A patient care report of a young crossbreed dog with a patent ductus arteriosus

A canine crossbreed, Honey, was presented for investigation of a grade VI/VI left-sided murmur, which had been auscultated at a general health check.

She had been in the owners' possession for 2 days after being adopted from rescue. No previous medical history was known. At home she was reported to be very bright with no exercise intolerance.

Signalment

Species: Canine

Breed: Crossbreed

Age: 2 years

Sex: Female neutered

Weight: 7 kg.

Patient status on presentation

On physical examination the patient was bright, alert and responsive. Heart rate was 88 beats per minute (bpm) (normal rate for adult canine 60–180 bpm dependent on breed, age etc), with a regular rhythm. A continuous grade VI/VI machinery like murmur was audible, with a point of maximal intensity at the left base radiating bilaterally and a palpable thrill. Respiratory rate (RR) was 24 breaths per minute (normal rate 10–30), with no increased effort. Femoral pulses were hyperdynamic but synchronous. Temperature was normothermic at 37.9°C, mucous membranes (MM) were pink and capillary refill time (CRT) was <2 seconds (normal rate 1–2 seconds). She weighed 7 kg, with a body condition score of 4/9.

Differential diagnosis

Given her age, congenital disease was considered the most likely cause of the murmur, with a left to right shunting patent ductus arteriosus (PDA) the top differential based on clinical examination.

The ductus arteriosus is a small duct that connects the aorta and pulmonary artery in the fetus, which should close after birth (Figure 1).

Other less likely diagnoses included concurrent aortic stenosis and insufficiency or an aorticopulmonary window.

Initial diagnostics and results

Echocardiogram

Conscious echocardiography revealed a dilated, volume overloaded left ventricle with left atrial enlargement in the right parasternal four chamber long axis view (Figure 2). Systolic function was good. In the short axis basilar view the main pulmonary artery (MPA) was dilated, a left to right shunting PDA was visible and colour flow Doppler revealed continuous turbulence (Figure 3). Normal sinus rhythm was evident throughout echocardiography on simultaneous echocardiogram (ECG).

Echocardiography is essential for the definitive diagnosis of cardiac disease, allowing for assessment of cardiac structures and function. Colour flow Doppler reveals abnormal flow and in cases of PDA, assessment of the main pulmonary artery will reveal continuous turbulent flow through the ductus. The size of the ductus can also be assessed in this way.

Thoracic radiographs

Lateral radiographs revealed cardiomegaly with a vertebral heart score of 12.3 (normal value <10.5). Left atrial and left ventricular enlargement were visible in both the lateral and dorsoventral (DV) view. The DV radiograph also revealed soft tissue densities in the region of the aorta and the MPA, representing pulmonary artery dilation and the PDA running into the wall of the aorta (Figure 4). There was no evidence of congestive heart failure (CHF).

Thoracic radiography is necessary to identify signs of CHF, which would require immediate treatment prior to ductal occlusion. Although echocardiography allows for more exact measurement and therefore diagnosis of specific chamber enlargement, radiography can also be used to assess cardiac size.

Medical management

The patient was prescribed pimobendan to maintain systolic function at a dose of 0.18 mg/kg per os (PO), twice daily (BID), to be given an hour before food.

Pimobendan is a positive inotrope with a dual mode of action, which increases myocardial contractility and has some vasodilating properties (Ware, 2008).

Initial nursing care

Cardiac patients require gentle handling in order to minimise stress. Increased stress levels trigger the sympathetic nervous system (or the ‘fight or flight’ response), which can result in vasoconstriction, increased heart rate and increased myocardial oxygen demand, which all have a negative impact on the heart. Echocardiography is generally performed without sedation and the majority of congenital patients are young, with low thresholds of tolerance. By maintaining a calm environment, using minimal restraint and calm, reassuring handling, it is generally possible to position patients for long enough for the cardiologist to achieve images necessary for diagnosis.

