THE CARDIAC RESERVE and

CARDIAC WORKby

Dr. Doaa Samy

Objectives:

• Define cardiac work • Define mechanical efficiency of the heart.• Define cardiac reserve, • List its parameters and limitations.• Explain the mechanism of cardiac reserve• Explain why trained athletes can achieve higher

cardiac output than non athletes can do• List the types of work done by the heart• Describe mechanical efficiency of the heart• Describe O2 consumption by the heart.• List the factors that affecting it.

The cardiac work

It is the amount of energy that the heart converts to work while pumping the blood

The cardiac work

This is the work needed to pump a certain volume of blood against the arterial blood pressure resistance.

1-Pressure volume work (pumping work)

pressure volume work/min= cardiac output X mean BP pressure volume work/beat= stroke volume X mean BP

The cardiac work

It consists of left ventricular work which is normally about 6Kg meter/minute at rest, and right work which is about 1/6 of that of the left ventricle (1Kg meter/minute) why?

1-Pressure volume work (pumping work)

The cardiac work

2-Kinetic work (acceleration work)

This is the work needed to give velocity to the blood stream . It is about 2% of the total work done by the heart during rest (about 0.14 Kg meter/minute).

The cardiac work

Therefore, the total work done by the heart during rest = 6+ 1+ 0.14 = 7.14 Kg meter/minute.

Total work done by the heart

Under basal conditions the fuel oxidized by the heart comes from

fatty acids(60%), carbohydrates(35%),

ketones and amino acids (5%)

The mechanical efficiency of the heart (M E)

In fact most of the potential energy is converted into wasted heat and a

small portion into work.

The mechanical efficiency of the heart (M E)

ME = Work done X100 Total energy expenditureDuring rest it is about 15-20%. It could be improved till it reaches 50%.

The mechanical efficiency of the heart (M E)

The cardiac reserve is the ability of the heart to

increase its work and output when required

(e.g. during muscular exercise).

Cardiac reserve

DefinitionCardiac reserve is the difference between the work of

the heart in a maximum activity and its work under basal condition.

Cardiac reserve

Cardiac reserve

The work of the heart during maximal activity (muscular exercise)

increases up to 13 folds the cardiac work at rest due to:

• Increased cardiac output.1-

• Increased mean arterial blood pressure.

2- 3-

This can be achieved by increased coronary blood flow

and increased coefficient of oxygen utilization by the

myocardial cells

Heart rate reserve

The mechanisms of cardiac reserve

Stroke volume reserve

Hypertrophy of the heart

But each mechanism has its limitation

The mechanisms of cardiac reserve

1- Heart rate reserve

The heart rate is increased through the following mechanisms:1- Increased sympathetic activity.2- Increased venous return with stimulation of the right atrial stretch receptors and direct stimulation of the SAnode.3- Increased body temperature.4- others.

Heart rate reserve limitation

Excessive increase in the heart rate

Shortens the diastolic time

1- decreases cardiac filling and stroke volume2- decreases coronary perfusion

The mechanisms of cardiac reserve

2- stroke volume reserve

The force of contraction and so the stroke volume can be increased through the following mechanisms1- cardiac dilatationDilatation of the heart by venous return EDV better systole (=Starling’s Law) ↓ESV.

2- sympathetic stimulation More efficient emptying of the ventricles ↓ESV.

Cardiac dilatation• Preload – Frank-Starling Law of the Heart

– Length tension relationship of heart– Length = EDV– Tension = SV

As the ventricles become overfilled, the heart becomes inefficient and stroke volume declines.

“cardiac reserve”

Sympathetic Stimulation

Positive Inotropic Effect

• increase the force of contraction without changing the length of the cardiac muscle cells

Excessive increase in the EDV

stroke volume reserve limitation

Decrease the force of myocardial contraction

Over stretching of myocardial fibers

The mechanisms of cardiac reserve

3- hypertrophy of the cardiac muscle

Increased size of each ventricular cell occurs when the heart is exposed to prolonged and sustained load which may be either: 1- Physiological cardiac hypertrophy2- Pathological cardiac hypertrophy

The mechanisms of cardiac reserve

3- hypertrophy of the cardiac muscle

1- Physiological cardiac hypertrophy® bulk of the heart muscle power of

cardiac contraction stroke volume.

® Hypertrophy is accompanied by dilatation EDV stroke volume

LEFT VENTRICULAR HYPERTROPHY

cardiac hypertrophy reserve limitation

Excessive hypertrophy of the heart without

corresponding increase in coronary

blood flow

Ischemia of the cardiac muscle due to inadequate perfusion

3- hypertrophy of the cardiac muscle

2- Pathological cardiac hypertrophya) Left ventricular hypertrophy

b) Right ventricular hypertrophy

Not a reserve

The mechanisms of cardiac reserve

. The mechanisms of cardiac reserve occur only during muscular exercise in normal hearts. However, in a diseased heart some of these mechanisms may be used during rest.

Cardiac output (L/min)

Heart rate (beat/min)

Stroke volume

(ml)

EDV(ml) ESV (ml)

Non-athlete

Rest 5 70 70 130 60

Maximum exercise

22 180 (*2.5) 120 (*1.7) 140 (+10) 20 (- 40)

Trained athlete

Rest 5 40 120 200 80

Maximum exercise

36 180 (*4.5) 200(*1.6) 220(+20) 20 ( -60)

COP HR SV EDV ESV

Non athleteAt rest

5 70-75 70 130 60

Max. exercise 22-25 180 120 140 20

Trained athlete At rest

5 40 120 200 80

Max. exercise 35-35 180 200 220 20

Conclusions

THANK YOU