Systolic pressure (aorta) = 124 mm Hg Diastolic pressure (aorta) =82 mm Hg R-R i
ID: 165862 • Letter: S
Question
Systolic pressure (aorta) = 124 mm Hg
Diastolic pressure (aorta) =82 mm Hg
R-R interval = 800 msec
Left ventricular end-diastolic volume = 140 mL
Left ventricular end-systolic volume = 70 mL
Mean pulmonary artery pressure = 15 mm Hg
Right atrial pressure = 2 mm Hg
Left atrial pressure = 5 mm Hg
O2 consumption (whole body) = 250 mL/min
O2 content of systemic arterial blood = 0.20 mL O2 /mL blood
O2 content of pulmonary arterial blood = 0.152 mL O2 /mL blood
A. Calculate the stroke volume, cardiac output, and ejection fraction of the left ventricle.
B.Calculate cardiac output using the Fick principle. (see page 148 in Costanzo)
C. Calculate TPR by using R = P/Q where Q = CO and P is MAP – right atrial pressure.
D. If a patient developed hypertrophy of the left ventricle so that the ejection fraction decreased 20%, what would you predict could happen next to the TPR and MAP?
Explanation / Answer
(A) The basic relationships are as follows:-
Stroke volume = end-diastolic volume – end-systolic volume
where ,
Stroke volume = volume ejected by the ventricle during systole (mL)
End-diastolic volume = volume in the ventricle before ejection (mL)
End-systolic volume = volume in the ventricle after ejection (mL)
Cardiac output = stroke volume x heart rate
where ,
Cardiac output = volume ejected by the ventricle per minute (mL/min)
Stroke volume = volume ejected by the ventricle (mL)
Heart rate = beats/min
Ejection fraction = stroke volume/end-diastolic volume
where ,
Ejection fraction = fraction of the end-diastolic volume ejected in one stroke
Now we can use these basic equations to calculate stroke volume, cardiac output, and ejection fraction in this case.
Stroke volume = left ventricular end-diastolic volume
– left ventricular end-systolic volume
= 140 mL – 70 mL = 70 mL
Cardiac output is the volume ejected by the left ventricle per minute. It is calculated as the product of stroke volume (determined to be 70 mL) and heart rate.it can be calculated from the R-R interval. "R" is the R wave on the electrocardiogram and represents electrical activation of the ventricles. The R-R interval is the time elapsed from one R wave to the next. It is also called cycle length (i.e., time elapsed in one cardiac cycle).
Heart rate = 1/cycle length
= 1/800 msec
= 1/0.8 sec
= 1.25 beats/sec
= 75 beats/min
Cardiac output = stroke volume x heart rate
= 70 mL x 75 beats/min
= 5250 mL/min
Ejection fraction = stroke volume/end-diastolic volume
= 70 mL/140 mL
= 0.5, or 50%
(B) we calculate cardiac output as the product of stroke volume and heart rate. However, we measure cardiac output by the Fick principle of conservation of mass. The Fick principle for measuring cardiac output employs two basic assumptions: (1) Pulmonary blood flow (the cardiac output of the right ventricle) equals systemic blood flow (the cardiac output of the left ventricle) in the steady state. (2) The rate of 02 utilization by the body is equal to the difference between the amount of 0 2 leaving the lungs in pulmonary venous blood and the amount of 02 returning to the lungs in pulmonary arterial blood. This relationship can be stated mathematically as follows:
- O2 consumption = cardiac output x [02]pulmonary vela – cardiac output x [02]pulmonary artery
Rearranging to solve for cardiac output:
Cardiac output = , O2consumption / [02 ]pulmonary vein — [02 'pulmonary artery
where ,
Cardiac output = cardiac output (mL/min)
02 consumption = 02 consumption by the body (mL 02/min)
[O2]pulmonary vein = 02 content of pulmonary venous blood (mL 0 2 /mL blood)
[021) pulmonary artery = 02 content of pulmonary arterial blood (mL 02/mL blood)
Cardiac output – 250 (mL/min) / 0.20 mL 02 /m1. blood – 0.152 mL 02 /mL blood
250 mL/min / 0.048 mL 02 /mL blood
= 5208 mL/min
Thus, the value for cardiac output measured by the Fick principle (5208 mL/min) is very close to the value of 5250 mL/min calculated as the product of stroke volume and heart rate.
(C) TPR is the collective resistance to blood flow that is provided by all of the blood vessels on the systemic side of the circulation. These blood vessels include the aorta, large and small arteries, arterioles, capillaries, venules, veins, and vena cava. Most of this resistance resides in the arterioles.
The fundamental equation of the cardiovascular system relates blood flow, blood pressure, and resistance. The relationship is analogous to the one that relates current (I), voltage (V), and resistance (R) in electrical circuits as expressed by Ohm's law (I = AV/R). Blood flow is analogous to current flow, blood pressure is analogous to voltage, and hemodynamic resistance is analogous to electrical resistance. Thus, the equation for blood flow is:
Q = P/R
or, rearranging and solving for R, R = P/Q
where Q = blood flow (mL/min) P = pressure difference (mm Hg) R = resistance (mm Hg/mL per min)
Therefore, to calculate total peripheral resistance (TPR), it is necessary to know the total blood flow through the systemic circulation (i.e., cardiac output of the left ventricle) and the pressure difference across the entire systemic circulation.
Cardiac output was calculated by different methods as 5250 mL/min and 5208 mL/m in, respectively. These values are similar, and we can (arbitrarily) take the average value (5229 mL/min) to represent cardiac output. The pressure difference across the systemic circulation ( P) is the difference in pressure at the inflow and outflow points. Inflow pressure is aortic pressure, and outflow pressure is right atrial pressure., mean aortic pressure was calculated as 96 mm Hg. Right atrial pressure is given as 2 mm Hg. Thus, P across the systemic circulation is 96 mm Hg – 2 mm Hg, or 94 inm Hg. Resistance (R), which represents TPR, is: -
R = P/Q
or
TPR = (mean arterial pressure – right atrial pressure)/cardiac output
= (96 mm Hg – 2 mm Hg)/5229 mL/min
= 94 mm Hg/5229 mL/min
= 0.018 mm Hg/mL per min
Related Questions
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.