Myocardial Oxygen Supply and Demand

Describe myocardial oxygen demand and supply, and the conditions that may alter each

• Myocardial oxygen supply is a function of coronary blood flow
• Myocardial oxygen demand is determined by myocardial work
• Myocardial ischaemia occurs when demand exceeds supply

Myocardial Oxygen Supply

• Myocardial oxygen supply is dependent on:
  • Coronary artery flow
  • Oxygen content of blood
  • Oxygen extraction

• Functionally, coronary artery flow is the determinant. This is because:
  • Oxygen content in individuals without pulmonary disease is maximal
  • Resting myocardial oxygen extraction is near-maximal (~70%)
    This high ER makes the heart less tolerant of anaemia than organs with a low ER.

• Therefore coronary blood flow is the limiting factor
  • Coronary blood flow is given by the equation:
    $CBF = {P_{Aortic \ Root} - P_{Cavity} \over Coronary \ Vascular \ Resistance}$
    • Aortic root pressure is the driving pressure for coronary flow
    • Cavity (ventricular) pressure acts as a Starling resistor for coronary flow
      • Note that if RAP exceeds cavity pressure, RAP will be the pressure opposing coronary flow (due to downstream pressure at the coronary sinus)
    • Note that cavity and aortic root pressure change throughout the cardiac cycle, therefore:
      • The flow to each ventricle is different during the cardiac cycle
      • The left ventricle is best perfused in diastole
        Therefore heart rate is an important determinant of coronary blood flow, as tachycardia will decrease coronary blood flow
• Flow to each ventricle is a function of how relationships change over the cardiac cycle

Left Ventricular Coronary Blood Flow:

Right Ventricular Coronary Blood Flow:

Myocardial Oxygen Demand

Normal myocardial oxygen consumption (MVO₂) is 21-27ml.min^-1. The three major determinants are:

• Heart rate
  A change in heart rate will change the number of tension-generating cycles, causing a proportional change in MVO₂.

• Contractility
  Refers to the rate of tension development as well as its magnitude. Changing $\Delta P \over \Delta t \ $ will change MVO₂.

• Ventricular wall tension
  Ventricular wall tension is pressure work, or the work done by the ventricle to generate pressure but not to eject volume.
  • Wall tension is given by the Law of LaPlace
    $Wall \ Tension = {P.r \over 2}$, where:
    • $P$ = Pressure during contraction
    • $r$ = Radius
  • Wall tension is therefore a function of:
    • Afterload
      Increasing afterload will increase the pressure during contraction.
    • Preload
      Increasing preload will increase radius, but to a lesser extent than increasing afterload.
      • This is because volume and radius are not directly proportional

Minor determinants of myocardial work are:

• External work
  External work can also be thought of as volume work, or the energy expended to eject blood from the ventricle.
  • This is encompassed by the area enclosed by the pressure-volume loop
    • Conversely, internal work is defined as the work required to change the shape of the ventricle and prepare it for ejection
      On the pressure-volume loop internal work is represented by a triangle between the point of 0 pressure and volume, the end systolic point, and the beginning of rapid ventricular filling.
  • This is a minor determinant because the majority of ventricular work is generating the pressure required to eject blood, not actually move volume
  • External work is of greater importance at high CO
  • External work is used to calculate cardiac efficiency, given by the equation:
    $Cardiac \ Efficiency = {External \ Work \over \ Myocardial \ O_2 \ Consumption }$

• Basal oxygen consumption
  Basal oxygen consumption (~8ml.min^-1.100g^-1) comprises ~25% of MVO₂.

References

1. Grossman W, Baim DS. Grossman's Cardiac Catheterization, Angiography, and Intervention. 7th Ed (revised). 2006. Lippincott Williams and Wilkins.
2. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.
3. Kam P, Power I. Principles of Physiology for the Anaesthetist. 3rd Ed. Hodder Education. 2012.
4. Miller RD, Eriksson LI, Fleisher LA, Weiner-Kronish JP, Cohen NH, Young WL. Miller's Anaesthesia. 8th Ed (Revised). Elsevier Health Sciences.