Potentially useful information: one mL one cm3 = 10-6 m3. One mmHg = 133.3 Pa (N

Question

Potentially useful information: one mL one cm3 = 10-6 m3. One mmHg = 133.3 Pa (N/m2). =EDV.ESV. CO HRXSV. Ejection Fraction = SV/EDV SV Hints: calculate heart rate for each condition. Remember to calculate work for the entire volume of blood that is pressurized i.e., end diastolic volume (EDV). Therefore you will need to use the data provided to calculate the EDV to answer this question. The work done by the heart is Pressure-Volume work. While both ventricles are doing work, in this problem you will just calculate the work done by the left (high pressure) ventricle. In lecture you saw that if both pressure and volume are expressed in ?I units, their product is N:m (work!) In this assignment you will calculate the work done by the left ventricle of the heart each minute to support a cardiac output of 5L/min. a) In a healthy person make the following assumptions: Stroke volume is 70mL and the Ejection Fraction is 58% (these are normal values). If systolic pressure is 120 mmHg, what is the work that the heart must perform each minute to produce a cardiac output of 5 L/min? b) Individuals with hypertension (high blood pressure) have a much greater resistance in their blood vessels and thus their heart not only has to produce greater pressure, it will have a much greater volume left following contractions (end systolic volume, ESV) Calculate the work the heart (for the identical cardiac output of 5L/min) in an pressure of 150 mmHg, an ejection fraction of 40% and a stroke volume of 60mL. 150mmHg is 25% greater than 120 mmHg. In this individual, how much greater is their individual with stage one hypertension for which these values apply: systolic blood c) cardiac work?

Explanation / Answer

a)

Stroke volume = 70mL

Ejection Fraction = 58%

End Diastolic Volume            = SV/Ejection Fraction

                                                = 70 × 100/58

                                                = 120 mL

Systolic Pressure = 120 mL/min

Cardiac Output = 5 L/min

Heart Rate = Cardiac output/Stroke volume = 5000/70 = 72 (approx.)

Work done per beat    = Systolic Pressure × EDV

                                    = 120 ×120

Work done per minute = 120 ×120 × 72 ×10-6 × 133.3

                                    = 138 N.m

So, the work done by the heart must be 138 N.m per minute to support a cardiac output of 5L.

b)

Ejection Fraction = 40%

Stroke volume = 60 ml

End Diastolic Volume            = SV/Ejection Fraction×

                                                = 60 × 100/40

                                                = 150 mL

Systolic Pressure = 150 mL/min

Cardiac Output = 5 L/min

Heart Rate = Cardiac output/Stroke volume = 5000/60 = 83 (approx.)

Work done per beat    = Systolic Pressure × EDV

                                    = 150 ×150

Work done per minute = 150 ×150 × 83 ×10-6 × 133.3

                                    = 249 N.m

So, the work done by the heart must be 249 N.m per minute to support a cardiac output of 5L.

c)

The change in work done by the heart

= (249 – 138) × 100/138

= 111 × 100 /138

= 80%

So, the work done will be 80% greater, when SBP changes by 25%.

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