What is Monitored Anesthesia Care (MAC) sedation and what are its cardiovascular risks?

Monitored Anesthesia Care (MAC) Sedation and Its Cardiovascular Risks

Monitored Anesthesia Care (MAC) sedation is a technique that involves administration of sedatives or dissociative agents with or without analgesics to induce a state that allows patients to tolerate unpleasant procedures while maintaining cardiorespiratory function, though it carries significant cardiovascular risks including hypotension that requires careful patient selection and monitoring.

Definition and Characteristics of MAC Sedation

MAC sedation refers to a specific anesthesia service provided by an anesthesia specialist to patients undergoing diagnostic or therapeutic procedures 1. It represents a continuum of sedation that often results in:

• A state where patients may be unresponsive to noxious stimuli while breathing spontaneously through their own airway 1
• A level of sedation that can range from moderate to deep, depending on the procedure requirements and patient needs 1
• A technique that combines local anesthesia with parenteral drugs for sedation and analgesia 2

Unlike general anesthesia, MAC sedation typically maintains spontaneous breathing but may still result in significant physiological changes requiring specialized monitoring.

Cardiovascular Risks of MAC Sedation

Primary Cardiovascular Concerns

1. Hypotension:

   • MAC sedation is associated with a lower incidence of hypotension (14%) compared to general anesthesia (59%) 1
   • However, more than one-third of patients undergoing moderate to deep sedation with propofol may experience hypotension (defined as systolic blood pressure <90 mmHg) 1
   • The duration of hypotension is typically brief, with mean arterial pressure <65 mmHg lasting approximately 1 minute during MAC 1

2. Hemodynamic Instability:

   • The incidence and severity of hypotension depend on multiple factors:
     • Patient characteristics (age, comorbidities, medication history)
     • Individual pharmacokinetics and pharmacodynamic responses
     • Specific drugs and combinations used 1
     • Depth of sedation achieved

3. Vasopressor Requirements:

   • Vasopressor use is less common during MAC (7%) compared to general anesthesia (44%) 1
   • This difference contributes to the perception of improved hemodynamic stability with MAC

Risk Factors for Cardiovascular Complications

Certain patient populations are at higher risk for cardiovascular complications during MAC sedation:

• Elderly patients: More sensitive to sedative effects and prone to hypotension 3
• Debilitated patients: Reduced physiological reserve increases risk 3
• ASA-PS III or IV patients: Pre-existing organ system dysfunction increases vulnerability 3
• Patients with cardiovascular disease: Including coronary artery disease or heart failure 4

Medication Considerations and Cardiovascular Effects

Different medications used for MAC sedation have varying cardiovascular profiles:

1. Propofol:

   • Most commonly used agent for MAC sedation
   • Can cause dose-dependent hypotension through direct myocardial depression and vasodilation
   • Rapid bolus administration particularly increases risk of cardiovascular depression 3
   • Risk of propofol infusion syndrome with prolonged use, especially in patients with head injury or septic shock receiving vasopressors 4

2. Benzodiazepines:

   • Provide good amnesia and relative cardiovascular safety
   • Can still cause significant hypotension in hemodynamically unstable patients 4

3. Opioids (e.g., morphine, fentanyl):

   • May provide cardioprotective effects but can cause respiratory depression
   • The CRUSADE study showed increased mortality in patients with non-ST segment elevation myocardial infarction who received morphine 4

4. Etomidate:

   • Offers rapid onset with hemodynamic stability even in shock and hypovolemia
   • Prolonged infusion (not bolus doses) may cause adrenal insufficiency affecting circulatory stability 4

Safe Administration Practices to Minimize Cardiovascular Risks

To minimize cardiovascular risks during MAC sedation:

1. Proper Administration Technique:

   • Avoid rapid bolus dosing, especially in high-risk patients 3
   • Use slow infusion or slow injection techniques during initiation 3
   • For initiation, administer propofol at 100-150 mcg/kg/min for 3-5 minutes while monitoring respiratory function 3
   • For maintenance, use variable rate infusion (25-75 mcg/kg/min) rather than intermittent bolus administration 3

2. Dose Adjustments for High-Risk Patients:

   • Reduce propofol dosage to approximately 80% of usual adult dosage in elderly, debilitated, or ASA-PS III/IV patients 3
   • Titrate to clinical effect, allowing approximately 2 minutes for onset of peak drug effect 3

3. Continuous Monitoring:

   • Monitor heart rate and blood pressure at 3-5 minute intervals throughout the procedure 1
   • Automated noninvasive blood pressure devices with audible alarms are recommended 1
   • Consider continuous electrocardiography for patients with significant cardiovascular disease 1

Comparison with General Anesthesia

When comparing MAC sedation to general anesthesia:

• MAC is associated with lower incidence of hypotension (14% vs 59%) 1
• MAC requires less vasopressor support (7% vs 44%) 1
• MAC may be associated with more adverse respiratory events than general anesthesia 1
• Recovery is typically faster with MAC due to use of relatively smaller amounts of sedatives and analgesics 2

Key Considerations for Clinical Practice

1. Patient Selection: Carefully evaluate cardiovascular status before choosing MAC sedation
2. Medication Choice: Select agents based on patient's cardiovascular profile
3. Administration Technique: Use slow infusion rather than bolus dosing
4. Monitoring: Maintain vigilant hemodynamic monitoring throughout the procedure
5. Rescue Capability: Be prepared to manage cardiovascular complications with appropriate medications and equipment

By understanding the cardiovascular risks associated with MAC sedation and implementing appropriate precautions, clinicians can safely provide this valuable anesthesia service while minimizing adverse outcomes.