The patient was referred for investigation of syncopal episodes, which began 2 months before presentation. She was reported to have weakness and collapse during periods of excitement or exertion. Her owner reported that during an episode she would become weak on her hindlimbs before falling onto her side. No loss of consciousness was reported and recovery was prompt, with normal behaviour and mentation regained within seconds.
Patient signalment
Species: Canine
Breed: Terrier cross
Age: 13 years
Sex: Female neutered
Weight: 9.5 kg.
Patient status on presentation
The patient was bright, alert and responsive (BAR) on presentation. She had a body condition score of 6/9 and weighed 9.5 kg. Heart rate (HR) was 88 beats per minute (bpm) (normal rate for adult canine 60–160 bpm) with synchronous femoral pulses. Respiratory rate (RR) was 24 breaths per minute (normal rate 10–30 breaths per minute) with no respiratory effort. Cardiac auscultation revealed no heart murmur, but an irregularly irregular rhythm was audible, with frequent pauses noted. No jugular venous distension or pulsation was present and abdominal palpation was unremarkable.
Differential diagnosis
Based on the presence of an irregularly irregular heart rhythm with frequent pauses alongside a history of syncope, the most likely differential diagnoses were sick sinus syndrome or atrioventricular (AV) block. In symptomatic patients with high grade second degree or third degree AV block, sick sinus syndrome, persistent atrial standstill and vasovagal syncope, pacemaker implantation is required to alleviate clinical signs and prolong survival (Santilli et al, 2019).
Initial diagnostics and results
It is essential that patients identified as possible candidates for pacemaker implantation have a thorough physical examination, cardiovascular and medical assessment in order to check for any disease which might require further investigation or require treatment prior to surgery.
Electrocardiography
Electrocardiogram (ECG) is necessary for definitive diagnosis of abnormal rhythms. ECG in this case revealed frequent pauses with otherwise normal complexes and normal P-QRS association. Heart rate was within normal limits at 86 bpm.
Echocardiography
Echocardiography allows for assessment of myocardial function, chamber size and cardiac remodelling. The patient's echo revealed moderate thickening of the mitral valve leaflets. Colour flow Doppler revealed a small regurgitant jet through both the mitral and tricuspid valves. Left atrial and ventricular measurements were within normal limits, and systolic function was good. The patient was diagnosed with AV valve disease stage B1 (asymptomatic dogs with valve disease but no cardiac remodelling requiring no treatment (Keene et al, 2019)).
Holter monitoring
Ambulatory ECG monitoring was performed in order to assess the cardiac rate and rhythm during syncope. The benefit of ambulatory monitoring over standard ECG is that intermittent arrhythmias can be identified that may not be noted during a short recording, the frequency and severity of the arrhythmia can be determined and the effects of exercise can be assessed. The ambulatory monitor identified predominant rhythms of normal sinus rhythm and sinus arrhythmia (a normal variation where heart rate increases on inspiration and slows on expiration). However, numerous long pauses were revealed, occurring during both higher and lower heart rates. The patient experienced one episode of collapse, which was related to a 6 second pause with no escape rhythm (Figure 1). There were no unconducted P waves identified, and a diagnosis of sick sinus syndrome was made.
Medical management
The patient was prescribed theophylline 10 mg/kg twice daily (BID) while her owner decided whether to proceed with surgery for permanent pacemaker implantation. Theophylline is a methylxanthine bronchodilator and beta-2 agonist with sympathomimetic effects. While commonly used in respiratory cases, its sympathomimetic effects can increase heart rate when used at higher doses in dogs with sick sinus syndrome as a result of stimulation of the central nervous system, although these effects may not be permanent.
Initial nursing care
One of the main areas in which veterinary nurses can provide care during the initial phase of investigation for syncopal or arrhythmic patients is placement of ambulatory or Holter monitors, which are externally-worn continuous ECG recorders (Figure 2). These monitors are generally left in place for 24 hours while the patient carries out normal daily activities, but monitoring can be continued for up to a week.
