Patients undergoing complex open-heart surgery will often be placed on cardiopulmonary bypass, which is a technique where a machine temporarily takes over the job of the heart and lungs, allowing a bloodless surgical field. Staff at the Royal Veterinary College have been performing open heart surgery since 2005 for correction of: pulmonic stenosis, double chambered right ventricle, atrial septal defect, ventricular septal defect, atrioventricular septal defect, Tetralogy of Fallot and mitral and tricuspid valve disease.
Cardiopulmonary bypass is a form of extracorporeal circulation where venous blood is drained via cannulation of the right atrium and right auricular appendage into a reservoir, oxygenated and returned to the body via a pump (Figure 1). Cardiopulmonary bypass is used for both beating and non-beating cardiac surgery.
Therapeutic hypothermia
Therapeutic hypothermia is applied via cooling of the blood in the cardiopulmonary bypass machine (Kanemoto, 2014). This reduces the metabolic rate and oxygen demand from the tissues reducing risk of ischaemic injury, and it also allows the use of slower artificial cardiac output settings, which is advantageous in decreasing air bubbles and therefore reducing the risk of embolic events and limiting trauma to blood cells. The core body temperature is usually only cooled by up to 8°C based on the patient's initial body temperature and is measured at various points in the body and closely monitored throughout surgery, to limit the risk of complications that can arise secondary to hypothermia which include:
- Acid–base disturbances
- Coagulopathy: in temperatures below 34°C thrombocytopathia can develop and platelets sequester in the spleen and other large organs and vessels
- Decreased blood flow to the kidneys
- Hormonal alterations (anti-diuretic hormone, insulin, renin–angiotensin–aldosterone system and cortisol are all affected and catecholamine production can be increased predisposing the patient to arrhythmias)
- Myocardial depression
- Vasoconstriction, which can lead to cold-induced diuresis. This occurs due to the body thinking it has a larger blood volume than it does
- Decreased sodium/potassium ATPase pump activation which raises intracellular sodium levels
- Can lead to impaired oxygen loading and left shift of the oxygen dissociation curve (Figure 2)
- Decreases red blood cell deformability, causing them to become stiff and easily damaged.
Cardioplegia
Once the heart is cannulated and cardiopulmonary bypass initiated to drain the heart, cardioplegia is applied to stop the heart to create a non-beating bloodless surgical field. This makes the heart more accessible and therefore minimizes surgical time, but it also limits myocardial damage by decreasing myocardial oxygen demand. Cardioplegia causes a reversible diastolic cardiac arrest which stops the heart in a flaccid state, which is preferable for surgical approach but also means that it is at its lowest state of energy demand (Carvajal et al, 2023; Greensmith and Barfield, 2023). A heart stopped in the systolic phase is dilated and can affect coronary perfusion, which can lead to irreversible myocardial damage or even death.
Blood cardioplegia is used, which is a mixture of St Thomas's solution (see Table 1), a high potassium solution with magnesium and local anaesthetic and the patient's own blood. This combination helps provide an energy substrate to the myocardium (glucose and lactate) and has been shown to decrease ventricular fibrillation when the heart is restarted (Ibrahim et al, 1999).
Table 1. An example of cardioplegia
| Cardioplegia solutions | St Thomas | Blood and St Thomas |
|---|---|---|
| K+ (mmol/l) | 20 | 20 |
| Na+ (mmol/l) | 144 | 142 |
| Mg2+ (mmol/l) | 16 | 16 |
| Ca2+ (mmol/l) | 2.2 | 1.7 |
| HCO3- (mmol/l) | Added prior to use | 30–40 |
| Procaine (mmol/l) | 1 | 1 |
| pH | 5.5–7.0 | 7.4 |
| Haematocrit (%) | 0 | 10–20 |
| Osmolarity (mOsm/kgH2O) | 300–320 | 310–330 |
Coming off bypass
After surgery is complete, slow measured warming of the patient is performed with the aim to reach around 36°C. The patient is weaned off bypass by incremental decreasing of the machine's flow, allowing the heart to take over. Electrolytes and blood gases are closely monitored to maintain as close to normal levels as possible.
