Cardiac Anaesthesia
(Dr. Joe Power)INTRODUCTION
Cardiac surgery is one of the most invasive, routine procedures carried out in modern medicine. It has a long history of development starting in the 1950's and continuing today. The two keys to the development of cardiac surgery has been the development of the heart lung machine and the use of heparin to reliably prevent clotting of the blood while the heart lung machine is in use. Cardiac surgery carries a risk greater then most other forms of surgery and for this reason it is only used as a treatment of last resort when all other non-surgical interventions have been exhausted. To minimize the risk, careful management of the anaesthetic and heart-lung machine is essential and the purpose of this discussion is to present an easy to understand description of the procedure.
ANAESTHETIC MANAGEMENT
All anaesthetics are preceded by a preoperative assessment. For cardiac surgery this requires admission to the hospital at least the day before the surgery. The anaesthetic assessment involves taking a history and examining the patient and review of the special investigations. The purpose of the history is to search for all the problems related to the heart condition and the other organ systems such as liver and kidney function. The examination is guided by the history and the aim is to seek further information that will confirm what is known at this stage. The history and examination will result in the formulation of a diagnosis and a treatment plan. The standard special investigations include blood tests, ECG (electrocardiogram) and chest xray. Additional investigations are guided by the particular problem and include an angiogram, echocardiography, lung function tests, computerized tomography (CT scan), Magnetic Resonance Imaging (MRI) and so on. Following the inclusion of all this information it is possible to then complete a detailed plan for the anaesthetic, the surgery, management of the heart lung machine and the post-operative Intensive Care Unit (ICU) treatment. The preoperative assessment is not complete without a detailed discussion with the patient about the procedure, the risks and the projected outcome. This is essential to allay fears, reassure the patient and to ensure that there is a common understanding of all the issues involved. Finally the anaesthetist will order a premedication which may include sleeping tablets, pain medication and an antacid for the stomach.
On arrival in the Operating Room (OR), the patient is transferred to the operating table and the essential non-invasive monitors such as the ECG and pulse oximeter are attached. The anaesthetist can then commence with the insertion of an intravenous catheter ("drip") and a catheter into one of the arteries to measure the blood pressure. These catheters are inserted after local anaesthetic has been injected under the skin so as to minimize the discomfort. At this stage it is usually possible to inject the drugs that will induce anaesthesia (sleep). However if the patient is having an emergency operation, it may be necessary to insert a central venous line (CVP) into one of the veins in the neck before the injection of the anaesthetic drugs. This is done to ensure the maximal monitoring during the potentially unstable period at the start of an anaesthetic. Once asleep the additional monitors will be inserted and these include a urinary catheter, temperature probes, stomach tube and a ultrasound probe placed in the oesophagus (food pipe). The exact monitors being used depends on the procedure being performed and the condition of the patient. The use of these monitors are the bedrock of performing a safe anaesthetic.
During the procedure further drugs are used to keep the patient asleep and these include anaesthetic gases or drugs infused into the veins. In addition a Bispectral Index (BIS) monitor is used to measure the depth of the anaesthetic so as to ensure adequate sleep depth throughout the procedure. This involves placing an electrode strip on the forehead of the patient. It is an additional monitor that helps to ensure the safety of the procedure.
Figure 1: Cardiac anaesthetic setup with anaesthetic machine,
infusion pumps and Transoesophageal Echocardiogram
Most cardiac surgery is performed with the use of the Heart-Lung Machine (HLM). This machine performs the function of the patients heart and lungs and allows the heart to be stopped so as to allow the surgeon a motionless and bloodless operating field. It involves placing cannulas (pipes) into the right atrium and aorta. The blood is drained away from the heart and pumped through a membrane oxygenator (artificial lungs) and then back into the patient. The heart can then be stopped to allow the surgery to proceed. The HLM is operated by a specially trained medical practitioner or a medical specialist (anaesthetist). Once the surgery is completed the patients heart is restarted and allowed to take over from the HLM in performing its normal function. Key to the functioning of the HLM is the use of heparin which is an anticoagulant that prevents the blood clotting when it comes into contact with the foreign surfaces of the HLM circuit. After the heart has restarted and the HLM is no longer needed, the anticoagulant effect of the heparin is terminated by injecting protamine which neutralizes the heparin. The reestablishment of normal clotting is essential to allow the procedure to be successfully concluded.
Figure 2: A Jostra Heart Lung Machine
Following completion of the surgery, the patient is maintained in an anaesthetised state and transferred on a ventilator to the ICU. It usually takes a few hours to normalize the patients condition during which time they are kept asleep and ventilated. Once the key parameters are normal it is possible to start waking the patient and allowing them to start breathing for themselves. If the surgery is uncomplicated and all goes according to plan it usually takes a few hours to reach the point where the ventilator can be removed. The ICU management continues in the form of balancing fluids, pain management, temperature control and possible blood transfusion. It is usual to remain in the ICU for one to two days post-operatively.
ANAESTHETIC AND SURGICAL RISK
Anaesthetic risk is dealt with in detail in the section found ..... However there are a few risks specific to cardiac anaesthesia that deserve mention and they are as follows.
(1) Invasive monitoring
The insertion of a central venous line is associated with the risk of puncturing the lung or the carotid artery in the neck. This can lead to a pneumothorax (air in the chest) or a haematoma (blood clot). The anaesthetist avoids this by following anatomical landmarks and certain safety procedures. If the central venous line is left in for more than a few days then there is the risk of infection.
(2) Neurocognitive problems
Cardiac surgery is unique in that operating on the heart and the use of a heart lung machine results in the potential for brain damage as a result of the procedure. The heart lung machine is a mechanical device that approximates the function of the heart and lungs but is not a true replacement. It is a mechanical device that drains blood away from the heart in plastic tubing and pumps it through an oxygenator. All of this creates the risk of introducing small particles and air into the circulation. Manipulation of the aorta (the largest blood vessel coming out of the heart) and opening the heart for the purposes of operating on the valves or internal structure also has the potential to dislodge atheroma and allow air in the circulation. The problems that may be caused by these factors range from a slight change in cognition (problems with memory, personality changes) to strokes and even brain death. The risk of a problem of this nature occurring ranges from 1 - 5%. This risk is influenced by many factors that include surgical technique, management of the heart lung machine and patient specific factors.
(3) Blood transfusion
The need for the use of blood products in cardiac surgery is greater than in other operations. The use of donated blood and blood products controversial and although it is regarded as generally safe there are risks and the consensus of opinion is to avoid transfusions as far as is possible. The risks of transmitting a disease such as hepatitis or HIV are extremely rare as the donated blood undergoes extensive screening before being used. The greater risk involves a mismatched transfusion that can cause a severe allergic reaction.
The surgical complications of cardiac surgery that can occur include excessive bleeding, infection... These are best discussed with your surgeon.
In summary cardiac surgery is a high risk procedure that is only embarked upon when all avenues of medical treatment have been exhausted. Safety of the procedure is dependent on paying the maximum attention to detail, the use of the best equipment and working as a team.
REFERENCES
(1) Clinical Anesthesiology
G. Edward Morgan, M.S. Mikhail, M.J. Murray
Lange Medical Books 4th Edition
(2) Cardiac Anesthesia
F.A. Hensley Jr, D.E. Martin, G.P. Gravlee
Wolters Kluwer/Lippincott Williams & Wilkins 4th Edition
INTERNET RESOURCES
(1) The Society of Cardiovascular Anesthesiologists
(2) Virtual Anaesthesia Textbook (very detailed)
http://www.virtual-anaesthesia-textbook.com/vat/cardiac.html