The quality of the Holter reading is dependent on the quality of placement, and preparation is key. An area each side if the chest should be clipped closely, cleaned thoroughly and wiped with surgical spirit. This helps to remove any dirt, grease or dead skin cells which may cause the ECG pads to not adhere sufficiently and result in loss of lead connection during the monitoring period. Once sufficiently dry, ECG pads are placed either side of the chest and leads attached. A new battery should be used for each patient to ensure adequate recording time can be achieved.
The monitor is then held in position with the use of specialised elasticated vests or T-shirts (Figure 3). In very active dogs where there is a risk that the leads may become detached during activity, it can be necessary to use wide tape over the top of the leads as an extra layer of security before covering with a Holter vest.
Owners are expected to complete a diary during the period that the patient is wearing the monitor to record all activity including exercise, sleeping and feeding alongside any symptoms such as weakness, abnormal behaviour or collapse. This provides the cardiologist with data that can be analysed alongside the ECG data. Symptoms can then be correlated with any rhythm abnormalities such as bradycardia during exercise or tachycardia during sleep. The veterinary nurse plays a key role in advising owners on the care of the monitor, appropriate removal and on the relevant information to include in the diary.
Initial outcome
A partial response was achieved with medical management but the patient continued to collapse with excitement and therefore her owners opted to proceed with pacemaker implantation.
Surgical admission
On presentation the patient was BAR, with a heart rate of 69 bpm, regular sinus rhythm and strong, synchronous pulses. RR was 40 breaths per minute with no respiratory effort. Mucous membranes were pink and moist, with a capillary refill time (CRT) under 2 seconds. Bodyweight was recorded as 9.4 kg with a body condition score of 5/9.
Preoperative nursing care
Prior to premedication an area over the right saphenous vein was clipped and local anaesthetic (EMLA 5% cream) applied underneath a plastic glove with a dressing over (to prevent absorption of the cream by the dressing). The patient was then clipped in preparation for the procedure from the base of the skull to the shoulder and on the right side just beyond the midline both dorsally and ventrally. Two large areas were clipped either side of the thorax to allow for external defibrillation in case of emergency. Veterinary nurses are generally responsible for patient preparation before surgery, and so having knowledge of the procedure being performed and the surgical site allows for appropriate clipping and skin preparation, reducing the risk of infection. This is particularly important with pacemaker implantation, where the lead remains in position in the ventricle permanently and infections can spread through the system, which can necessitate removal.
Anaesthesia and surgery
Pre-anaesthesia assessment was performed before premedication and the patient was classified with an American Society of Anaesthesiologists (ASA) status of III (moderate risk) as systemic disease can complicate anaesthesia. An intravenous cannula was placed in the left cephalic vein and methadone 0.2 mg/kg administered intravenously (IV) for premedication, providing mild sedation and a good level of analgesia.
The patient was moved into theatre while still conscious for placement of transvenous temporary pacing leads before induction of anaesthesia. Patients with sick sinus syndrome have a higher risk of mortality during anaesthesia as physiological changes caused by anaesthesia drugs can supress the escape focus resulting in asystole.
The patient was positioned in left lateral recumbency and restrained gently while a 5 French introducer catheter was placed in the right saphenous. The temporary lead was then guided fluoroscopically into the right ventricle (Figure 4) and connected to a temporary pulse generator. ECG was monitored closely to ensure consistent pacing had been achieved and heart rate maintained at 90 bpm. Prior to initiation of temporary pacing, several pauses were noted on ECG, with a reduction on systolic blood pressure, but the patient stabilised with pacing.
Once control of the heart rhythm was established, anaesthesia was induced with alfaxalone 2 mg/kg IV and midazolam 0.25 mg/kg IV. The patient was intubated and anaesthesia maintained with sevofluorane and oxygen. A prophylactic dose of clavulanate potentiated amoxicillin was administered at 20 mg/kg IV.
The patient was positioned with a wedge under the neck in order to ease jugular access and a final surgical scrub was performed before draping.
