Cardiac Output

Definition

Cardiac output is the quantity of blood pumped by either ventricle in one minute. It is 5L/min in a healthy adult at rest.

Cardiac output = Stroke volume * Heart rate

( stroke volume is a volume of blood pumped out by each ventricle per beat. It is about 70 - 80 ml.  stroke volume = End diastolic volume - End systolic volume )

Factors that affect the cardiac output

There are 4 major factors. They are,

                                        1. Heart rate

                                        2. Preload

                                        3. Inotropic state of the heart

                                        4. Afterload

1.Heart rate

Heart rate is the number of beats of the heart per minute. A normal resting heart rate for adults is 70 beats per minute. Heart rate is regulated by cardiac innervation and hormones. sympathetic discharge increases the heart rate and parasympathetic discharge decreases the heart rate. Thyroxine, epinephrine and noepinephrine are the hormones which cause for the increasing heart rate. If the heart rate is more than 180/min, cardiac output decreases as diastolic shortness.

2. Preload

Preload is the degree to which the myocardium is stretched before it contracts.

According to frank starling's law, states that the energy of contraction is proportional to the initial length of the muscle fibre within physiological limits. length is proportional to ventricular end diastolic volume. (VEDV). VEDV is the volume of blood in the ventricles just before contraction. changes in preload alter the number of cross bridges that are available, by changing the overlap of thick and thin filaments in cardiac muscle.

Preload is increased by, 

- increased total blood volume

- increased venous tone

- increased pumping action of skeletal muscles

- increased negative intrathoracic pressure

- stronger atrial contraction

Prelaod is decreased by,

- decreased ventricular complance

- increased intrapericardial pressure

3. Inotropic State of the heart

Inotropic state is the strength of myocardial muscle contraction which affects the stroke volume. This is dependent on the intracellular calcium concentration.

Force of contraction is increased by, 

- symphathetic nerve stimulation

- circulating catecholamines

- hormorns ( glucagon )

- inotropic drugs - theophylline

Force of concentration is decreased by, 

- hypoxia

- hypercapnia

- acidosis

- vagal stimulation

- myocardial ischemia

- drugs quinidine

4. Afterload

Afterload is the load or resistance against which the ventricles contract to expel the blood. Pulmonary and systemic resistance are the two types of resistances. It depends on viscosity of blood ejected, physical characteristics of blood vessles (diameter etc). When afterload is increased, cardiac output is decreased.

Effects of various conditions on cardiac output.

Cardiac output is increased in,

- exercises

- eating 

- pregnancy

- high environmental temperature

- anxiety , excitement

- epinephrine

Cardiac output is no change 

- in sleep

- moderate changes of environmental temperature

• Muscular exercises will results in increase in cardiac output

1. Increased sympathetic discharge

      increased heart rate, myocardial contractibility

2. Increased venous return, VEDV contractibility

3. Decrease afterload due to vasodilation increased  stroke volume, cardiac output,