Myocardial Oxygen Demand

Introduction:

Energy needs: 

• Basal metabolism represents the energy consumed in cellular processes, other than contraction.
• Done by energy-dependent ion pumps etc.

Normal myocardial oxygen demand in resting individuals:

• Normal basal cardiac metabolism accounts 25%.

Factors determining:

• Myocytes (cardiac) contraction – Primary factor (75%)
• Ie., directly related to external cardiac work (cardiac contraction), but relationship is not constant.

Myocardial wall tension:

• Directly proportional to intraventricular pressure (P), ventricular radius (R), and inversely proportional to myocardial wall thickness (myocardial mass).
• Intraventricular (intracavitary) pressure – Determined by aortic pressure (i.e., afterload)
• Ventricular radius – Determined by end-diastolic ventricular volume (i.e., preload)
• Heart rate (Chronotropy)
• Ionotropic state (contractility)

Assessment of myocardial oxygen demand:

• Myocardial oxygen demand is roughly proportional to ventricular systolic pressure generated and systole duration.
• Ie., Area under real-time pressure curve of ventricle in systole –> Referred as “systolic pressure – time index” (SPTI).

Clinical assessment methods:

Double product: 

• Systolic BP x Heart rate.

Triple product: 

• Systolic BP x Heart rate x Systolic ejection time.

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