Generally veterinary nurses will perform radiography in practice, and the same considerations apply for handling of patients whether this is performed conscious or with mild sedation. Good DV and lateral radiographs can usually be obtained with patience. Appropriate collimation and positioning are essential, using foam wedges and sandbags where necessary to achieve non-rotated images. All thoracic radiographs should be taken on peak inspiration to allow for the best visualisation of intrathoracic structures.

Initial outcome

With a diagnosis of PDA confirmed, the patient was referred for ductal occlusion with an Amplatz Canine Ductal Occluder (ACDO). She was discharged to her owners that afternoon with an appointment arranged for surgery 2 weeks later.

PDA occlusion may be performed either via surgical ligation or placement of embolisation coils or ACDO, which is a self-expanding device made of nitinol wire mesh designed specifically for canine PDAs. Transcatheter occlusion is much less invasive as thoracotomy is not required, and has been shown to have lower mortality and complication rates (Ranganathan et al, 2018). In patients under 2.5 kg whose vessels are not large enough to accommodate the size of the catheters required to perform the procedure it is not possible, and surgical ligation is necessary. In this patient, the ACDO was considered the most suitable method of occlusion. With no intervention, patients are likely to deteriorate into heart failure.

Surgical admission

On presentation for occlusion of her PDA the patient was bright, alert and responsive. Heart rate was 96 bpm, with a grade VI/VI left basilar murmur radiating across the thorax and a precordial thrill. RR was 28 breaths per minute with no increased respiratory effort or adventitious lung sounds. MM were pink, with a CRT <2 seconds. Her owners reported she was doing well at home, with no deterioration in exercise tolerance. Weight was recorded at 7.1 kg. She had been starved since midnight in preparation for surgery.

Diagnostics and results

Repeat echocardiography to make initial measurements for sizing of the ACDO revealed a moderately large funnel-shaped PDA. Left ventricular volume overload was evident as previously, with mild left atrial enlargement.

Pre-anaesthesia assessment was performed before premedication and the patient was classified with an American Society of Anaesthesiologists (ASA) status of III (moderate risk) (ASA, 2020), as systemic disease can complicate anaesthesia. An intravenous cannula was placed in the right cephalic vein, and methadone 0.3 mg/kg and acepromazine 0.01 mg/kg were administered intravenously (IV) an hour before surgery. This provided mild sedation.

Anaesthesia and surgery

Anaesthesia was induced with 4 mg/kg propofol, given IV slowly to effect. The patient was intubated and anaesthesia maintained with isoflurane and oxygen. A prophylactic dose of clavulanate potentiated amoxicillin was administered at 16 mg/kg IV, and doses calculated for adrenaline 0.01 mg/kg, atropine 0.04 mg/kg and lidocaine 1 mg/kg in case of emergency during surgery, e.g. bradycardia, haemorrhage, cardiac arrest etc. A wide area over the femoral artery was clipped and prepped for aseptic surgery with surgical scrub. The patient was moved to theatre, where she was positioned in right lateral recumbency and placed on a ventilator to maintain normal respiration during surgery. The area over the right femoral artery was given a final surgical scrub, and a fenestrated drape placed over the site. The femoral artery was catheterised with a vascular access sheath using the modified Seldinger technique. A 6 French pigtail catheter was passed into the aorta just cranial to the ductal ampulla, and angiography performed with 1 ml/kg non-ionic contrast delivered via power injector at 20 ml/second. The PDA was visualised under fluoroscopic image intensification and a moderately large duct revealed, with an ostium measuring 3.9 mm. Following angiography, the pigtail catheter was removed, and a 7 mm ACDO was deployed into the ductus (Figure 5). Repeat angiography confirmed good positioning of the device prior to detachment (Figure 6). Blood pressure was recorded on deployment at 145 mmHg systolic, 60 mmHg diastolic (mean 85 mmHg). Monitoring of heart rate and blood pressure during duct occlusion are critical to monitor for Branham reflex; on closure of the duct, patients may become bradycardic with an increase in blood pressure as a result of a sudden rise in aortic pressures (Madruga et al, 2021). As occlusion of the ductus prevents blood from shunting through the PDA, the heart murmur will no longer be heard as the flow of blood is corrected through the vessels.

