Coronary Circulation and Autoregulation

🫀LCx (left circumflex) supplies the lateral LV wall (I, aVL, V5-V6).

RCA (right coronary artery) supplies the RV + inferior LV wall + SA node (~60% of population) + AV node (90% of population) (II, III, aVF).

DOMINANCE refers to which artery gives rise to the posterior descending artery (PDA) — RCA-dominant in ~85% of population (most), LCx-dominant in ~8%, co-dominant in ~7%.

RCA dominance + occlusion is associated with bradyarrhythmias because both nodes are typically in RCA territory.

⚡During systole the contracting myocardium physically compresses intramyocardial vessels — flow is briefly minimal or even reversed, especially in subendocardium.

Diastole provides 70-80% of total left coronary flow.

RIGHT coronaries get more uniform perfusion across the cardiac cycle because RV pressure is much lower than LV.

Subendocardium is the most vulnerable layer because it's perfused last and pressure-compressed first.

Normal: 60-80 mmHg.

CPP falls when aortic diastolic drops (vasodilation, aortic regurgitation, severe systemic hypotension, sepsis) OR LVEDP rises (CHF, volume overload, myocardial ischemia → diastolic dysfunction → high filling pressure).

PHENYLEPHRINE raises aortic diastolic without raising HR or contractility → first-line for CAD hypotension.

Norepinephrine acceptable as second line.

Coronary flow stays constant across MAP 60-140 mmHg via metabolic (adenosine, lactate, K+), myogenic (vascular smooth muscle stretch-response), and endothelial (NO, prostacyclin) mechanisms. β-adrenergic dilation and increased demand from exercise/sympathetic activation match flow to demand.

Outside this autoregulatory range, perfusion becomes PRESSURE-PASSIVE.

DIABETIC and chronically HYPERTENSIVE patients have RIGHT-SHIFTED autoregulation curves — they need HIGHER MAPs (often 80-100 mmHg) to maintain perfusion than the normal-curve textbook would suggest.

Intra-op MAP target should be within 10-20% of patient's known awake baseline, not a universal number.

⚡Classic example: adenosine — used diagnostically in stress testing precisely because it causes steal in CAD.

Isoflurane was historically thought to cause clinically significant steal but modern data show no clinical relevance at typical MAC values.

In severe LV hypertrophy or aortic stenosis where subendocardium is already chronically underperfused, any vasodilator can theoretically worsen subendo perfusion — pick agents that preserve SVR (etomidate over propofol) and use phenylephrine liberally.

SUPPLY factors: HR (longer diastole = more perfusion), aortic diastolic pressure (CPP), hemoglobin (O₂ carrying capacity), SaO₂, vessel patency.

DEMAND factors: HR (more beats per minute = more O₂ used), wall stress (preload × afterload), contractility, basal metabolic rate.

The 'double product' (HR × SBP) is a clinical surrogate for myocardial O₂ demand.

GOALS in CAD: HR 50-70 (longer diastole + lower demand), MAP within 20% of awake baseline, avoid hypoxia, anemia (target Hb ≥8 in CAD; ≥9-10 if active ischemia), and hypothermia. β-blocker continuation through the perioperative period is critical — POISE-1 trial showed that initiating de novo high-dose metoprolol pre-op caused harm (hypotension + stroke), but continuing chronic β-blocker is protective.

Phenylephrine 100 mcg boluses for hypotension preserves CPP without HR rise.

Pretreatment with IV fentanyl + IV esmolol blunts laryngoscopy hypertensive response.

Avoid simultaneous steep Trendelenburg + tachycardia (worst combo for subendo perfusion).

AORTIC STENOSIS = same principles but hyper-amplified; severe AS patients tolerate hypotension extremely poorly because the hypertrophied LV is exquisitely CPP-dependent.

🔎ECG signs (V5 + II monitoring catches ~95% of intra-op ischemia events): ST depression (subendocardial), ST elevation (transmural), new T-wave inversion, new arrhythmia (bigeminy, VT).

Treat suspected intra-op ischemia: 100% O₂, hold β-blocker if bradycardic but otherwise continue, treat HTN/HR aggressively, phenylephrine for hypotension, nitroglycerin gtt 0.5-5 mcg/kg/min if BP allows, consider deepening anesthesia if light, send postop ECG + troponin, cardiology consult.