Case 11 Flashcards
What are the major determinants of myocardial oxygen supply and demand?
The myocardium normally extracts 65% of the oxygen in arterial blood compared with extraction of 25% by other tissues. An increase in myocardial demand for oxygen can be met only by an increase in coronary blood flow, not by increased oxygen extraction. The major determinants of myocardial oxygen supply include the following: • Coronary anatomy. Myocardial blood supply derives from the left and right coronary arteries. The right coronary artery normally supplies the right atrium and most of the right ventricle and in right dominant circulation (85% of people) gives rise to the posterior descending artery, which supplies part of the interventricular septum and the inferior wall of the left ventricle. It also usually supplies the sinoatrial node (60% of patients) and the atrioventricular node (85% of patients). The left coronary artery bifurcates into the left anterior descending artery and the circumflex arteries. The left anterior descending artery supplies the septum of the left ventricle and the anterior wall, whereas the circumflex 5. What intraoperative monitoring techniques can be used to detect myocardial ischemia? 6. What are the advantages and disadvantages of on-pump versus off-pump coronary artery bypass grafting? 7. What are unique anesthetic considerations related to off-pump coronary artery bypass grafting? artery supplies the lateral wall and in left dominant circulation (15% of people) gives rise to the posterior descending artery. • Coronary perfusion pressure (CPP). CPP is determined by the difference between aortic pressure and ventricular pressure. During systolic contraction, the pressure in the left ventricle approaches aortic pressure, so no coronary perfusion occurs during this time. The left ventricle is almost entirely perfused during diastole, whereas the right ventricle is perfused during both systole and diastole. CPP for the left ventricle is the difference between the aortic diastolic pressure (ADP) and the left ventricular end-diastolic pressure (LVEDP): CPP 5 ADP 2 LVEDP • Heart rate. Because nearly all perfusion of the left ventricle occurs during diastole, the duration of diastole is also a significant determinant of coronary perfusion. Increases in heart rate result in a disproportionately greater decrease in diastolic time resulting in decreased coronary perfusion. • Arterial oxygen content. Hemoglobin concentration and oxygen saturation need to be optimized to maintain adequate supply. The optimal hemoglobin concentration and transfusion trigger remain a matter of debate. The major determinants of myocardial oxygen demand include the following: • Basal oxygen requirements. Basal requirements account for about 20% of the oxygen requirements. • Left ventricular wall tension (T). T is directly proportional to the intraventricular pressure (P) and ventricular radius (r) and inversely proportional to ventricular wall thickness (h): T 5 Pr/2h. Increases in either preload (increased radius) or afterload (increased pressure) increase T and oxygen demand. The increase in oxygen demand secondary to an increase in afterload is usually compensated by an increase in CPP and oxygen supply. • Heart rate. Increases in heart rate increase oxygen demand, while decreasing oxygen supply, increasing the potential for myocardial ischemia. • Contractility. Decreased myocardial contractility is associated with smaller oxygen requirements. However, if CPP is decreased, the oxygen supply is decreased.
How is coronary artery disease treated, and which medications should be continued perioperatively?
To avoid ischemia, a balance between myocardial oxygen supply and demand must be maintained. The medications used to treat CAD attempt to maintain this balance by either reducing the demand for or increasing the supply of myocardial oxygen. b-Adrenergic Blockers By antagonizing the effects at b receptors, these agents reduce heart rate (increasing diastolic time) and contractility. Myocardial oxygen consumption is decreased, and coronary perfusion is improved. b Blockers are beneficial for most patients with CAD, particularly if their heart rates are increased. When acutely administered in adequate dosage, b blockers have been shown to significantly reduce myocardial oxygen demand and the incidence of atrial and ventricular arrhythmias. The current American College of Cardiology/American Heart Association (ACC/AHA) guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery recommend that b blockers should be continued in patients who have been taking them before surgery and should be started in patients who are at high risk for cardiovascular disease, as defined by the presence of more than one clinical risk factor (Box 11-1), and who are undergoing high-risk or intermediate-risk surgery. Antiplatelet Agents Many patients present for coronary artery bypass grafting (CABG) while being treated with platelet inhibitors. Aspirin is a well-recognized component of primary and secondary prevention strategies for all patients with CAD. The combination of aspirin and clopidogrel has been shown to improve outcome after acute coronary syndrome. Many patients presenting for CABG underwent a previous coronary artery stent procedure, necessitating treatment with aspirin and clopidogrel to prevent early stent thrombosis. Antiplatelet therapy is also used after CABG to reduce postoperative ischemic complications. Aspirin