The delivery mechanism, catheter and vascular access sheath were removed, and the femoral artery was ligated. The wound was sutured with non-absorbable suture material and a light adhesive dressing was applied. The patient was moved to the hospital following extubation, and temperature, pulse, RR, CRT and MM colour were monitored every 15 minutes until she was completely recovered. She was hospitalised overnight and repeat echocardiography booked for the following morning before discharge.

Patient status on recovery

Overnight the patient was quiet, alert and responsive. HR was 76 bpm with a regular rhythm with no murmur auscultated. Resting RR in her kennel was 20 breaths per minute with normal effort. MM were pink and CRT <2 seconds. She was normothermic at 38.1°C.

Resting energy requirement was calculated as 283 Kcal ([7.1 kg x 30] + 70 = 283 Kcal), 85.5 g per day of her normal diet of Royal Canin Sensitivity Control dry food. She was offered 25 g on recovery, which she ate well, and another two meals of 30 g each through the evening, which she also ate well. She was walked out every 4 hours and passed normal urine each time. Normal faeces were passed at 7 pm.

Surgical nursing care

Veterinary nurses have many responsibilities during cardiac interventional surgeries. Thorough preparation of the surgical suite, including interventional catheters, allows for smooth running of the surgery. Size of catheters will depend on the size and weight of the patient and size of the PDA. The ACDO selected is generally 1.5–2 times the minimal size of the ductus, as measured on echocardiography, but fluoroscopic measurement may differ. Allowing for a few sizes either side of echocardiographic measurement is therefore useful. A list of equipment and consumables that should be prepared is included in Table 1.

Safety sheets should be prepared for each patient, with admission weight and maximum contrast and heparin doses calculated and recorded prior to surgery. Exposure time during fluoroscopy should also be recorded on this form postoperatively.

The veterinary nurse is also responsible for patient preparation pre-operatively. A wide area should be clipped over the femoral artery with care taken to avoid skin irritation. The area needs to be large enough to avoid contamination from hair during placement of catheters. Once clipped, initial skin preparation can be done to remove the worst contamination prior to moving to theatre for final surgical preparation.

Postoperative nursing care

Prior to moving from theatre, a small sterile dressing should be applied to the wound. Careful monitoring should be undertaken until the patient is recovered and ambulatory.

Adequate nutrition should be calculated and provided during hospitalisation, and bedding kept clean and dry. The wound dressing should be monitored closely and changed if necessary to protect the wound.

A key area where veterinary nurses can provide vital support is at discharge. The majority of cases will be more active postoperatively than they were prior to ductal occlusion because of improved cardiac output, and owners should be prepared for this. Exercise restriction should be enforced initially postoperatively to allow for appropriate embolisation of the ACDO and healing of the wound over the femoral artery. The reasons for this should be explained clearly to the owners. Wound care including how to monitor for signs of infection, when to remove the dressing and the use of a buster collar to prevent interference should be discussed, and an appointment made for suture removal.

Outcome

Echocardiography the following morning revealed no residual ductal flow. The patient remained bright, alert and responsive and was discharged into her owners' care that afternoon. Repeat echocardiography was advised 3 months postoperatively to assess cardiac size and function.

Medications

The patient was discharged with 10 days of clavulanate potentiated amoxicillin for prophylactic antibiotic cover. Her owners were advised to continue pimobendan until her next echocardiogram.

Home care instructions

Restricted exercise was advised for 2 weeks postoperatively, with short lead walks only. Her owners were advised to keep her quiet in the house and to use a buster collar to prevent her from licking her sutures. They were advised to visit their first opinion veterinary practice in 10 days for suture removal.

Case discussion

PDA is one of the most common congenital diseases in canine patients (Durham, 2017).

