Cardiac output and its determinants Flashcards
Draw diagrams illustrating the relationship between stroke volume and its determinants
Draw a diagram comparing cardiac output with HR
Describe the relationship between stroke volume and preload
Stroke volume increases with increased preload, up to a plateau, beyond which it begins to decrease again
Desribe the relationship between stroke volume and afterload
Stroke volume decreased with increased afterload, in a fairly linear fashion
Describe the relationship between stroke volume and contractility
Stroke volume increases with increased contractility, for any given preload and afterload value
Draw a diagram reflecting the relationship between stroke volume and end diastolic volume
Frank starling curve
What surrogate volume is used for prelaod in measurements
EDV
How is the action of SNS different between cardiac and heart muscle
Heart muscle increase the strength and rapidity of isometric twitch and speeds up both the onset and relaxation fo the muscle twitch
It has no such effect in skeletal muscle
What is a molecular target of enhanced myocardial relaxation by norepinephrine/adrenaline
Phospholamban and troponin I
How is Frank-Starling different to contractility increase directly?
Frank starling mechanism increases the velocity of contraction and maximal isometric force, but does not change the maximum valocity of contraction at zero load
Noradrenaline in contrast increases Vmax, velocity of contraction at other loads and maximal isometric force
Relate cardiac output to HR
◦ Higher heart rate increases cardiac output as it multiplies stroke volume
◦ At very high heart rates stroke volume may be decreased due to decreased preload - all people have a maximum HR for maximum cardiac output that decreases with age; up to HR 140 CO generally increases with increasing HR, and above 140 it begins to impair diastolic filling
What is the general maximal threshold of increased cardiac output with increased HR
◦ Higher heart rate increases cardiac output as it multiplies stroke volume
◦ At very high heart rates stroke volume may be decreased due to decreased preload - all people have a maximum HR for maximum cardiac output that decreases with age; up to HR 140 CO generally increases with increasing HR, and above 140 it begins to impair diastolic filling
Preload is determined by?
◦ Intrathoracic pressure
◦ Atrial contribution (“atrial kick”)
◦ Central venous pressure (RA pressure)
◦ Mean systemic filling pressure which depends on total venous blood volume and venous vascular compliance
◦ Compliance of the ventricle and pericardium
◦ Duration of ventricular diastole
◦ End-systolic volume of the ventricle
Draw a graph describing the relationship between preload and SV
How does afterload change preload
Increased ESV –> increased EDV
How does afterload affect contractility
Anrep effect
What factors affect aferload
- Tramsural wall stress - Law of Laplace
- Transmural pressure - cavity pressure, intrathoracic pressure
- Ventricular wall radius - Arterial impedence factors
- Hagen Poiseulle factors
- Arterial elastance - proximal distension, as well as returning pressure wave
- inertia
What are the 5 factors affecting contractility
◦ Heart rate (Bowditch effect)
◦ Afterload (Anrep effect)
◦ Preload (Frank-Starling mechanism)
◦ Cellular and extracellular calcium concentrations
◦ Temperature
Define contraciity
Increased contractility improves SV for any given preload or afterload
Draw a set of diagrams illustrating the effect of afterload and preload on SV and illustrate how contractility affects these
Stroke volume increases with increased preload, up to a plateau, beyond which it begins to decrease again
Stroke volume decreased with increased afterload, in a fairly linear fashion
Stroke volume increases with increased contractility, for any given preload and afterload value
How is cardiac output different with age?
Higher proportionally in children
How is cardiac output affected by pregnancy
50% increase at term
How is cardiac output affected by eating
25% increase while eating
What has a greater influence on CO - HR or SV
HR - can change by a factor of 3
SV can only increase by up to 50%