (Beckett, 2016). Timely recovery must be balanced with avoiding excessive stimulation, which could cause coughing or hypertension, increasing ICP (Raisis and Musk, 2017). Recovery in this case was smooth, the patient was extubated when swallow returned and she remained calm and settled on her way back to the dog ward.
When recovering seizuring patients with intracranial disease it is important any potential excitement or agitation is minimised. This can be achieved by recovering individuals in a quiet, dimly lit, warm room, with the capacity to be monitored closely (Sines, 2018). In this case the patient was recovered in the dog ward, often a noisy, bright and a busy area of the hospital. In hindsight it would have been better for this patient to recover in the intensive care unit (ICU), a smaller, quieter room, with adjustable lighting and a dedicated nurse looking after fewer patients. This type of recovery has been proven to reduce stress, incidence of seizure activity, and may be the only treatment required for some agitated patients (Forsgård et al, 2019). In the future the author would recommend all seizure patients be recovered in ICU for these reasons.
Conclusions
This case highlights the importance of veterinary nurses having a comprehensive understanding of the relevant pathophysiology, pharmacokinetics, and effects of anaesthesia when nursing patients with suspected intracranial disease. Creation and implementation of a suitable anaesthetic plan to include clear intervention points will facilitate a smoother induction, maintenance and recovery from anaesthesia, ensuring the best possible outcome for the patient. Multiparameter monitoring throughout each of these stages is vital to allow swift recognition and rectification of potentially life-threatening complications. Recovery of patients with intracranial disease should be conducted in a calm, quiet environment to prevent excitement or agitation to reduce incidence of seizure activity.
KEY POINTS
- Investigation of suspected intracranial disease often requires general anaesthesia for advanced imaging. Physiologic and homeostatic changes often make anaesthetic management of these cases challenging.
- The main aims of anaesthesia in these patients are to maintain cerebral blood flow (CBF) and prevent life-threatening increases in intracranial pressure (ICP), which could lead to brain herniation.
- Creating and implementing suitable anaesthetic management plans will facilitate smoother induction, maintenance and recovery from anaesthesia in these cases.
- Multiparameter monitoring of these patients throughout all stages of anaesthesia is imperative to enable timely interventions and prevent life-threatening complications.