Cardio phys: Exam #2 review Flashcards
What is the formula for cardiac output? What is the average cardiac output in adults?
Cardiac output = Stroke volume (SV) x Heart rate
Normal cardiac output for adults is 4,900 - 5,600 mL/min
What is the formula for ejection fraction (EF)? What is the normal ejection fraction for adults?
EF = stroke volume (SV)/initial volume.
Normal is 55-70% in adults.
Define cardiac preload
Preload is the end DIASTOLIC pressure that stretches the R or L ventricle of the heart to its greatest geometric dimensions under variable physiologic demand. IN OTHER WORDS: It is the initial stretching of cardiomyocytes PRIOR to contraction; therefore, it is related to the sarcomere length at the end of diastole.
What are the effects of increased preload on SV, CO, and work?
SV (stroke volume): Increased
CO (cardiac output): Increased
Work: Increased
- ) If all variables remain constant, what does an increase in left ventricular end diastolic volume do?
- ) What variables must remain constant?
- ) Increased cardiac output.
2. ) Afterload and contractility
What is the consequence of an increased preload with a constant ejection fraction?
Increased cardiac output due to increased stroke volume
What is an analogous term for cardiac preload?
Ventricular filling (end diastolic volume)
Give an equation for myocardial “work”
Stroke volume X aortic pressure
- ) What are consequences of increased myocardial work?
2. ) How can work be reduced?
- ) Death…High BP exacerbates this
- ) Afterload reduction, vasodilation
* Less work = less O2 consumed*
Aortic stenosis (AS) equates to an increased _______ = MORE WHAT?
INCREASED Afterload = MORE WORK
How many mL/min of O2 do we consume?
250mL O2/min
What is the law of Laplace (in words and equation)?
The law of Laplace for a sphere (i.e. the approximate shape of the heart) states that pressure correlates directly with tension and wall thickness and correlates inversely with radius –In other words, the law of Laplace for a sphere states that the greater the thickness of the wall of the sphere (e.g., left ventricle), the greater the pressure that can be developed.
Wall tension (T) = Pressure (P) X Radius (r)
Isovolumetric ventricular contraction begins with which EKG phase?
QRS
What is the most important factor contributing to cardiac output?
End-diastolic volume
What is the maximum right atrial pressure for filling?
4 mmHg
What is the minimum volume of blood needed to fill venous capacity and therefore detect a pulse?
4 L
- ) By what means can you empty the heart fastest? Why?
2. ) What types of medications do this?
- ) Venous dilation, i.e. increasing unstressed volume, because the veins contain the greatest volume of blood (60% of total vasculature).
- ) Venous dilators –> Nitrates/nitroglycerine/amyl nitrate, Morphine (acute pulmonary edema), furosemide, Sildenafil (nitric oxide drugs), Prostacyclin
What effect does a diuretic have on preload?
Decreases preload by lowering volume
Describe the speed and mediation of the baroreceptor reflex
Attempts to restore Pa to set-point in A MATTER OF SECONDS.
Neurally mediated
Regarding baroreceptors (mechanoreceptors), what effect does stretching have on their action potential?
Increased stretch from increased ARTERIAL PRESSURE causes an INCREASE IN FIRING RATE of the afferent nerves.
What are baroreceptors most sensitive to (2)?
While they are sensitive to absolute level of pressure, they are EVEN MORE SENSITIVE to CHANGES IN PRESSURE (∆P) and RATE OF CHANGE IN PRESSURE.
What is the strongest stimulus for baroreceptors?
RAPID change in arterial pressure
What are two potential physiological causes for hypertension?
- ) A defect that decreases sensitivity of baroreceptors to increased BP, or…
- ) Increase in the BP set-point of the brain stem centers.
Describe the parasympathetic outflow that controls BP
The PS outflow is the effect of the VAGUS NERVE on the SA node to decrease the heart rate.