# Anesthesia for Coronary Artery Bypass Surgery

## Preoperative Assessment

### Cardiac Evaluation

Preoperative evaluation for CABG centers on understanding the patient's coronary anatomy and ventricular function. The catheterization report should be reviewed for the number of diseased vessels, the presence of left main disease, and the ejection fraction. Echocardiography provides essential information about regional wall motion abnormalities, ejection fraction, diastolic function, and any coexisting valvular pathology. The timing of any recent myocardial infarction matters significantly, as surgery within 30 days of an MI carries increased risk. Medications require careful management: beta-blockers and statins should be continued through the morning of surgery, the decision to hold ACE inhibitors on the day of surgery remains debated, and antiplatelet agents should be discontinued according to a timeline coordinated with the surgical team. Baseline laboratory studies include hemoglobin, coagulation parameters, and a type and screen or crossmatch.

### Risk Stratification

Formal risk stratification tools such as the EuroSCORE II or STS risk calculator predict perioperative mortality and help guide discussions with the patient and surgical team. Features that place patients in a high-risk category include left main coronary disease, ejection fraction below 30%, redo sternotomy, renal dysfunction, emergency surgery, and advanced age.

## Hemodynamic Goals

### General Principles for CAD

The hemodynamic goals for patients with coronary artery disease revolve around protecting the myocardial oxygen supply-demand balance. **Heart rate** should be kept low-normal, ideally 50-70 bpm, because tachycardia simultaneously increases oxygen demand and reduces diastolic perfusion time. **Blood pressure** management requires maintaining adequate diastolic pressure for coronary perfusion while avoiding both hypotension and severe hypertension, the latter of which increases afterload and oxygen consumption. **Preload** should be maintained at adequate levels, avoiding hypovolemia. **Contractility** should be preserved, avoiding excessive negative inotropy and supporting with inotropes if needed. **Sinus rhythm** is preferable because the atrial kick contributes 15-25% of cardiac output.

## Intraoperative Management

### Monitoring

CABG surgery requires comprehensive monitoring beyond standard ASA monitors. An arterial line, typically radial and often placed before induction, provides continuous blood pressure measurement and access for blood gas sampling; a femoral site may be preferred if radial artery harvest is planned. A central venous catheter provides CVP monitoring, a route for vasoactive drug administration, and large-bore access for volume resuscitation. The use of a pulmonary artery catheter varies by institution but is most useful in patients with low ejection fraction, redo surgery, or combined valve procedures. Transesophageal echocardiography is near-routine at many centers, providing real-time assessment of regional wall motion, valvular function, and air detection after cardiopulmonary bypass. Additional monitoring includes a urinary catheter with temperature probe, processed EEG monitoring (BIS or SedLine) during CPB, and increasingly, cerebral oximetry via near-infrared spectroscopy.

### Induction

Induction must be smooth and hemodynamically stable, specifically avoiding tachycardia and hypotension. Common induction strategies include reduced-dose propofol, etomidate, or a combination of midazolam with a high-dose opioid. High-dose fentanyl (10-15 mcg/kg) or sufentanil (1-3 mcg/kg) effectively blunts the sympathetic response to laryngoscopy and intubation. Muscle relaxation is provided by rocuronium, vecuronium, or pancuronium, though pancuronium's mild vagolytic effect may cause undesirable tachycardia and should be used cautiously. Phenylephrine and ephedrine should be drawn and immediately available before induction.

### Maintenance

Anesthesia during the pre-CPB period is maintained with a volatile agent (isoflurane or sevoflurane) or total intravenous anesthesia, supplemented by an opioid infusion (fentanyl, sufentanil, or remifentanil). During cardiopulmonary bypass, anesthesia is delivered either via a vaporizer connected to the CPB circuit's gas line or through a propofol infusion. Neuromuscular blockade is maintained throughout the procedure. BIS monitoring helps prevent awareness, which is a particular concern during CPB when anesthetic delivery pathways may be altered.

## Cardiopulmonary Bypass (CPB)

### Components

The CPB circuit consists of several essential components working in series. A venous cannula placed in the right atrium drains blood by gravity into a venous reservoir. From there, blood passes through a membrane oxygenator for gas exchange and a heat exchanger for temperature management. An arterial pump, either roller or centrifugal, returns oxygenated blood to the patient via an aortic cannula. A separate cardioplegia delivery system provides myocardial protection. Filters and bubble traps are integrated throughout the circuit to prevent particulate and air embolism.

### Initiation of CPB

Before cannulation, full heparinization is achieved with a bolus of 300-400 units/kg of heparin, targeting an activated clotting time (ACT) above 480 seconds (some centers accept above 400). The ACT must be confirmed before the surgeon proceeds with cannulation. Venous cannulation is performed first, followed by arterial cannulation. When "going on bypass," the perfusionist assumes control of the circulation while the anesthesiologist stops ventilation and allows the lungs to deflate. Adequate venous drainage is verified by observing the CVP decrease to near zero. Anesthesia must be maintained via the CPB circuit throughout the bypass period.