An incision was made over the right jugular vein, which was exteriorised for introduction of the permanent pacing lead. A Medtronic 52 cm bipolar active fixation pacing lead (Figure 5) was advanced under fluoroscopy and positioned in the apex of the right ventricle.
The permanent lead was then connected to a temporary pacing box and threshold and settings checked to confirm appropriate positioning. A Medtronic Sphera pacemaker generator (Figure 6) was then implanted in a small pocket under the cutaneous trunci muscle. With this in place, the permanent lead was tunnelled under the skin to the generator and connected via set screws. With permanent pacing established, the temporary lead was removed and the pulse generator pocket and incision site were closed routinely.
The surgical site was bandaged with an absorbent foam dressing directly over the wounds, secured with padded bandage, conforming bandage and cohesive bandage to protect the wound and to help prevent the formation of a seroma. Final interrogation of the pacemaker was performed and parameters set using the external programmer.
Surgical nursing care
Veterinary nurses are responsible for several key roles in interventional cardiac surgeries, from setting up the surgical suite, preparing interventional kits and catheters, positioning the patient for surgery and assisting during the procedures. In pacemaker implantation the veterinary nurses may be responsible for restraining the patient for placement of the temporary lead prior to anaesthesia. Care must be taken during this phase to keep the patient calm and still. All equipment should be opened as quietly as is possible and conversation kept to a minimum as sudden or loud noises may startle the patient.
The veterinary nurse may also be expected to assist during temporary pacing by handling the non-sterile end of the leads used to connect the temporary pulse generator to the leads, adjusting the pacing rate as directed. Final interrogation and programming of the pacemaker may also be performed by trained veterinary nurses prior to recovery.
An essential role for the veterinary nurse is aseptic bandaging of the patient, ensuring appropriate pressure is applied to help to protect the wound and prevent seroma formation (Figure 7).
Patient status on recovery
The patient recovered from the anaesthetic uneventfully, and remained comfortable overnight. Meloxicam 0.2 mg/kg was administered IV for pain relief. The patient was quiet, alert and responsive. Her HR was between 72 and 88 bpm, resting RR was between 16 and 28 breath per minute with normal effort, and mucus membranes were pink with CRT <2 seconds. Temperature was not monitored to avoid patient stress. She ate well overnight.
The following morning the dressing was replaced and the pacemaker was interrogated — parameters were assessed to check for any irregularities, e.g. loss of capture (where the pacemaker output fails to depolarise the ventricle). A threshold and sensing test were performed, lead impedance assessed, and battery life recorded.
Threshold is the minimum amount of energy required for the pacemaker to cause the ventricle to depolarise and a threshold test involves a gradual reduction in current (or amplitude) until capture is lost. Resistance to this flow of current (called impedance) is created by the lead and heart tissue, and is measured in ohms. During the initial healing period while the lead becomes surrounded by tissue, resistance will increase meaning that less current reaches the myocardium. For this reason, threshold is initially set to at least three times threshold measured to allow a safety margin. This is later reduced to at least twice threshold once impedance reduces.
If the threshold is too low, depolarisation will not occur and if it is set too high then the battery life will be reduced.
A sensing test ensures that the pacemaker is correctly identifying the patient's own ‘intrinsic’ heart beats. This can be thought of as a wall — any complexes with an amplitude lower than the set sensitivity (2.8–5.0 mV normal) will not be seen as the ‘wall’ is above this height (Figure 8; Swift, 2018). If the sensitivity is too high, the pacemaker will not identify intrinsic beats and the pacemaker will continue pacing unnecessarily. If sensitivity is too low, the pacemaker may identify P or T waves as R waves, and prevent the pacemaker from pacing when it is needed.
Postoperative nursing care
It is essential that patients remain calm postoperatively following pacemaker implantation because of the risk of lead dislodgement. Patients that jump or climb up the front of the kennels or that are very active may require sedation during the initial 24-hour recovery period.
