G3iv / 16B07: Compare and Contrast the supply and demand of O2 for the RV and LV

16B07: Exam Report

Compare and contrast the supply and demand of oxygen for the right and left ventricle.

29% of candidates passed this question.

An integrated answer to supply and demand of oxygen was expected, as a comparison between the right and left ventricles. Many candidates concentrated on differences not similarities. Myocardial oxygen demand was in general poorly described.

About 85 – 90% of oxygen demand is for internal work (major determinants wall tension 30 – 40%, heart rate 15 – 25%, myocardial contractility 10 – 15%, basal metabolism 25%). 10 – 15% of oxygen demand for external work or pressure volume work, determined by MPAP x CO.

It was expected answers would comment on the phasic nature of coronary blood flow which differs between left and right and the consequence of this to subendocardial oxygen supply during systole usually. Coronary blood flow is affected by coronary perfusion pressure (determined by aortic pressure and RV pressure) & coronary vascular resistance (determined by autoregulation, metabolic factors, humoral factors, nervous control interacting with local endothelial factors)

Generally, coronary blood flow is tightly coupled to oxygen demand/consumption due to high basal oxygen consumption (8 – 10 ml/ min/100g) and high oxygen extraction ratio (75%). Better answers noted that oxygen supply can only be increased to cope with increased demand only by increased coronary blood flow.

G3iv / 16B07: Compare and Contrast the supply and demand of O2 for the RV and LV

Define

Heart = aerobic organ → CANNOT TOLERATE O2 DEBT

  • Will use any substrate available
  • Glucose, lactate, ketones, FA
  • 40% glucose, 60% other
  • Insulin ­glucose uptake by myocytes

Quantify

Blood flow = 250ml/min

5% CO

O2 consumption 30ml/min

10% of O2 consumption

High extraction ratio = 80%

If demands increase it cannot extract any higher, it is FLOW LIMITED

Classify

Coronary Blood Supply

Coronary Anatomy

Right Ventricle

R cusp of AoV

Gives off

  • Conus Branch
  • SA branch
  • AV nodal branch
  • R Marginal
  • Post descending A (dominance

Left Ventricle

L cusp of AoV

  • LCx
  • LAD

Right Ventricle

Left Ventricle

Coronary

Blood Flow

CBF = CPP/CVR

 CPP = Aortic P – (larger of) Ventricular P or RAP

Coronary

Perfusion Pressure

Right Ventricle

Throughout systole & diastole

Driven by Aortic pressures

RV SYST CPP

= Aortic Syst P – RVP

= 120 – 25

= 95mmHg

RV DIAST CPP

= Aortic Diast P – RAP

= 80 – 5

= 75mmHg

Left Ventricle

Early systole → compression → CBF briefly reverses

Mid-late systole → follows aortic P

Diastole → maximal BP as compression lifted, then falls as aortic P falls

LV SYST CPP

= Aortic Syst P – LVP

= 120-120

= 0mmHg

LV DIAST CPP

= Aortic Diast  P – RAP

= 80-5

= 75mmHg

Coronary Blood Flow

Right Ventricle

Left Ventricle

Coronary

Vascular

Resistance

R = 8hL/pr4

Radius is biggest influence

Determined by:

  • Metabolic
  • Pathology
  • Autoregulation
  • ANS
  • Humoral
  • Drugs

Ventricular Demand

Myocardial O2 consumption determined by:

Wall Tension (40%)

Basal Metabolic Activity (25%)

HR (25%)

Contractility (15%)

External work (10-15%)

Electrical Depolarisation (1%)

Right Ventricle

Left Ventricle

Wall Tension

AfterL gives rise to Wall Tension in the ventricle

∴can be thought of as wall tension required to overcome impedence to eject blood into ventricle

WALL TENSION

Wall T = (Transmural P x Radius) / (2 x Thickness)

Transmural P

Radius

Thickness

–          Thickness of LV greater due to higher afterload it works against

OUTFLOW TRACT

RV pumps blood against a low resistance circuit (PVR) and therefore in health should not require as much as the LV

SVR is primary determinant of LV afterL

LV has to pump against a much higher afterload and thus has much more demand

Right Ventricle

Left Ventricle

Basal Metabolic Activity

Basal Metabolic activity = basal O2 consumption = 8ml/100g/min both ventricles

Right Ventricle

Left Ventricle

Heart Rate

Majority of CBF occurs in diastole;

In LV blood supply occurs only in diastole, so tachycardia reduces diastolic filling time and greatly compromises the actual supply →  Subendocardium most susceptible, no flow during systole

Tachycardia will also increase the energy requirements of myofibrils from lengthening and shortening and increase demand for substrate and O2

Right Ventricle

Left Ventricle

Contractility

Intrinsic ability of myocardial fibres to shorten, independent of preL & afterL

LV will have more demand than LV owing to larger m mass

Right Ventricle

Left Ventricle

External work

Does not correlate well w O2 consumption

Refers to volume work, both ventricles eject similar volume so this work is similar

Right Ventricle

Left Ventricle

Electrical depolarisation

Similar level of contribution

Measuring Myocardial O2 Consumption

  • Requires measuring arterial + venous O2 content & application of FICK PRINCIPLE
    • Unrealistic to achieve
  • When myocardial CONTRACTILITY is steady, measuring TENSION TIME INDEX will reflect myocardial O2 consumption
TENSION TIME INDEX