Principles of Cardiac Output Study Guide Flashcards
Cardiac Output (CO)
the amount of blood pumped by each ventricle per minute 5-6L/min
Stroke Volume (SV)
the amount of blood pumped by each ventricle per beat
- Correlates with strength of ventricular contraction
- Typically, about ~70mL
equation for CO.
CO = Heart Rate (HR) x Stroke Volume (SV)
- ex: CO = 75 bpm x 70mL/min = 5250mL/min = 5.25L/min
cardiac reserve
the difference in resting CO and maximal CO (typically 4-5x resting CO, but in athletes can be as much as 7x CO)
How do EDV and ESV relate to SV and therefore CO?
EDV - ESV = SV
Bigger SV = bigger CO
typical ejection fraction for a healthy heart
Each ventricle pumps abt 60% of its blood w each contraction 70mL/120mL x 100 = 60%
3 factors that regulate stroke volume
preload, afterload, contractility
Preload
the degree to which muscle cells are stretched before contracting
- Higher Preload = Higher SV
- Preload increases with increased venous return – through exercise w increased SNS activity, and increased filling time
Afterload
the pressure the ventricles must overcome to eject blood, ‘back pressure’ on the aortic and pulmonary valves 80 mmHg in aorta and 10 mmHg in pulmonary trunk
- Hypertension increases afterload - ventricles have to work harder to eject blood
Contractility
the contractile strength achieved at a given muscle length, increases with rises in ca2+ and increased SNS activity
Frank-Starling Law
A length tension relationship – cardiac muscle cells are stretched to their optimal length for maximal contraction
Ionotropic agents
increase contractility
Positive ionotropic agents
Epinephrine, norepinephrine, thyroxine, glucagon, high levels of extracellular ca2+, and the drug digitalis
negative ionotropic agents.
acidosis, rising extracellular K+ levels, and Ca2+ channel blocker class of drugs (amlodipine, cardizem)
Chronotropic agents
increase/decrease heart rate
positive chronotropic agents
epinephrine, thyroxine, hypercalcemia
negative chronotropic agents
hypocalcemia
How to calculate your maximal heart rate
Age - 220
How does maximal heart rate guide your exercise routines?
You want to be in the 50-85% range for exercise, so for example, a 20 year old would subtract 220 from their age (=200) and then aim to have the heart rate be 100-170 bpm
how SNS regulates heart rate
- Emotional and physical stressors activate the SNS – epinephrine is released, the SA Node depolarizes more rapidly
- SNS also increases heart contractility and speeds heart relaxation via enhanced Ca2+ movement
- Enhanced contractility lowers ESV so SV doesn’t decline as it typically does with an increased HR
how PNS regulates heart rate
- Reduces heart rate, mediated by Acetylcholine
- Acetylcholine hyperpolarizes the membranes of its effector cells by opening K+ channels
vagal tone
Both the SNS and PNS are continuously sending signals to the heart – typically, the PNS predominates (lowers heart rate)
- An impairment of the vagus nerve will increase resting HR by ~25 bpm (75 bpm to 100 bpm)
* When either the SNS or PNS is activated more strongly, the other is inhibited
Define the atrial or Bainbridge reflex.
- An autonomic reflex initiated by increased venous return and increased atrial filling
- Stretching of the atrial walls increases heart rate by stimulating the SA node and atrial stretch receptors
- Stretch receptor activation triggers reflexive adjustments of autonomic output to the SA node – increased HR
Epinephrine
increases both heart rate and contractility