The heart may restart spontaneously, fibrillate or beat with an abnormal rhythm. In some instances, internal pacing wires are placed to restart the heart while the patient is warming (Figure 3). These are usually removed before the patient is recovered to avoid any uncontrolled blood loss during removal.
Once the arterial and venous bypass lines are removed from the patient and chest closure is being performed, protamine sulfate is administered to reverse the action of perioperative heparinisation. Heparin binds to antithrombin, enhancing its inhibition of coagulation factors. Protamine displaces and binds to heparin, reversing its action by forming a protamine–heparin complex that is devoid of anticoagulant activity (Silverstein and Hopper, 2015a). This stable salt aggregate can then be removed via the kidneys (Applefield and Krishnan, 2023). Rapid injection of protamine can cause histamine release by mast cells and can cause hypotension, bradycardia, pulmonary hypertension and respiratory distress (Macintire et al, 2006).
Patients may receive fresh whole blood or packed red blood cell transfusions as required in the postoperative stage. Patients are blood typed prior to surgery and the dedicated blood transfusion team place on hold units of packed red blood cells stored in a specialised blood fridge. Also available are a variety of size units of fresh frozen plasma for management of any ongoing coagulopathies. The units of packed red blood cells can be split to make smaller units according to the patien'ts demand calculated from the patient's packed cell volume and any ongoing blood loss.
Postoperative care in the intensive care unit
Oxygen and warming
The patient is brought through into the ICU and recovered in an oxygen kennel for supplemental oxygen therapy and warming (Figure 4). Patients are initially recovered in 80% oxygen; however, each time that the kennel is opened for patient interactions the oxygen levels will drop, so treatments are grouped together to minimize this. A multiparameter monitor is placed on the patient so heart rate, invasive blood pressure and temperature can be monitored, alongside visual respiratory rate and effort checks from outside of the kennel (Figure 5).
Rebound hypothermia can occur due to cold peripheral blood accumulated during surgery now being recirculated and going to the body core, which, if not supported and monitored, can cause re-warming shock. The majority of the patient warming is done at the end of surgery but some residual effects may last in the postoperative recovery stage. Shivering and pain increases oxygen consumption, so it is important to maintain core body temperature and to control pain postoperatively to limit additional demand on the body. Oxygenation is typically at its lowest between 8 and 12 hours postoperatively but may last up to 48 hours. Thoracic wall pain, atelectasis and surgery itself can all have detrimental effects, so oxygen levels are monitored via arterial blood gases during the first 12–18 hours (Figures 6 and 7). There is a reduction in functional residual capacity up to 50% during the postoperative phase (Polese et al, 1999; von Ungern-Sternberg et al, 2007; Greensmith and Barfield, 2023), which is not fully understood, but may be caused by pulmonary collapse while on cardiopulmonary bypass resulting in inflammation and increased work of breathing. Nasal cannulae are avoided as a source of oxygen delivery due to risk of haemorrhage during placement. Oxygen therapy is slowly weaned over-night and is tailored to the patient's needs based on blood gas results, respiratory rate and effort. Over-oxygenation increases free radicals, can lead to oxygen toxicity and worsen reperfusion-mediated injuries that can occur post-cardiopulmonary bypass (McBride, 2023).