The wound dressing should be removed the day after surgery and the wound assessed and cleaned if necessary before redressing. This should then be repeated every 2–4 days until suture removal, generally 10–14 days post-operatively. Seroma and signs of infection should be monitored for carefully as infection may not only delay healing time, but also carries the risk of infection travelling down the lead which could result in fever, lethargy, embolic events, endocarditis and sepsis. Infection may necessitate lead removal and replacement at a later date (which may not be feasible in pacing-dependent patients).
It is important that it is clearly noted on the patient's records that a pacemaker has been implanted, and communication by the veterinary nurse with the referring veterinary practice to ensure this information is accurately recorded is essential. Jugular blood samples should never be taken from the right jugular in patients with a pacemaker because of the risk of lead damage, and peripheral samples are therefore favoured. The use of electrocautery should be avoided during any subsequent surgeries. Following the death of these patients, the pacemaker generator must always be removed before cremation because of the risk of explosion.
Owner education postoperatively is also an important part of the veterinary nurse's role. Patients should be walked on a carefully fitted harness that does not rub on the pacemaker lead, and should never wear collars or neck leads because of the risk of lead fracture. They should never wear electric shock collars and should remain clear of strong magnets (e.g. magnetic resonance imaging (MRI)) unless the pacemaker is an MRI safe model, as most newer ones are. Patients must be rested strictly for at least the first 4 weeks following implantation to allow the lead to embed sufficiently and avoid the risk of lead dislodgement. Only short lead walks should be undertaken with no running or jumping up. Crate training before surgery can be useful.
Medication
The patient was discharged with amoxicillin clavulanate 10 mg/kg per os (PO) BID and robenacoxib 1 mg/kg PO once daily for 7 days for pain relief; theophylline was discontinued.
Outcome
A follow-up appointment for pacemaker interrogation was booked for 6 weeks postoperatively, with instructions for the patient to attend her referring veteterinary practice for suture removal 10 days following discharge.
Case discussion
Sick sinus syndrome is a term given to a number of ECG abnormalities of the sinoatrial (SA) node, including severe sinus bradycardia and severe SA block (Tilley, 1992). One or more areas of the conduction system may be involved.
ECG features can be variable and include severe bradycardia not induced by drugs or other systemic disease, long pauses, which may follow atrial premature complexes, and escape rhythms can be seen. In some cases, severe bradycardia may alternate with supraventricular tachycardia, known as ‘bradycardia-tachycardia syndrome’.
Symptoms depend on the ECG abnormalities, but include weakness, lethargy, exercise intolerance and syncope caused by reduced cardiac output.
Breeds commonly affected include West Highland White Terriers, Cocker Spaniels and Miniature Schnauzers. Females are reported to be more commonly affected (Willis, 2018).
Permanent pacemakers are the treatment of choice for symptomatic patients with sick sinus syndrome. While sudden death is a rare outcome in these patients, quality of life can be severely affected. Medical management may be possible in asymptomatic and less severely affected patients, but sick sinus syndrome will generally worsen within a short period of time (Kraus and Gelzer, 2016) and permanent pacemaker implantation is necessary for symptomatic patients. Following the necessary period of rest postoperatively, patients are reported to have an improved quality of life (Ward et al, 2016)
Conclusions
Nursing care for patients requiring pacemaker implantation requires varied skills in many areas including gentle patient handling, performing varied diagnostic tests, surgical nursing and wound care. Good communication skills are essential in these cases as there is a large amount of owner compliance required through all stages, and communication with referring practices is essential to ensure long-term patient safety.
KEY POINTS
- Pacemaker therapy is the treatment of choice for patients with symptomatic bradycardia.
- Patients should receive a full medical and cardiovascular work up prior to surgery to identify any possible conditions that may contribute to symptoms or complicate surgery.
- Anaesthesia drugs may supress the patient's inherent escape focus, which is their rescue mechanism to counteract dangerous pauses or bradycardia. This may result in asystole. For this reason, heart rate control is implemented with temporary pacing leads prior to anaesthesia induction.
- Owner education is essential for successful patient recovery. Postoperative care considerations such as exercise restriction, the use of harnesses rather than leads and long-term considerations such as blood sampling require good, clear communication.
- Following recovery patients can lead fuller, more active lives.