Cardiac out put and its regulation

Cardiac Output Dr Raghuveer Choudhary

Objectives ❖ Define stroke volume, end-systolic volume, and end-diastolic volume . ❖ Define cardiac output, venous return, cardiac index & cardiac reserve . ❖ Understand the concept of preload and afterload . ❖ Understand the factors affecting the EDV (the venous return ). ❖ Understand the factors affecting the ESV . ❖ Know how cardiac contractility & heart rate changes affect CO . ❖ Identify the factors that affect heart rate . ❖Know the method for measurement of CO (The direct Fick’s method ) . ❖ Describe the factors affecting the SV&CO

CARDIAC OUTPUT Cardiac output is defined as: The volume of the blood pumped out by each ventricle per minute . Cardiac output = Stroke volume x Heart rate CO = SV x HR = 70 x 70 = 4900ml/min (5000ml/min)

1- Cardiac Output It is the amount of blood pumped by each ventricle per minute. It keeps the arteries full of blood. An increase in cardiac output results in increased blood pressure. Anything that decreases cardiac output also decreases blood pressure, because there is less pressure on the vessel walls. Cardiac Output = Heart Rate X Stroke Volume Anything that affects heart rate or stroke volume affects cardiac output and thus blood pressure. 21/06/2021 7

STROKE VOLUME (SV) Volume of blood pumped out by each ventricle per beat. 70ml / beat (70 to 80ml / beat) SV = EDV – ESV END-DIASTOLIC VOLUME (EDV) Volume of blood present in the ventricular cavity at the end of ventricular diastole. EDV = 120 to 130ml

END-SYSTOLIC VOLUME (ESV) Volume of blood present in the ventricular cavity at the end of ventricular systole. ESV = 40 to 50ml SV = EDV – ESV More EDV, more SV More ESV, less SV

EJECTION FRACTION Percentage (fraction) of EDV ejected out by each ventricle per beat. It is 60 to 65% of EDV Ejection Fraction is a valuable index of ventricular function

CARDIAC INDEX It is the cardiac output with respect to body surface area, i.e ; cardiac output per square meter of body surface area. 70 kg adult male = 1.7 sq m of body surface area. CO 5 CI = ------------------------ = ------------ Body Surface area 1.7 = 3 lit /min/ m 2 80 years man :- CI = 2.4 lit/min/m 2 10 years child :- CI = 4 lit/min/m 2

During the first 3 hours after meals, the CO increases by ≈ 30% to enhance blood flow in the intestinal circulation . Later months of pregnancy are accompanied by 30% increase in CO due to increased uterine blood flow . At environmental temperature above 30°C, the CO is increased due to increased skin blood flow. Also at low environmental temperature CO is increased due to shivering that increases blood flow to the muscles . Increased sympathetic activity during anxiety and excitement (enhances the CO up to 50% - 100 %.) Sitting or standing from the lying position deceases the CO by 20-30%.

CO is crucial since it is also the amount of blood that flows into the circulation and is responsible for transporting substances to and from the tissues. Thus, the body has strict control mechanisms that maintain adequate CO.

↑VR → ↑EDV → ↑initial length → ↑force on contraction → ↑SV → ↑CO when it exceeds physiological limits: ↑↑EDV → sarcomere disruption & loss of its function → ↓force of contraction → ↓SV

The force of contraction is proportional to the initial length of the cardiac muscle within physiological limits end diastolic volume (we will discuss EDV later in cardiac cycles) Cardiac muscle accommodates itself to the changes in venous return up to certain limits (excessive stretch causes damage to heart muscles which decreases the force of contraction) The force of contraction is proportional to the initial length of the cardiac muscle within physiological limits end diastolic volume (we will discuss EDV later in cardiac cycles) Cardiac muscle accommodates itself to the changes in venous return up to certain limits (excessive stretch causes damage to heart muscles which decreases the force of contraction)

Normally, Rt atrial pressure (RAP) fluctuates with atrial contraction and respiration. When the mean RAP is about 0 mmHg, the CO in an adult is about 5 L/min. Because of the steepness of the cardiac function curve, very small changes in RAP (just a few mmHg), can lead to large changes in cardiac output .