Thoracic drains
A unilateral thoracic drain is placed at the end of surgery to remove air and blood from the thoracic cavity and to monitor ongoing losses in the postoperative phase. Initially the thorax is drained every 1–2 hours, monitoring for fluid and any air accumulation, output is recorded and trends monitored. The thoracic fluid is very haemorrhagic in consistency, so the packed cell volume of the fluid is monitored to compare to the patient's peripheral packed cell volume. It is very common for the thoracic fluid to have a high packed cell volume similar to that of the circulating blood due to haemorrhage during placement onto cardiopulmonary bypass, intraoperative heparinisation and coagulation disturbances (Figure 8). The packed cell volume of the thoracic fluid usually trends down over the first 12 hours postoperatively and the fluid is monitored for clotting. If fluid from the thorax clots, then it can be indicative of active/severe haemorrhage that may require surgical intervention. Fluid from a body cavity does not clot due to a lack of prothrombin activation and fibrinolytic activity occurring in a body cavity when blood remains in a cavity for greater than an hour.
Heparinisation
Strikethrough on surgical dressings is also important to monitor for any ongoing losses and, alongside thoracic drain output, will determine if the patient receives any prophylactic low molecular weight heparin therapy within the first 12 hours post-surgery.
Protamine to reverse intraoperative heparinisation is administered in theatre at the end of surgery; however, rebound heparinisation can occur between 2–18 hours post-protamine therapy. This can occur due to intra/extravascular heparin deposits or due to rapid degradation of protamine. Treatment for rebound heparinisation is subsequent therapy with more protamine. Fresh frozen plasma provides further antithrombin substrate for heparin to bind to (Silverstein and Hopper, 2015b), so fresh frozen plasma transfusions are only administered if enough time has passed since the last heparin dose was administered. Time frame varies by each case – it is very much dose and case dependent. There are no data on dogs undergoing cardiopulmonary bypass on rebound heparinsation and factors that can affect it include dose/s of heparin, duration of bypass, heparin deposition, organ clearance and temperature. Monitoring using other means like activated clotting times may also be inaccurate due to platelets numbers and function. In the author's experience, a criticalist that specialises in perfusion and the care of the patients that undergo cardiopulmonary bypass makes a clinical decision based on the above critieria but plasma would not be given in at least the first 6 hours if rebound heparinisation was considered.
Low-molecular-weight heparin therapy is usually initiated after thoracic drain output is reduced or the drains are removed. Patients are at risk of developing thrombosis on the repaired valve and chordae tendineae due to the body's reaction to artificial chords/sutures and systemic responses from being placed on cardiopulmonary bypass. Low molecular weight heparin provides a safer more predictable anticoagulant therapy, longer bioavailability and lower requirement for consistent anticoagulation monitoring than unfractionated heparin (Merli and Groce, 2010).
Arrhythmias
Common arrhythmias that can develop post operatively are atrial premature contractions and ventricular premature contractions (Figure 9), which as long as they do not impact blood pressure will not require treatment. Atrio-ventricular block (Figure 10) can commonly be seen and is often related to opioids or related to cannulation of the atrium during surgery, causing temporary electrical conduction disturbances. Marked tachycardia, atrial fibrillation and ventricular fibrillation can also be seen. Troubleshooting pain/analgesia levels and any electrolyte disturbances are the first-line treatments; however, tachycardia can also be due to the effects of cardiopulmonary bypass and surgical stimulation releasing catecholamines and increasing cortisol levels which usually normalize after 24 hours. If arrhythmias persist and blood pressure is altered then they will be treated accordingly with anti-arrhythmic therapies.
Blood pressure
Blood pressure is closely monitored throughout the post-operative period via an arterial catheter, or serial non-invasive blood pressure readings via Doppler or veterinary calibrated oscillometric monitor. Hypertension usually occurs due to stress, pain or fear, so 2–4 hourly pain scoring and constant rate infusion of an analgesia like fentanyl in the immediate post-operative phase is needed. Intrapleural local analgesia is instilled every 6 hours after thoracic drainage to provide local regional analgesia to the thorax (Figure 11). Analgesia is tapered based on the patient's pain scores and demeanour.
Some altered pharmokinetic effects with analgesia have been seen in people undergoing cardiopulmonary bypass, especially with fentanyl (Gedney and Ghosh, 1995; Pea et al, 2008; Greensmith and Barfield, 2023). It is reported that large swings between sedation and lack of analgesia can be seen. Although there is no evidence that this occurs in dogs, it is something that should be considered when monitoring our patients.