### Management During CPB

During bypass, the **mean arterial pressure** is targeted between 50-80 mmHg, with institutional variation; higher targets are used for elderly patients and those with cerebrovascular disease or diabetes. **Flow rate** is maintained at 2.2-2.5 L/min/m2, the equivalent of a normal cardiac index. The **hematocrit** target varies, but most centers tolerate 21-25% on CPB and transfuse if it falls below 21%. **Temperature** management commonly involves cooling to 32-34 degrees Celsius (moderate hypothermia), though some centers use deeper cooling to 28 degrees or normothermic bypass. **Glucose** is monitored and hyperglycemia treated with an insulin infusion targeting values below 180 mg/dL. **Acid-base management** follows either alpha-stat or pH-stat principles, with alpha-stat being more common for adult cardiac surgery. **Urine output** is monitored, with the understanding that oliguria is expected during hypothermic CPB. Anesthesia is maintained via a vaporizer on the CPB circuit or a propofol infusion with continued opioid supplementation.

### Cardioplegia

Cardioplegia is a solution delivered to the coronary arteries to arrest the heart in diastole, providing myocardial protection during the period of aortic cross-clamping. The high-potassium concentration causes diastolic arrest, while cold temperature reduces the metabolic rate. Cardioplegia solutions may be crystalloid-based (such as St. Thomas or Bretschneider solutions) or blood-based. Delivery can be antegrade through the aortic root, retrograde through the coronary sinus, or a combination of both. Repeated doses are administered every 15-20 minutes during cross-clamping to maintain myocardial protection.

### Weaning from CPB

Weaning from CPB follows a systematic sequence. Rewarming to a core temperature of 36-37 degrees Celsius is completed first. A checklist is then reviewed: electrolytes (potassium, calcium, magnesium), acid-base status, hematocrit, and pacing capability. The heart is de-aired under TEE guidance. Ventilation is resumed with lung recruitment maneuvers. Adequate heart rate and rhythm are ensured, with pacing if needed. CPB flow is then gradually reduced while the heart is allowed to fill and eject. Ventricular function is assessed both visually and with TEE. Vasopressor or inotrope support is initiated as needed. At separation, CPB is stopped and the patient assumes full circulation. A comprehensive assessment of hemodynamics, TEE findings, and hemostasis follows.

### Failure to Wean

When the heart cannot sustain adequate circulation after CPB, causes include myocardial stunning, incomplete revascularization, air embolism, graft failure, new or worsened valvular dysfunction, protamine reaction, and right ventricular failure. Interventions include inotropes (epinephrine, milrinone, dobutamine), vasopressors, pacing optimization, and correction of metabolic derangements. If these measures fail, mechanical support with an intra-aortic balloon pump or ventricular assist device should be considered.

## Heparin-Protamine Protocol

### Heparin Management

Full anticoagulation is established with a heparin bolus of 300-400 U/kg before CPB, and additional heparin is administered during bypass to maintain the ACT above 480 seconds. The ACT is monitored every 30 minutes during the bypass period.

### Protamine Reversal

Protamine neutralizes heparin at a dose of 1 mg protamine per 100 units of heparin administered, typically resulting in a total dose of 3-4 mg/kg. It must be administered slowly over 10-15 minutes after separation from CPB. Adequate reversal is confirmed by the ACT returning to its baseline value.

### Protamine Reactions

Protamine can cause three distinct types of adverse reactions. **Type I** reactions involve hypotension from histamine release caused by rapid administration; the treatment is simply slowing the infusion rate. **Type II** reactions are anaphylactoid or anaphylactic, mediated by IgE antibodies, with risk factors including fish allergy, prior protamine exposure, NPH insulin use, and vasectomy. **Type III** reactions are the most catastrophic, producing acute pulmonary vasoconstriction with resultant pulmonary hypertension, right ventricular failure, and systemic hypotension; this complement-mediated response may require returning to CPB. Management across all types begins with stopping the protamine infusion and supporting hemodynamics, with epinephrine for anaphylaxis and consideration of returning to CPB for Type III reactions.

| Reaction Type | Mechanism | Presentation | Risk Factors | Treatment |
|---|---|---|---|---|
| Type I | Histamine release (rapid administration) | Hypotension | Rapid infusion | Slow infusion; phenylephrine |
| Type II | IgE-mediated anaphylaxis | Urticaria, bronchospasm, cardiovascular collapse | Fish allergy, prior protamine, NPH insulin, vasectomy | Stop infusion; epinephrine; supportive care |
| Type III | Complement-mediated pulmonary vasoconstriction | Acute pulmonary hypertension, RV failure, systemic hypotension | Unknown predisposition | Stop infusion; return to CPB; inhaled NO; epinephrine |

## Temperature Management

Moderate hypothermia during CPB (32-34 degrees Celsius) reduces metabolic rate and provides organ protection. Rewarming must be gradual, at less than 1 degree Celsius per 5 minutes, to avoid cerebral hyperthermia. Rewarming overshoot above 37.5 degrees Celsius is associated with cognitive injury. After CPB, active warming maintains normothermia and avoids post-bypass hypothermia, which contributes to coagulopathy and shivering.