has strong efficacy in the prevention of early graft thrombosis after CABG. However, controversy exists in regard to preoperative antiplatelet therapy. The risk of hemorrhagic complications needs to be weighed against the potential antiischemic benefits of antiplatelet therapy. The premature discontinuation of antiplatelet therapy in patients with coronary artery stents presenting for noncardiac surgery increases the risk of stent thrombosis, myocardial infarction, and death. It is ideal to delay elective noncardiac surgery for 4 to 6 weeks after bare metal stent implantation and at least 12 months after drug-eluting stent implantation. If surgery cannot be postponed, a discussion with the surgeon and the patient’s cardiologist is necessary to determine the risk versus benefit of temporarily stopping antiplatelet therapy compared with the risk of bleeding complications. If it is determined that clopidogrel therapy needs to be interrupted, owing to the risk of bleeding complications, aspirin therapy should be continued if possible, and clopidogrel should be restarted as soon as possible. 3-Hydroxy-3-methylglutaryl-coenzyme A Reductase Inhibitors (Statins) 3-Hydroxy-3-methylglutaryl-coenzyme A (HMG CoA) reductase inhibitors have a variety of important effects independent of their primary purpose as lipid-lowering agents. They appear to possess potent antiinflammatory and antithrombotic effects. Statins have also been shown to decrease myocardial reperfusion injury after cardiac surgery. The current ACC/AHA guidelines on perioperative cardiovascular evaluation and care for noncardiac surgery recommend continuing statins in patients currently taking them and considering starting statins in patients with at least one clinical risk factor (see Box 11-1) undergoing intermediate-risk surgery. Postoperative withdrawal of statin treatment is independently associated with increased in-hospital mortality after CABG. Intraaortic Balloon Pump An intraaortic balloon pump is a mechanical device that increases myocardial oxygen perfusion while increasing cardiac output and myocardial oxygen delivery. It actively deflates in systole, increasing forward blood flow by reducing afterload. It actively inflates in diastole, increasing blood flow to the coronary arteries. These actions combine to decrease myocardial oxygen demand and increase myocardial oxygen supply. The preoperative use of intraaortic balloon pumps in highrisk patients undergoing CABG has shown promising results in improving outcomes. 3. What are the preanesthetic concerns in a patient with coronary artery disease? The anesthesiologist needs to be aware of any comorbidities the patient has and plan accordingly. Patients with CAD often have a history of hypertension, diabetes, cerebrovascular disease, and chronic renal disease. Many also have a history of smoking. Patients with hypertension have intravascular hypovolemia and may become hypotensive after induction of anesthesia. Patients with severe diabetes mellitus are at risk for autonomic and peripheral neuropathy that can lead to silent myocardial ischemia and delayed gastric emptying. Appropriate prophylaxis for a “full stomach” should be considered. If a patient has significant cerebrovascular disease, a higher blood pressure is required to maintain cerebral perfusion pressure. Chronic renal disease may affect the elimination of certain medications. Premedication in the preoperative period may be very important in a patient presenting for CABG. Premedication with benzodiazepines and opioids reduces apprehension and provides analgesia for potentially painful events before induction (e.g., vascular cannulation). This premedication may help prevent preoperative anginal episodes elicited by tachycardia secondary to anxiety or painful stimuli. After premedication, all patients should receive supplemental oxygen and monitoring with pulse oximetry, electrocardiogram (ECG), and noninvasive blood pressure.
What are the preanesthetic concerns in a patient with coronary artery disease?
What anesthetic technique is used for coronary artery bypass grafting?
The anesthesiologist needs to be aware of any comorbidities the patient has and plan accordingly. Patients with CAD often have a history of hypertension, diabetes, cerebrovascular disease, and chronic renal disease. Many also have a history of smoking. Patients with hypertension have intravascular hypovolemia and may become hypotensive after induction of anesthesia. Patients with severe diabetes mellitus are at risk for autonomic and peripheral neuropathy that can lead to silent myocardial ischemia and delayed gastric emptying. Appropriate prophylaxis for a “full stomach” should be considered. If a patient has significant cerebrovascular disease, a higher blood pressure is required to maintain cerebral perfusion pressure. Chronic renal disease may affect the elimination of certain medications. Premedication in the preoperative period may be very important in a patient presenting for CABG. Premedication with benzodiazepines and opioids reduces apprehension and provides analgesia for potentially painful events before induction (e.g., vascular cannulation). This premedication may help prevent preoperative anginal episodes elicited by tachycardia secondary to anxiety or painful stimuli. After premedication, all patients should receive supplemental oxygen and monitoring with pulse oximetry, electrocardiogram (ECG), and noninvasive blood pressure.