Transfusion medicine
Patients undergoing complex heart surgeries with the use of cardiopulmonary bypass require varying degrees of supportive transfusion therapies. Previously, fresh whole blood was used at the end of surgery to aid with primary haemostatic dysfunction that occurs due to cardiopulmonary by-pass (Greensmith and Barfield, 2023), but in the absence of fresh whole blood being available, packed red blood cells have been used with no change to outcome in the patients. Further transfusion therapy is then guided by cardiovascular stability, patients' packed cell volume and thoracic drain outputs/patients' ongoing losses in the post operative period. Fresh frozen plasma is only administered if ongoing coagulopathies are contributing to significant blood loss.
Any blood component transfusion is monitored closely initially at 5-, 15- and 30-minute intervals for the first hour, followed by hourly checks of heart rate, respiratory rate, temperature and blood pressure until an hour post transfusion. Blood products are usually administered over a 4-hourly period.
Complications that can occur are transfusion reactions, transfusion-related acute lung injury and transfusion-related circulatory overload. Symptoms of which are pyrexia, vomiting, facial oedema (mainly seen in plasma transfusions), urticaria, increase in respiratory rate and effort, hypotension and haemolysis (McMicheal, 2015).
Electrolyte disturbances
Electrolyte disturbances are commonly seen postoperatively due to chronic diuretic use prior to surgery for treatment of cardiac disease (Greensmith and Barfield, 2023). Potassium levels are often depleted in these patients and so it is common to require potassium supplementation (Table 2).
Table 2. Fluid therapy potassium supplementation
| KCI 20% | ||||
|---|---|---|---|---|
| Radiometer potassium level | add to 250 ml bag | add to 500 ml bag | add to 1 L bag | add to 2 L bag |
| < 2.0 mmol/L | 20 mmol | 40 mmol | 80 mmol | 160 mmol |
| 2.0–2.5 mmol/L | 15 mmol | 30 mmol | 60 mmol | 120 mmol |
| 2.6–3.0 mmol/L | 10 mmol | 20 mmol | 40 mmol | 80 mmol |
| 3.1–3.5 mmol/L | 7 mmol | 14 mmol | 28 mmol | 56 mmol |
| 3.6–4.6 mmol/L | 5 mmol | 10 mmol | 20 mmol | 40 mmol |
Sodium levels can be elevated in the first 8–12 hours postoperatively, which is usually normalised by encouraging the patient to drink and correct any free fluid deficits. However, if sodium values reach in excess of 165 mmol/L then the patient is changed from compound sodium lactate to sodium chloride 0.45%.
Intravenous fluid therapy is usually administered at a rate of 1 ml/kg/hr while the patient is recovering and not eating.
Infection control
Patients require strict barrier nursing due to multi-factorial risk of infection – patients have multiple indwelling lines (central line, arterial line, urinary catheter, peripheral intravenous catheter and a thoracic drain), immunodeficiency due to white blood cell function impairment, consumption of the complement system and dilution of immunoglobulins all related to cardiopulmonary bypass. To decrease the risk of infection, patients receive prophylactic antibiotic therapy for the initial period and all indwelling lines are removed as soon as feasibly possible. Gloves and aprons are worn when handling at all times and thoracic drains are handled aseptically.
Urinary output
Urinary output is closely monitored every 2–4 hours via the patient's indwelling urinary catheter. Urine output is often raised initially, due to neurohormonal and osmolar changes secondary to surgery and cardiopulmonary bypass therapy (including the use of cardioplegia and crystalloid fluid used to prime the extracorporeal circuit) and previous diuretic therapy. Output and specific gravity of urine is monitored in trends and usually decreases in the first 12 hours. Low urinary outputs (below 1 ml/kg/hr) are rarely seen and are usually related to urinary catheter patency, so are always troubleshooted aseptically first. Colour of urine is monitored for haemolysis, which can occur in instances of blood transfusion reaction or due to lysed cells from damage by the cardiopulmonary bypass pump.
