Contractility Flashcards
systole - defined
*left ventricle (and right ventricle) contract
*sending blood out the pulmonic and aortic valves
diastole - defined
*left ventricle (and right ventricle) relax
*ventricles allow themselves to fill with blood to prepare for the next heartbeat
LV ejection fraction (EF) - defined
*EF is an index of ventricular contractility (how much blood the left ventricle ejects in 1 heartbeat)
*normal EF is > 50% (generally 50-70%)
*in essence: EF = (amount of blood pumped out of the ventricle) / (total amount of blood in ventricle)
LV ejection fraction - formula
EF = (EDV - ESV) / EDV = SV / EDV
recall: stroke volume: SV = EDV - ESV
EDV = end diastolic volume (represents when the LV has the most blood in it)
ESV = end systolic volume
SV = stroke volume
stroke volume (SV) - formula
SV = EDV - ESV
EDV = end diastolic volume
ESV = end systolic volume
gold standard test used to measure LV ejection fraction
cardiac MRI
cardiac output - defined
*the amount of fluid pumped through the heart in 1 minute (measured in L/min)
-note: organs receive different amounts of cardiac output (ex. brain receives 15%, kidneys receive 20-25%, heart receives 5%)
-normal range: 4-8 L/min (but there is significant variation related to body size)
cardiac output - formula
CO = SV x HR
CO: cardiac output (measured in L/min)
SV: stroke volume (SV = EDV - ESV)
HR: heart rate
cardiac index
*a measure used in many intensive care settings to account for body size by normalizing the cardiac output relative to body surface area
*cardiac index = CO / body surface area
*normal cardiac index ranges from 2.5 L/min/m2 to 4.5 L/min/m2
note: a cardiac index < 1 L/min/m2 is not compatible with life
a cardiac index < ? is incompatible with life?
cardiac index < 1 L/min/m2 is not compatible with life
Swan-Ganz catheter
*small tube, typically placed in someone’s jugular vein or subclavian vein
*allows for the direct measurement of pressure and obtain blood samples throughout the right-sided circulation system
*used to calculate cardiac output (based on Fick Principle)
Fick Principle as a measure of cardiac output
CO = (rate of O2 consumption) / (arterial O2 content - venous O2 content)
examples of how different factors can affect CO using Fick Principle
*if the rate of O2 consumption increases (exercise, increased metabolism, etc), the cardiac output increases
*if cardiac output decreases (and metabolism remains the same), then the cells/tissues of the body get their oxygen by attempting to extract more oxygen from the bloodstream
Fick Principle: obtaining the measures
*arterial O2 content: measured from a peripheral artery
*venous O2 content: measured from the pulmonary artery (via the Swan-Ganz catheter)
*a commonly used value for rate of O2 consumption at rest is 125 mL O2/min/m2
*however, OXYGEN CONTENT can be DIRECTLY CALCULATED using:
([Hb] x 1.34 x O2 sat) + 0.003(PaO2)
thermodilution method for measuring CO
*measures CO (using Swan-Ganz catheter) by extrapolating flow based on the change in temperature at distal post after injection of a cooled solution at a more proximal port
factors that affect stroke volume
1. contractility: increase in contractility increases SV
2. preload: increase in preload increases SV
3. afterload: DECREASE in afterload increases SV
sarcomeres in the heart - general principles
*sarcomeres are the heart’s building blocks
*many sarcomeres are contained within a single muscle cell
*muscle cells are organized into myofibrils, which are themselves organized into muscle fibers
*muscle fibers are organized into different regions of the heart, such as the endocardium, myocardium, and epicardium
sarcomeres - bands & filaments
*recall: myosin is the thick filaments, and actin is the thin filaments
*I band: actin + titin
*H band: myosin only
*A band: actin-myosin overlap + myosin only
*with contraction, both the H and I bands narrow, but the A band stays the same
sarcomere electron micrograph
troponin complex
*composed of troponin C, troponin I, and troponin T
*when calcium becomes bound to specific sites in the troponin complex, a series of protein structural changes such that tropomyosin is rolled away from myosin-binding sites on actin, allowing myosin to attach to the thin filament and SHORTEN THE SARCOMERE, which produces force
troponin C
*binds to calcium ions to produce a conformational change in troponin I
*part of the troponin complex
troponin T
*binds to tropomyosin, interlocking them to form a troponin-tropomyosin complex
*part of the troponin complex
troponin I
*binds to actin in thin myofilaments to hold the troponin-tropomyosin complex in place
*part of the troponin complex
actin and myosin cross-links during contraction
*for most circumstances, the more actin-myosin cross-links, the more force is generated
*actin-myosin will continue to form cross-links until enough force has been generated for the sarcomere to shorten
*if only 1 actin-myosin cross-link is needed to generate sufficient force to shorten, then it will obviously shorten faster than if 4 actin-myosin cross-links were needed