## Fast-Track Cardiac Anesthesia

The modern approach to uncomplicated CABG targets early extubation within 4-8 hours postoperatively. This is achieved by using lower opioid doses (total fentanyl below 15 mcg/kg) and supplementing with regional techniques or multimodal analgesia. The benefits include reduced ICU length of stay, cost savings, and earlier mobilization. Appropriate patient selection is critical: candidates should have uncomplicated CABG anatomy, good ventricular function, hemodynamic stability, and normothermia at the end of the case. Contraindications include hemodynamic instability, ongoing bleeding, hypothermia, and respiratory compromise.

<image>A schematic diagram of the cardiopulmonary bypass circuit showing the complete pathway: venous drainage from the right atrium through a venous cannula to the venous reservoir, through the membrane oxygenator and heat exchanger, through the arterial pump (roller or centrifugal), through arterial filters, and returning to the patient via the aortic cannula. Cardioplegia circuit shown as a branch with antegrade and retrograde delivery paths. Monitoring points labeled: arterial line pressure, venous reservoir level, mixed venous oxygen saturation, temperature. Vaporizer attachment point on the oxygenator gas line for volatile agent delivery during CPB.</image>

<image>A step-by-step timeline of the weaning process from cardiopulmonary bypass: (1) rewarming phase with temperature targets, (2) checklist of prerequisites (K+, Ca2+, Hct, pacing, ventilation, de-airing), (3) TEE assessment showing de-airing views and ventricular function, (4) gradual flow reduction with hemodynamic monitoring, (5) separation from CPB with first independent arterial waveform, (6) post-separation assessment (hemodynamics, TEE, hematology). Inotrope/vasopressor decision tree shown alongside for each hemodynamic scenario (low CO, vasodilation, RV failure).</image>

<image>A protamine reaction classification diagram showing three types: Type I (histamine-mediated hypotension with treatment of slowing infusion and phenylephrine), Type II (anaphylactic with urticaria, bronchospasm, cardiovascular collapse treated with epinephrine), and Type III (pulmonary vasoconstriction with acute RV failure shown on TEE with RV dilation and septal shift, treated with return to CPB, inhaled nitric oxide, and epinephrine). Risk factors listed for each type. A management algorithm connects recognition to intervention for each type.</image>

## Clinical Pearls

The hemodynamic goal for CABG is simple in principle but requires constant vigilance: maintain a slow heart rate, adequate diastolic pressure for coronary perfusion, and avoidance of excessive myocardial oxygen demand. The ACT must exceed 480 seconds before initiating CPB; inadequate anticoagulation on bypass causes consumptive coagulopathy and circuit thrombosis. Heparin reversal should always be confirmed with an ACT after protamine administration rather than assuming the calculated dose is adequate. TEE is invaluable throughout CPB management for confirming de-airing, assessing ventricular function before separation, diagnosing regional wall motion abnormalities that may indicate graft failure, and evaluating for new valvular dysfunction. Fast-track cardiac anesthesia with early extubation is the modern standard for uncomplicated CABG, reducing ICU costs and improving outcomes, but patient selection remains the key to safety. Protamine reactions can be catastrophic; the drug should always be administered slowly, epinephrine should be immediately available, and the team must be prepared to return to CPB if a Type III reaction occurs.

## References

- Kaplan JA, et al. *Kaplan's Cardiac Anesthesia*, 7th edition. Chapters on CABG and CPB Management.
- Engelman DT, et al. The Society of Thoracic Surgeons, The Society of Cardiovascular Anesthesiologists, and The American Society of ExtraCorporeal Technology: clinical practice guidelines for cardiopulmonary bypass. *Ann Thorac Surg*. 2019;108(5):1617-1638.
- Zhu J, et al. Fast-track cardiac anesthesia: a systematic review and meta-analysis. *Br J Anaesth*. 2012;109(2):154-162.
- Horrow JC. Protamine: a review of its toxicity. *Anesth Analg*. 1985;64(3):348-361.
- Miller RD, et al. *Miller's Anesthesia*, 9th edition. Chapter on Cardiac Anesthesia.