Nursing care
Alongside all the intensive monitoring that the patients receive, they also require lots of nursing care, recumbency care, analgesia monitoring and encouragement to eat and drink and, when ready, mobilisation. These patients can be very rewarding but due to the critical nature of their surgery nurses must be prepared for complications to arise and although the success rate is currently 80–90% (author's observations), these patients can also very suddenly deteriorate.
Resuscitation
Resuscitation attempts on cardiothoracic patients are limited. Recover guidelines (Fletcher and Boller, 2023) are followed with the exception of chest compressions, due to the risk of compressions causing irreversible damage to the surgically repaired heart. Therefore, the ECG rhythm is prioritized, with external pacing pads (Figure 12) being used to pace the heart and administer defibrillation if the patient is in a shockable rhythm (pulseless ventricular tachycardia or ventricular fibrillation). Resuscitation attempts carry a poor prognosis: statistics for survival in a non-anaesthetic related arrest are just 6–7% for patient to survive to discharge (Fletcher and Boller, 2023) and coupled with patient age and co-morbidities survival rates can be much lower in these patients.
Biological changes seen in the postoperative stage
Electrolyte disturbances seen during the postoperative phase are caused by metabolic effects of cardiopulmonary bypass.
Vasoconstriction, fluid shifts and hyperglycaemia can all occur and are related to elevated catecholamine release caused by surgical stimulation and cardiopulmonary bypass (Vaska, 1992; Polderman and Girbes, 2004). Anti-diuretic hormone levels increase initially during surgery, but return to normal around 4 hours after surgery. Cortisol levels are elevated and there is an upregulation of atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and the renin–angiotensin–aldosterone system (RAAS), which usually normalise within 24 hours.
Ionised hypocalaemia can be seen, causes of which are chelation of calcium by the anticoagulant ACDA/citrate in the blood products administered, but also due to different biochemical conditions during cardiopulmonary bypass. This does not usually require any treatment and self resolves.
Hypomagnesemia can occur alongside increased renal excretion of magnesium. Magnesium levels are slow to return to normal, but supplementation is rarely required unless consistent arrhythmias occur.
Complications of cardiopulmonary bypass
There are many postoperative complications that can be seen with cardiopulmonary bypass:
- Systemic inflammatory response after bypass syndrome
- Myocardial stunning
- Organ ischaemia and reperfusion injury
- Coagulopathy/thrombocytopathia
- Rebound heparinisation
- Embolism/thrombosis (air, fat, tissue aggregates, white cells, platelets, clots)
- Acid–base and electrolyte disturbances
- Complications from hypothermia (immunopathy, hormonal alterations, diuresis)
- Infections
- Arrhythmias.
Patients are at risk of developing systemic inflammatory response after bypass syndrome. Symptoms include hypothermia or pyrexia, tachycardia, tachypnoea, hypotension, leukocytosis or leukopenia. Causes can be related to:
- Surgical stimulation
- Inflammation due to contact with the extracorporeal circuit
- Blood trauma caused by the cardiopulmonary bypass pump
- Ischaemia
- Reperfusion injury
- Fibrinolytic activation.
Coagulopathies can also occur in patients, the causes of which are:
- Activation of coagulation cascade due to circuit contact
- Fibrinolysis and low fibrinogen
- Fibrin deposition on circuit
- Dilution of clotting factors
- Anaemia
- Heparin therapy
- Hypothermia
- Thrombocytopaenia/thrombocytopathia
- Inflammation-induced endothelial dysfunction.
Conclusions
While these cases are challenging and there are many biological factors that may affect the patient, the key thing as a veterinary nurse is to always focus on the patient and communicate with the team, so any problems can be dealt with quickly to avoid deterioration. Knowing that patients have been given the very best chance of a good quality of life is what makes managing these patients very rewarding.