Sympathetic stimulation affects both the heart and the systemic circulation: (1) It makes the heart a stronger pump. (2) in the systemic circulation, it increases the Psf because of contraction of the peripheral vessels, especially the veins, and it increases the resistance to venous return.

It is expressed as tension which must be developed in the wall of ventricles during systole, i.e the load the heart needs to overcome to open the semilunar ( Aortic and pulmonary ) valves and eject blood to aorta/pulmonary artery . - Left ventricular afterload represents the force that the muscle must generate to eject the blood into the aorta. - When the aortic pressure ( afterload ) is reduced, the velocity of shortening of the LV myocardial fibers increases ( the ventricles will contract easily ) . Hence, the LV can eject blood more rapidly → ↑ SV → ↓ ESV . - The opposite is true with increased LV afterload .

Venous Return

CARDIAC OUTPUT MEASUREMENT DIRECT METHOD Animal anesthetized Direct cannulation of great vessels Electromagnetic / Doppler flowmeter

MEASUREMENT OF CARDIAC OUTPUT Direct Methods Cardiac output can be measured directly by placing an electromagnetic flow meter in the ascending aorta or by using a cardiometer . These are accurate methods of measuring cardiac output. However, these direct methods are applicable only in experimental animals or in humans (in patients) undergoing open thoracic surgery. In humans, cardiac output is usually determined by using Doppler combined with echocardiography .

Indirect Methods 1. Fick method 2. Indicator dilution method 3. Thermo-dilution method 4. Ballistocardiography 5. Echocardiography 6. X-ray method 7. Pulse-pressure method

Fick Method Definition: Fick principle is defined as the amount of a substance taken up by an organ or by the whole body per unit of time is equal to the arteriovenous difference of the substance times blood flow. Procedure: Cardiac output can be measured by measuring the amount of oxygen consumed by the body in a given period and dividing this value by the arteriovenous difference of oxygen across the lungs. The oxygen consumption of the body is measured by spirometry. As the arterial content of oxygen is same in all parts of the body, for measuring oxygen content of the arterial blood, the blood is obtained from any peripheral artery . The venous blood is collected from the pulmonary artery by placing a catheter into it through the heart.

Measurement of cardiac output Fick Method adding 10 beads per minute

Measurement of cardiac output Fick Method adding 10 beads per minute concentration is 2 beads per litre Flow = Rate added Concentration = 10 beads/min 2 beads/litre = 5 litres/min

OXYGEN FICK METHOD: Fick’s Principle The amount of a substance taken up by an organ or the whole body in a unit time is equal to arteriovenous difference times the blood flow.

Measurement of cardiac output Fick Method lung rate of O 2 consumption Flow = rate of O 2 consumption [O 2 ] leaving – [O 2 ] entering = 250 ml/min 190 – 140 ml/litre = 5 litres/min O 2 concentration of blood leaving lung O 2 concentration of blood entering lung

Table 31–3 Effect of Various Conditions on Cardiac Output. Condition or Factor a No change Sleep Moderate changes in environmental temperature Increase Anxiety and excitement (50–100%) Eating (30%) Exercise (up to 700%) High environmental temperature Pregnancy Epinephrine Decrease Sitting or standing from lying position (20–30%) Rapid arrhythmias Heart disease

The cardiac output is calculated as: O2 consumption (mL/min) Output of left ventricle = —————————————— (AO2 )  (VO2) Advantages 1. Result is accurate 2. No chemical is injected Disadvantages 1. Catheterization should be done by expert hand 2. Hospitalization is required for catheterization 3. Patient may be apprehensive of catheterization that increases cardiac output 4. Simultaneous measurement of oxygen consumption makes the process difficult. 5. It is difficult to measure cardiac output by this method in ambulatory patients and during exercise.

