Cardiac Anesthesia

Question 1

Determinants of Myocardial Oxygen Demand:

Answer 1

The principle determinants of MVO2 are wall tension and contractility.

Myocardial Demand
o Wall Stress: PR/2h (where P is intracavitary pressure,
R is Ventricular radius, h is wall thickness)
o Heart Rate
o Contractility

Question 2

Myocardial Oxygen Supply

Answer 2

• Increases in myocardial oxygen requirements can be met only by increasing the coronary blood flow
• NB arterial blood oxygen content and myocardial oxygen extraction are infrequent reasons for intraoperative myocardial ischemia because oxygenation and blood volume are usually well controlled during anesthesia.
• In addition since the coronary sinus blood is desaturated (PO2 in the range of 15 to 20mmHg) further oxygen extaction is inadequate to meet a significantly increased MVO2
• Therefore, the principle mechanism for matching oxygen supply to alterations in MVO2 is exquisite regulation and control of coronary blood flow

Question 3

Coronary Blood Flow

Answer 3

The critical factors that modify coronary blood flow are the perfusion pressure and the vascular tone of the coronary circulation, the time available for perfusion (diminished mainly by heart rate)
The severity of intraluminal obstructions, and the presence of (any) collateral circulation.

Suppply summarized:
o Coronary blood flow (AoDP-LVEDP / Coronary vascular resistance)
 Diastolic time
 Collaterals, capillary density

o Oxygen Content Hb x SatO2
o Hb-O2 dissociation curve
o O2 extraction

Question 4

What are of the myocardium is the most vulnerable to ischemia?

Answer 4

Subendocardium of the left ventricle

Question 5

Coronary Artery Disease and Hemodynamic Goals:

Answer 5

Preload
o Decreased
 A smaller heart size (TEE dimensions) decreases wall tension and PADP (pulmonary artery diastolic pressure) and increases perfusion pressure gradient

Afterload
o Maintain
 Hypertension is better than hypotension (coronaries fill off diastolic pressure)

Contractility
o Decrease
 If LV function is normal

Rate
o Decrease
 Optimise diastolic filling of coronaries

Rhythm
o Sinus
 Correct Arrhythmias

MVO2
o Monitor for and treat “supply” related disturbances

Post CPB
o No need for increased PADP (LVEDP)

Question 6

Treatment of Intraoperative ischemia

Answer 6

Treatment of Intraoperative Ischemia:
Increased Demand increased HR
o Treat usual reason including inadequate anesthesia, administer Beta Blocker

Increased Demand increased BP
o Increase anesthetic depth

Increased Demand increase PCWP
o Nitroglycerin

Decreased Supply decreased HR
o Atropine or pacing

Decreased Supply decreased BP
o Decreased anesthetic depth, administer vasoconstrictor

Decreased Supply increased PCWP
o Nitroglycerin, calcium channel blockers, consider heparin

Question 7

Aortic Stenosis Severity by gradients

Answer 7

Aortic Stenosis Severity by gradients:

Mild
o Less than 20 mmHg mean gradient

Moderate
o 20-40 mmHg mean gradient

Severe
o >40 mmHg mean gradient

Critical
o >50 mmHg mean gradient

Question 8

Aortic Stenosis Severity by valve area

Answer 8

Aortic Stenosis Severity by valve area:
Mild
o >1.5 cm2

Moderate
o 1.0-1.5 cm2

Severe
o <1.0 cm2

Critical
o <0.75 cm2

Question 9

Aortic Stenosis Hemodynamic Goals:

Answer 9

Aortic Stenosis Hemodynamic Goals:

Preload
o Maintain or increase

Afterload
o Maintain or increase (to maintain coronary perfusion gradient)

Contractility
o Usually not a problem, may require inotropic support if hypotension persists

Rate
o Avoid bradycardia (decreased CO) and tachycardia (ischemia)

Rhythm
o	Sinus (may need cardioversion or beta blockers in on sinus or fast rhythms)

MVO2
o Treat tachycardia and hypotension (ischemia is an ever present risk)

Post CPB
o Increase contractility (Increased myocardial stunning)