Appendix C: Postoperative complications in cardiac èurgery
Postoperative problems common to all patients occur following cardiac surgery (chest infection, wound infection, etc.) but some are characteristically seen in cardiac patients.
Neurological sequelae
For most patients, cardiopulmonary bypass leaves no obvious neurological deficit but, on detailed psychological testing, subtle abnormalities may be detected that often persist for up to 1 year. There is a risk of permanent stroke in the order of 1-2 per cent depending on age and carotid disease. This fact should be mentioned to all patients undergoing any cardiac surgery.
Poor cardiac output
There are several mechanisms for this complication in the early postoperative period, namely:
· poor myocardial function;
· infarction;
· tamponade or bleeding;
· intravascular volume depletion.
The diagnosis is usually simple as the patient is peripherally cold, with a low blood pressure and poor urine output.
Treatment
In the absence of tamponade, any underfilling should be corrected. Rhythm disturbances are treated and, if the low output state persists, the heart may require pharmacological or mechanical support.
Pharmacological support. Inotropic drugs act in a variety of ways to alter the systemic vascular resistance, increase the heart rate and increase the force of myocardial contraction. Commonly used inotropes indude isoprenaline, dopamine, dobutamine, adrenaline and noradrenaline; they are commonly used in conjunction with vasodilating agents which decrease the afterload.
Dobutamine is a 3-agonist with chronotropic and inotropic properties and limited vasodilatation. The dose is titrated to the haemodynamic response and the usual range is 5-15 mg/kg per minute. Dopamine at low doses (<5 mg/kg per minute) has the beneficial effect of augmenting renal blood flow At higher doses it constricts the renal arteries and has an inotropic effect (Table 48.1).
Judicious use of inotropes and vasodilators, with the use of physiological measures (heart rate, blood pressure, central venous pressure and Swan-Ganz measurements), help to optimise the cardiac output.
Rhythm disturbances. Atrial arrhythmias are common after cardiac surgery and must be treated if persistent or causing haemodynamic compromise. The most common
arrhythmia is atrial fibrillation which occurs in 20-3 0 per cent of postoperative patients. Digoxin is a common treatment but calcium channel blockers or membrane stabilisers may be used. DC shock is only used if urgent control is necessary. Bradycardia is seldom seen, but temporary followed by permanent pacing may be required. Any arrhythmias may occur and the surgeon must be familiar with their cause, pattern, treatment and prevention.
Cardiac support. There is a number of ways to support the failing heart in the anticipation that it will (1) recover some or all of its function or (2) maintain the circulation long enough for cardiac transplantation.
There are circumstances when conventional cardiopulmonary bypass (from right atrium to aorta) is either not feasible, as in aortic dissection, or not advisable (multiple injury), and other means of supporting the circulation are employed.
Left and right heart bypass. This employs a similar technique to cardiopulmonary bypass except oxygenation is not employed and the returning blood is channelled into the same side of the circulation. In right ventricular dysfunction, blood is drained from the venous system, passed through the pump and returned to the pulmonary artery, thereby bypassing the right ventricle. It is useful when temporary dysfunction of a ventricle occurs and full recovery is expected. It has also been used on occasions to support the heart until transplantation.
Mechanical support (Fig. 48.63). If low cardiac output persists in spite of inotropic support, the heart may require mechanical support while it recovers its function. The intraaortic balloon pump is a device that is inserted, either percutaneously or under direct vision, into the common femoral artery. It is threaded into the aorta until its tip lies just distal to the arch vessels. The balloon is triggered by the EGG to deflate in ventricular systole (thus reducing afterload) and inflate in diastole (displacing blood which perfuses the coronary arteries retrogradely). When the heart has recovered sufficiently, the balloon is removed. Complications include damage to the femoral artery, leg ischaemia and haemolysis.
Other means of augmenting ventricular performance
Intra-aortic balloon counterpulsation may not be sufficient to support the failing ventricle and is only suitable for the left-sided circulation. The ventricle (right or left or both) may be bypassed by draining blood from the proximal atrium, passing it through a centrifugal pump and returning it to the distal great vessel thereby bypassing the affected ventricle altogether. As the ventricle recovers, the flow in the pump is gradually reduced so that the ventricle slowly takes over the circulation. Once weaning is complete, the assist device is removed. Biventricular support is possible as a bridge to transplantation.
Artificial hearts
It has proved impossible to find a perfect mechanical substitute for the native heart. The totally artificial heart is limited by thrombosis and infection, although survival of over 3 months has been documented. The energy supply to drive the implanted heart often weighs more than the patient and therefore mobility is restricted. The Jarvik total artificial heart has had limited success in temporarily supporting the circulation while heart transplantation is awaited. Further research is required before the expense of this technology can be justified for routine use.