Anaesthesia

19. Anaesthesia for Cardiac Surgery

19. Anaesthesia forCardiac Surgery The anaesthetic management will largely depend on the nature of the procedure, the state of the patient and his age. It can safely be said that amongst the most spectacular advance s i n recent year s are thos e pertaining t o cardia c surger y an d extra corporeal circulation, but in this section only the outlines can be given. The occasions when anaesthesia would be required ca n conveniently b e divided into 3 stages: (a) Investigation s (b) 'Closed ' cardiac surgery (c) 'Open ' cardiac surgery (a) Investigations It i s o f paramoun t importanc e t o obtai n a s fa r a s possibl e a n answe r t o th e following questions: (i) What is the cardiac lesion and what form of surgery is proposed? (ii) What sequelae have resulted in the individual patient and how rapidly is the condition changing? (iii) Are there any other features of importance, not necessarily related to cardiac disease? (iv) Has optimal pre-operative preparation been achieved? (Branthwaite, 1977). The investigations include those that are carried out without anaesthesia, i.e. history and examination, radiology of chest, ECG, laboratory investigation s (haemotology , biochemistry, bacteriology) and assessment of pulmonary function . Amongst the most valuable diagnostic tools are those that involve invasive techniques (cardiac catheterization, angiocardiography and coronary arteriography) and the non-invasive echocardiography. In adults, invasive techniques may be performed under a combination of sedation and local anaesthesia, while infants and young children would require general anaesthesia . The aims of the invasive techniques are mainly to measure the intracardiac pressures, oxygen saturation of the blood in the various compartments and interpret them i n conjunctio n wit h th e angiocardiographies . Complication s includ e distur bances of blood volume, mechanical trauma, dysrhythmias, myocardial infarction , adverse reactions to contrast media, embolism and various other possible mishaps (as many are carried out in a darkened room, impeccable care is essential). The oxygen saturation of blood samples withdrawn from various sites in the heart and great vessels is usually calculated by oximetry. An unexpected change in oxygen 314 Anaesthesia and Other Specialties saturation betwee n sample s withdraw n fro m adjacen t chamber s woul d sugges t mixing of blood originatin g fro m a different site . For example , a sudde n increase in the saturation betwee n righ t atria l an d righ t ventricular sample s would indicat e th e presence of a left to right shunt through a ventricular septa l defect. The difference i n oxygen saturatio n ma y als o b e use d t o asses s th e rati o o f pulmonar y t o systemi c blood flow—the Qp/Q s ratio . Bloo d flow i s inversely relate d t o th e arteriovenou s oxygen conten t differenc e betwee n the sites of sampling. Qp/Qs ratio may b e state d as: (Arterial-mixed venous ) oxygen saturation (Pulmonary venous-pulmonary arterial ) saturatio n Once diagnosis is established and the decision to operate taken, there are basically two possible managements: 'close' or 'open' cardia c surgery (b) 'Closed' Cardiac Surgery The chest is opened through a thoracotomy incisio n and the operation carried ou t with th e patient' s hear t beating . A numbe r o f procedure s includin g close d mitra l valvotomy, resectio n o f coarctatio n o f th e aorta , closur e an d divisio n o f paten t ductus arteriosus , lef t atrio-femora l bypass , pericardia l procedure s (cardia c tam ponade , constrictiv e pericarditis) , insertio n o f pace-maker s an d cardioversio n ma y then be managed. Conventiona l pre-operativ e medication , an d anaesthesi a throug h an endotrachea l tub e an d IPP V ar e mos t commonl y employe d fo r procedure s requiring thoracotomy . Correctio n o f coarctatio n o f th e aort a ma y occasionall y require hypotensiv e anaesthesi a (sodiu m nitroprussid e i s suitable ) whil e cardio version requires somewhat different handling . Cardioversion entails synchronous DC defibrillation t o convert a supraventricula r dysrhythmia to sinus rhythm. There is a transitory asystole due to the passage of the current. I f successful , th e S A nod e wil l initiat e mor e regula r beats . Th e procedur e may be...