Historical Perspective Adolf Fick: 1829- 1901 Born in Sep. 1829, in Kassel, Germany Earned MD in 1851 1855- Introduced a law of diffusion called Fick law of diffusion 1870- Was the first one to develop a technique for measuring cardiac output Fick was died in Aug. 1901
Cardiac output Volume of blood ejected from left ventricle in one minute It is the determinant of global oxygen transport from the heart to the body It reflects the efficiency of cardiovascular system There no absolute value for cardiac output measurement
Cardiac output influencing factors Ventricular Preload - Volume of blood in the ventricle at the end of diastole - Any changes in the ventricular preload will affect the ventricular stroke volume
Frank- Starling Mechanism The ability of the heart to change its force of contraction and therefore stroke volume in response to changes in venous return I.E ( the greater the diastolic volume or fiber stretch at the end of diastole the stronger the force of contraction at systole The force of contraction will decline once this physiological limit has reached
Cardiac output influencing factors Preload assessment It has been very difficult to measure the fiber length or volume at the bedside It is clinically acceptable to measure preload as a pressure. CVP is used to evaluate right ventricular preload Pulmonary artery diastolic pressure or PCWP are used to evaluate the left ventricular preload
Cardiac output influencing factors Contractility Increased contractility, will shift Starling’s curve to the left Decreased contractility will shift Starling’s curve to the right
Cardiac output influencing factors Assessment of contractility Stroke volume (SV) SV = EDV - ESV Stroke volume index (SVI) SVI = CI / HR Left ventricular stroke work index LVSWI = MAP – PCWP x SVI x (0.0136)
Cardiac output influencing factors Afterload Related to ventricular wall stress Laplace Law: Tension (T)= Pr/t Afterload per se does not Alter preload
Clinical Measurement of Afterload Right ventricular afterload -Pulmonary vascular resistance (PVR) -PVR = MPAP- PAWP / CO x 80 -Normal:< 250 dynes/ sec /cm -5
Clinical Measurement of Afterload Left ventricular afterload -Systemic vascular resistance (SVR) -SVR = MAP- RAP / CO x 80 Normal: 800 – 1200 dynes / sec / cm -5
Ventricular compliance Compliance = V/P Given a change in the pressure cause a change in the volume
Ejection Fraction Is a fraction of blood ejected by the ventricle relative to its end-diastolic volume EF= SV / EDV. 100 Ejection fraction is used as a clinical index to evaluate the inotropic status of the heart
Methods of calculating and measuring cardiac output Simple method: CO = SV x HR SV = 2ml x pulse pressure CO = [2ml x pulse pressure] x HR
Methods of calculating and measuring cardiac output Fick Principle: “gold standard” CO = VO 2 / O 2 art – O 2 ven Arterial O 2 = Hb x 1.34 x O 2 sat. Venous O 2 = Mixed venous blood VO 2 = Oxygen consumption Fick Principle relies on the total uptake of a substances by peripheral tissue is equal to the product of blood flow to the peripheral tissue and arterial – venous concentration difference of the substances Fick cardiac outputs are infrequently used because difficulties in collecting and analyzing exhaled gas conc. In critically ill patients because may not have normal VO 2 value
Methods of calculating and measuring cardiac output Thermodilution Method Based on how fast the flowing blood can dilute the substances introduced into the circulation Stewart-Hamilton Equation: CO = I x 60 / cm x t x 1/k Area under the curve is inversely proportion to the rate of blood flow. This flow is equivalent to cardiac output in the absence of shunt
Methods of calculating and measuring cardiac output Thermodilution Curve
Thermal dilution method Limitations Affected by the phase of respiration and should be measured at the same point of respiratory cycle Variations in the speed of cold water injection can result in altered measurement
Non Invasive Methods Doppler Method Based on measuring the length of blood flow- velocity in the ascending aorta in unit time. Multiplied by the cross- sectional area of the aorta to give stroke volume. Multiplied to heart rate to give cardiac output.
Methods of calculating and measuring cardiac output Impedance plethysmography This technique was developed by NASA. Cardiac output can be measured across externally electrodes. It measures a changing impedance in the chest (blood volume). The rate of change of impedance is a reflection of cardiac output. CO = Art. BP / total peripheral vasc. resistance
Questions Question 1 During a cardiac output reading the thermistor of the PAC measures the temperature of the: a.Blood b.Injectate c.Pulmonary artery blood d.Mixing of blood and injectate
Questions Question 2 Which of the following statements is correct As it relates to cardiac output curve ? a.The larger the curve, the larger the output b.The smaller the curve, the smaller the output c.The larger the curve, the smaller the output
Questions Question 3 When patients take a spontaneous breath, Venous return and cardiac output: a.Increase b.Decrease
Questions Question 4 A 50- year-old male with myocardial ischemia presents with the following: HR 128 BPM CO 6.2 L/min MAP 88 mmHg SVO 2 51% CVP 6 mmHg PAWP 16 SaO 2 94% Is the cardiac output for his tissue is adequate ?
Questions Question 5 Same patient after treatment with NTG and morphine, his numbers are as follow: HR 115 BPM CO 5.1L/min MAP 84 mmHg PAWP 13 mmHg SVO 2 61% CVP 5 mmHg SaO 2 95% Is he better or worse ?
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