What intraoperative monitoring techniques can be used to detect myocardial ischemia?p5
Intraoperative detection of ischemia depends on the recognition of ischemic changes by ECG, hemodynamic manifestations, and new-onset regional wall motion abnormalities (RWMAs) seen on transesophageal echocardiography (TEE). • ECG. The simultaneous monitoring of leads V5 (anterior wall) and II (inferior wall) allows for the detection of 90% of ischemic episodes. In addition, this monitoring allows for detection of atrial and ventricular dysrhythmias, another possible manifestation of ischemia. Early ischemic changes usually involve T-wave inversions followed by ST segment depression. • TEE. The earliest sign of myocardial ischemia is systolic RWMAs, which occur within seconds of reduced coronary perfusion. TEE is more sensitive than ECG at detecting myocardial ischemia. The routine use of TEE is now recommended for all patients undergoing CABG or OPCAB surgery. The transgastric short-axis midpapillary muscle view of the left ventricle is commonly used because it visualizes myocardium supplied by all three major coronary arteries. New RWMAs detected intraoperatively are not specific for myocardial ischemia because they may frequently occur secondary to nonischemic causes, such as changes in loading conditions, alteration in cardiac electrical conduction, post-CPB pacing, or poor myocardial preservation.
What are the advantages and disadvantages of on-pump versus off-pump coronary artery bypass grafting?
For many years, CABG was primarily performed with the use of CPB, which allowed for a bloodless and immobile surgical field. Conventional CPB and aortic cross-clamping risks generalized systemic inflammatory responses that cause hemodynamic instability and affects all organs, especially the heart, lungs, and kidneys. Cerebral dysfunction likely results from emboli that dislodge from the aorta during cross-clamping, cannulation, and proximal vein graft anastomosis. The introduction of OPCAB into practice was an attempt to decrease the adverse clinical consequences of conventional CPB and aortic cross-clamping. The introduction of mechanical stabilizer devices that minimized motion around the anastomosis site (independent of heart rate) allowed for performing CABG without the use of CPB (i.e., “off-pump”). The ability to expose the posterior surface of the heart to access the posterior descending and circumflex arteries was vital for the multivessel application of this technique. This ability depends on placing suction devices on the apex of the heart without producing major hemodynamic compromise. About 30% of CABG procedures are likely performed off-pump. Numerous studies analyzing whether OPCAB reduces morbidity and mortality compared with conventional CABG have reported conflicting results. The final word regarding difference in outcome and which patients may benefit from an OPCAB technique is still years away.
What are unique anesthetic considerations related to off-pump coronary artery bypass grafting?
The continuous, intensive involvement of the anesthesiologist is perhaps more crucial during OPCAB than during on-pump CABG. Relative to on-pump CABG, OPCAB extends the range of surgeon-induced hemodynamic changes the anesthesiologist encounters. Surgical manipulations can cause various cardiac anatomic distortions including compression of the right ventricle and distortion of the mitral valve anulus. Anesthesiologists must be able to anticipate and communicate with surgeons to minimize the adverse impact of these changes on the heart and other vital organs. With a skilled surgeon, the changes are usually modest or easily treated by the Trendelenburg position, judicious volume expansion, and use of vasoconstrictors or inotropes. However, severe changes secondary to acute ischemia, unrecognized right ventricular compression, or severe mitral regurgitation may occur, necessitating emergent conversion to CPB. The anesthesia technique in patients undergoing OPCAB does not differ much from the anesthesia technique for on-pump CABG. It can depend on the indication for OPCAB. Patients with advanced age, significant ascending aortic disease, poor left ventricular function, and multiple comorbidities may be scheduled for OPCAB to avoid CPB and aortic cross-clamping. Anticoagulation in patients undergoing OPCAB is an area of controversy. Some surgeons prefer low-dose heparinization (e.g., 100 to 200 units/kg heparin) with a target activated clotting time (ACT) of 250 to 300 seconds, whereas others choose full heparinization (e.g., 300 units/kg) with a target ACT of greater than 480 seconds during the procedure. ACT is measured every 30 minutes, and heparin is administered to maintain the target ACT.