In the fetus oxygen is supplied via the placenta and umbilical cord. The ductus arteriosus is a muscular blood vessel that links the aorta (which carries oxygenated blood from the heart to the body post-partum) and the pulmonary artery (which carries deoxygenated blood to the lungs post-partum) allowing blood flow to bypass the non-functional lungs of the fetus. This duct should close within 24 hours of birth, diverting blood flow to the now functional lungs. Failure of the duct to close results in communication between the aorta and MPA. Provided pulmonary vascular pressures are normal, blood flow shunts through the PDA ‘left to right’, from the high-pressure systemic circulation to the low-pressure pulmonic circulation. Oxygenated blood leaving the left ventricle is shunted from the descending aorta via the PDA into the MPA distal to the pulmonic valve. As a consequence, additional blood passes through the lungs and back to the left side of the heart, causing a volume overload of the left atrium and left ventricle. Surgical or interventional occlusion of a left to right shunt is always indicated in these cases, as disease progression nearly always results in CHF. A continuous murmur is heard as blood flows in the same direction in both systole and diastole. Hyperdynamic or ‘water hammer’ pulses can be palpated, caused by increased systolic pressures as a result of volume overload and rapid fall in pulse pressure caused by diastolic run-off of blood through the PDA (Bonagura and Lehmkuhl, 1999).

If pulmonary vascular pressure increases and systemic pressure becomes equal to or less than pulmonary pressures, then the shunt can reverse and blood can flow from ‘right to left’. This is rare and generally occurs as a result of a large ductus or co-existing pulmonary hypertension. In these patients no murmur is generally auscultated as there is no pressure gradient between systemic and pulmonary circulation.

Disease progression

The size of the PDA will affect disease progression. A small ductus with minimal shunting may have minimal effect on cardiac structure and function, whereas a larger ductus will result in a greater volume of shunting and associated cardiac remodelling. Left untreated, increased circulating blood volume will result in volume overload of the left atrium and left ventricle. This eventually increases left ventricular end-diastolic pressure, leading to CHF.

Aetiology

Certain breeds have been shown to have an increased risk of PDA, including the Bichon Frisé, Chihuahua, Cocker Spaniel, Collie, German Shepherd, Maltese, Pomeranian, Poodle, Shetland Sheepdog and Yorkshire Terrier (Strickland and Oyama, 2016). The disease has a hereditary trait. Females present with a higher incidence than males (Strickland and Oyama, 2016).

Conclusion

Provided that a left to right shunting PDA is treated promptly before permanent cardiac remodelling occurs, prognosis is good. Treatment via either surgical ligation or transcatheter occlusion is curative, and patients generally go on to live normal lives with no negative impact on life expectancy. These are therefore satisfying cases to provide nursing care for.

KEY POINTS

• Patent ductus arteriosus is the most common congenital heart defect in dogs. It is more common in certain breeds (particularly in Poodles, Maltese, Shetland Sheepdogs and Collies). Females are affected more often than males.
• The ductus arteriosus is a normal blood vessel in the fetus, allowing blood ejected from the right ventricle in systole to shunt into the descending aorta, bypassing the non functional lungs. Closure of the ductus should occur within 24 hours of birth.
• Failure of the ductus arteriosus to close results in oxygenated blood shunting from the high pressure systemic circulation of the aorta through the patent ductus arteriosus into the low pressure circulation of the pulmonary artery. This results in additional blood passing back through the lungs and to the left side of the heart, causing a volume overload.
• Common findings at clinical examination include a constant ‘machinery’ like murmur with point of maximal intensity at the left heart base, and hyperkinetic femoral pulses.
• Left untreated, increased circulating blood volume will result in volume overload of the left atrium and left ventricle eventually resulting in congestive heart failure.
• With treatment (closure of the ductus), patients can go on to live full and normal lives provided this occurs prior to permanent cardiac remodelling. Patients with signs of heart failure must be stabilised before surgery, and those with more severe failure or advanced cardiac remodelling have a poorer prognosis. Treatment should therefore be carried out as soon as possible following diagnosis.