DYE DILUTION (HAMILTON’S DILUTION) METHOD (Indicator Dilution Method) Evan’s blue (T-1824) Albumin labeled with I 131 Cardiogreen Brilliant violet red

Indicator Dilution Method Principle: A known amount of an indicator is injected into circulation usually through an arm vein and the concentration of the indicator is measured in serial samples of the arterial blood. The output of the heart is equal to the amount of indicator injected divided by its average concentration in arterial blood after a single circulation through the heart . Procedure This method is popularly known as Hamilton’s dye dilution method . The dye injected is usually the Evans’ blue or indocyanine green . Before injection of the dye, 10 ml of peripheral venous blood is withdrawn and divided equally into two samples. In one sample of 5 ml, enough quantity of the dye is injected to give a concentration of 0.5 mg/100 ml. (used as standard). The other sample is used as blank.

One ml of the dye solution containing 5 mg is injected rapidly into the basilic vein. From a limb artery, the blood samples are collected at an interval of 2 s in serial tubes. The tubes are then centrifuged together with the standard and blank tubes, following which the concentration of dye is determined photocolorimetrically . The concentration of the successive samples is plotted on a semi-log paper . The resulting concentration of dye in the arterial blood changes with time. First , the concentration rises as the indicator carried by the fast-moving blood reaches the arterial sampling point Second, reaches a peak as majority of indicator substance arrives at the sampling point

Finally , the concentration falls as indicator carried by slow moving blood arrives at the point. Thus, the result obtained gives a curve with an ascending limb , a peak , and a descending limb . But the descending limb ends with a rise. The slope of descending limb is extrapolated to the abscissa. The point on the time scale at which it touches the abscissa, gives the time of first passage of the dye through the artery (t). Advantages: Accurate method . Disadvantages Should not be repeated in short time as the concentration of the dye of the earlier use may give errors.

Thermodilution Method Principle Same as indicator dilution technique . In this method, the cold saline is used as the indicator. A double lumen catheter is used. Following catheterization, the cold saline is injected into the right atrium through one side of the catheter. A thermistor is placed in the other end of the other side of the catheter. The change in temperature of the blood is recorded in the pulmonary artery through the thermistor placed in the catheter. The change in temperature is inversely proportional to the amount of blood flowing through the pulmonary artery.

Advantages 1. Saline is harmless 2. Cold is dissipated, so recirculation is not a problem 3. Can be repeated many times, if needed 4. Usually preferred for children as saline is nontoxic5. Useful in severely sick patients (serious patients in intensive care units) Disadvantages 1. Cardiac catheterization is required.

OTHER METHODS 1) Echocardiography: This is a noninvasive technique in which ultrasonic waves emitted from a transducer detects waves reflected from various parts of the heart . When it is combined with Doppler technique it determines the velocity and volume of flow of blood through various cardiac valves. Principle Ultrasound waves are transmitted into the chest and the reflection of these waves off the various parts of the heart is analyzed. A transducer, which is a small microphone-like device, is held against the chest. The transducer sends and receives the ultrasound waves . By moving the transducer to various positions on the chest, different structures of the heart and the blood flow are analyzed. The computer software assembles the reflected ultrasound waves to create an image of the heart. These images appear on a television screen.

Parameters Seen 1. Size and shape of the heart 2. Pumping efficiency of the heart: 3. Valve abnormalities Advantages It is a non-invasive test . Accurate diagnosis and develop a treatment plan that is best. It is a safe and painless way Transthoracic echocardiography is used in critically ill patients Limitations The major limitation is that it is often difficult to obtain good quality images from persons who have broad chests, are obese, or are suffering from chronic lung disease.

2) Ballistocardiography In this method, the vibrations generated by each heart-beat are received and converted into waveforms by a transducer that records the cardiac activities on an ink recorder . From the recording, cardiac output is calculated by using a special formula by analyzing the recorded waves. However, cardiac output measured by this method is not an accurate one. 3) X-ray Method In this method, a radio-opaque dye is injected intravenously and then the size of the heart is detected by serial x-rays in systole and diastole from which cardiac output is measured using computer programme . 4) Pulse-Pressure Method Pulse pressure (difference between systolic and diastolic pressures) provides a rough idea of cardiac output.

INDIRECT METHODS Oxygen Fick Method Dye Dilution Method Thermal Dilution Method Doppler’s Method Ballistocardiography Echocardiography Radionucleotide angiography