Coronary Disease in Noncardiac Surgery
When taking care of a patient requiring surgery who had a recent heart attack, there are three main things to consider before bringing them into the operating room: When was their heart attack? Was there any intervention? Are they on dual antiplatelets? If the surgery is emergent, that supersedes these questions and you continue to the OR. If the surgery can wait weeks to months, then you need to assess when it would be safest to proceed. Once in the operating room, it is the Anesthesiologist’s job to maintain a balance of myocardial oxygen supply and demand to reduce any further injury or ischemia to the heart.
Patients can have heart attacks (myocardial infarctions - MI) without the need for revascularization. If this is the case, it is generally recommended to wait 60 days before having the patient undergo elective surgery. If a patient goes undergoes a percutaneous intervention (PCI) without a stent placed, it is recommended for them to wait at least 2 weeks prior to surgery. This allows for healing at the balloon site in the coronary vessel. Patients who get a bare metal stent should wait at least 30 days before stopping dual antiplatelet therapy (DAPT) and undergoing elective surgery. Ideally, this population waits at least 3 months before having surgery. If a patient receives a drug eluding stent, they should remain on DAPT for at least 3 months. It can be stopped after 6 months. If a surgery is required between 3 and 6 months from revascularization with a DES, then it is up to the Cardiologist and Surgeon to decide if the benefits out-weigh the risk of discontinuing one of the antiplatelet therapies. The concern of stopping the DAPT early is stent failure caused by re-endothelialization leading to thrombosis. Ideally, these patients wait at least 6 months and potentially up to a year before undergoing surgery.
In the operating room, we want to optimize the patients cardiac conditions to reduce risk of any further damage. We do this by balancing myocardial oxygen supply and demand. When oxygen demand exceeds oxygen supply, it leads to ischemia.
The factors that contribute to supply include coronary perfusion pressure and arterial blood oxygen content. Coronary perfusion pressure is the diastolic blood pressure of the aorta minus the left ventricular end diastolic pressure (CBF = DBPao - LVEDP). The coronary arteries are perfused during diastole (except for the RCA which is perfused in both systole and diastole). Things that can decrease CBF include aortic hypotension, elevated LV diastolic pressure (ie. from LVH), tachycardia (reduce time in diastole), atherosclerosis of the vessels. Arterial blood oxygen content is determine by amount of hemoglobin and O2 saturation [CaO2 = (Hgb x 1.38)(SaO2) + (PaO2)(0.003)]. Both anemia and hypoxemia can contribute to low oxygen supply.
The factors that determine demand are heart rate, contractility, systemic vascular resistance, and LV volume. Heart rate determines the frequency that the heart is working at. Contractility describes the velocity of the myocardium increasing in wall stress. Increased LV volume (preload) and systemic vascular resistance (afterload) will both increase the LV wall stress and lead to a higher demand.
We use standard ASA monitoring for these patients with consideration of an invasive arterial blood pressure if the risk of the surgery or the patient’s other comorbidities warrant it. The continuous ECG monitor will monitor leads II and V5. These are the best leads for detecting ST changes indicating ischemia, and for ensuring the patient is in a sinus rhythm. A pulmonary artery catheter (PAC, Swan Ganz) is not recommended for routine use. Transesophageal echocardiography (TEE) is only recommended in situations of persistent, unexplainable hypotension to help aid with resuscitation. The ultimate goal is to keep myocardial oxygen supply greater than demand.
This goal carries into the post-operative period. This is the highest risk for a patient to experience an MI. There are 4 main concerns postoperatively that you want to pay close attention to avoid to decrease a demand > supply scenario. These include:
Hypotension
Severe Pain
Anemia
Hypothermia
Hypotension will decrease the myocardial supply by decreasing the aortic diastolic pressure, reducing the coronary perfusion pressure. Severe pain can lead to an increase in stimulation of the sympathetic nervous system. This causes tachycardia and increased myocardial contractility - both increase oxygen demand. Anemia can lead to decrease oxygen supply for the myocardium and should be suspected after surgeries with major blood loss and hemodynamic instability. Hypothermia can lead to a hypermetabolic state and potentially tachycardia, raising the oxygen demand in myocardium and skeletal muscles.
