26. Parameters of the cardiac cycle; volume fractions; factors influencing cardiac output

Question 1

What should be mentioned?

Answer 1

Volume fractions:
EDV, ESV, SV, CO
Measuring CO (cardiac output)

Parameters describing the heart cycle
Ventricular compliance
Work and performance of the heart

Factors influencing cardiac output

Question 2

Volume fractions

Answer 2

• *EDV (end diastolic volume)**
• At the end of diastole (maximal relaxation, right before the next heart contraction), the ventricles are maximally filled.
• *ESV (end systolic volume)**
• When the ventricles are maximally emptied (end of systole, right before relaxation), there is still some blood remaining in them.
• *Stroke Volume (SV)**
• This volume fraction passes into the aorta at each cycle. − EDV-ESV

Cardiac output (CO)

• the volume of blood pumped into the circulation by the heart in one minute.
• heart rate times stroke volume:

CO = (EDV – ESV) × Frequency = SV × Frequency

• Cardiac output is one of the most important physiological and also clinical parameter describing the performance of the heart. It equals the amount of blood forwarded into the aorta from the left ventricle per unit time.
• At the end of systole certain amount of blood still remains in the heart. That phenomenon plays an important role in the adaptation of the heart to the increased stretch.
• The amount of blood at the end of diastole is called EDV (end diastolic volume), while the amount at the end of systole is called ESV (end systolic volume). The difference between the two gets into the periphery during the cardiac cycle: it is called Stroke Volume (SV).

Question 3

Measuring of cardiac output

Answer 3

The cardiac output is a good measure of cardiac performance.

-Using Fick’s principle

Cardiac output is based on the fact that the amount of oxygen that is taken up by the lung per unit time should equal the amount taken up by the tissues.

-CO= total O2 uptake (l/min) / arterio-venous O2 difference (l/l)

-Using Stewart’s principle

− Injecting Evans-blue i.v., sample collection & analysis, plot curve, just before recirculation do extrapolation. Area under the extrapolated curve = CO

Question 4

Ventricular compliance (picture)

Answer 4

Question 5

Ventricular compliance

Answer 5

Dilating capacity (volume change taking place due to unit-pressure change = ΔV/ΔP)

-Important parameter of the adaptability of the heart. Closely related to the dilating ability of ventricles.

-In rest:
the pressure maintained by blood arriving from periphery (EDV ventricular pressure) is 5 mmHg. − That creates 60 ml EDV.

-Increase of EDVP:

• linearly(proportionally)increasesEDV(+SV),butonly
• until 25 mmHg (from here Collagen fibers prevent further dilation).

In elderly animal compliance curve shifts to right:

• The same EDVP can’t dilate the heart to the same EDV.
• At least two-fold EDVP is needed to achieve the same EDV.

Rational:

-Decreased compliance in old animals due to increasing rigidity of elastic fibers and the aging

of muscle cells

Question 6

Rushmer diagram

Answer 6

The Rushmer diagram analyses the cardiac work (Wo) as a function of volume and pressure in the left ventricle during cardiac cycle.

A-At the beginning of Systole: mitral valves are closed, isovolumetric contraction.

B-When aortic pressure is reached: semilunar valves open, ejection begins.

C-At end of systole: ventricular P ≤ aortic, so semilunars close, but the P is still a bit ≥ than Atrial Pressure, so mitrals are still closed, isovolumetric relaxation

D.-When ventricular P ≤ atrial, mitral valves open and filling starts.

Question 7

The work of the heart

Answer 7

Total work of the heart

W t = W outer (mechanical) + W inner (heat production)

• Mechanical work serves mostly to maintain average pressure, and only 4% is the kinetic energy (moving the fluid (blood) in the system). Therefore the kinetic component is neglected, so: Wouter= SV × ΔP (arterial average pressure)
• It is also known that burning 1 l oxygen produces 20 kJ of energy in the animals. Wt = oxygen consumption × energy equivalent of O2 .
• The efficiency of the heart is cca. 10-20%, Knowing Wo and Wt the efficiency (E) of the heart can also be calculated. The inner work (heat production) can also be calculated, Wi= Wt - Wo.
• Since the heart gains its energy from oxidative processes, the total work of the heart can be determined by measuring the total oxygen consumption of this tissue.

Question 8

The performance of the heart

Answer 8

Performance = Work/time
W/t=P×V/t

P = cardiac output (CO)

-Since the heart maintains ± constant arterial pressure (Pa), the performance (W/t) is mostly determined by volume flow of the unit time (V/t), so performance is equivalent with cardiac output!

Question 9

Rushmer diagram (picture)

Answer 9

Question 10

Factors influencing the cardiac output

Answer 10

Cardiac output is the blood volume that is pumped into the systemic circulation in one minute.

-CO = (EDV-ESV) × Frequency (heart rate)

• *-EDV is influenced by:**
• Ventricular compliance (decreases with age) − Ventricular filling time in diastole.Central venous pressure (CVP; v. cava + atrial pressure) = Preload

ESV is influenced by:

• Aortic pressure (afterload), when the semilunar valves are still open.
• Contractility ( isometric maximal tension and maximal contraction velocity )
• Contractility increases by sympathetic, decreases by parasympathetic stimulation

Frequency is influenced by:

-Sympathetic effects

• artificial increase (i.e.by Pacemaker): duration of diastole decreased so CO decreased.
• natural increase (sympathetic stimuli): reduced systolic time, and proportionally reduced diastolic time (so Starling works) → SV increases → CO increases

-Parasympathetic effects

• Normally: heart is under parasympathetic control.
• Constant firing of n. vagus decreases frequency, decreases contractility

Question 11

Factors influencing Cardiac Output (CO) (picture)

Answer 11

Question 12

Effects of autonomous nervous system on the heart frequency (picture)

Answer 12