Myocardial Ischemic Preconditioning

Definition

• Morgan & Mikhail's Clinical Anesthesiology, 7e - a brief ischemic episode "protects a cell from future, more pronounced ischemic events"
• Fuster and Hurst's The Heart, 15th ed - "repeated brief episodes of coronary occlusion preceding a prolonged coronary occlusion with reperfusion reduce infarct size"

Two Windows (Phases) of Protection

Mechanism / Signaling Cascade

• Adenosine (A1 receptor activation)
• Opioids (k- and delta-opioid receptor activation)
• Bradykinin
• Reactive oxygen species (ROS) - paradoxically protective in small amounts
• Nitric oxide

• These triggers activate Protein Kinase C (PKC) and PI3K/Akt/eNOS pathways
• PKC activation opens sarcolemmal and mitochondrial KATP channels

• KATP channel opening --> reduced mitochondrial Ca²+ overload
• Reduced Ca²+ accumulation --> improved contractile recovery post-reperfusion
• Mitochondria become more permeable to ATP precursors
• Stabilization of the mitochondrial permeability transition pore (mPTP) - prevents its opening on reperfusion, which is the final common pathway of cell death
• 30-40% of cardioprotection from volatile anesthetics is attributed to reduced Ca²+ loading

Forms of Preconditioning Relevant to Anesthesia

1. Classical (Direct) IPC

• Brief coronary occlusion + reperfusion cycles performed surgically before a sustained ischemic period (e.g., during CABG)

2. Anesthetic Preconditioning (APC)

• Volatile anesthetics (isoflurane, sevoflurane, desflurane) mimic IPC - termed "anesthetic preconditioning"
• Mechanism: diffuse through myocardial cell membranes --> alter mitochondrial electron transport --> form ROS --> trigger PKC --> open KATP channels
• Can be applied before the ischemic insult (preconditioning) or immediately after (postconditioning)
• Cardioprotection persists beyond elimination of the anesthetic
• Studies in CABG patients confirm reduced myocardial injury markers with volatile agents vs. propofol-based TIVA

3. Opioid-induced Preconditioning

• Morphine and other opioids mimic IPC through k- and delta-opioid receptor activation
• Reduces infarct size comparable to classic IPC
• Involves adenosine A1 receptors and PKC
• Late preconditioning is also produced by morphine (24 hours after administration)
• Intrathecal morphine has also been shown to produce comparable cardioprotection

4. Remote Ischemic Preconditioning (RIPC)

• Brief ischemia of a distant organ (limb, intestine, kidney) provides equivalent cardioprotection to direct IPC
• Clinical application: inflating a blood pressure cuff on the arm to 200 mmHg for 5-minute cycles
• Mediated by myocardial k-opioid receptors
• Multiple RCTs in CABG patients have evaluated RIPC; results have been inconsistent

Postconditioning

• Brief coronary occlusion cycles applied during early reperfusion (not before ischemia) also reduces infarct size
• This is the reperfusion analog of preconditioning
• Volatile anesthetics administered solely on reperfusion also confer postconditioning protection
• Morphine enhances isoflurane-induced postconditioning via PI3K and opioid receptor activation

Clinical Relevance in Anesthesia

Summary Points for Exam

1. First described in canine myocardium (Murry, 1986)
2. Two windows - early (1-4 hr, pre-existing molecules) and late (24hr onset, 2-4 days, requires gene expression)
3. Key mediators: Adenosine, opioids, bradykinin, ROS, NO
4. Signal transduction: Triggers --> PKC --> KATP channel opening --> mitochondrial Ca²+ protection --> mPTP stabilization
5. Volatile anesthetics = anesthetic preconditioning (preferred over propofol in cardiac surgery)
6. RIPC = simple limb cuff cycles; clinical benefit inconsistent in large RCTs
7. Postconditioning = same protection applied at the START of reperfusion
8. Opioids, adenosine, nicorandil (KATP channel opener), and volatile agents are all pharmacologic mimics of IPC