Cardiac Performance Flashcards
Cardiac performance
. How well the heart performs this pump function
. Described in terms of functional parameters (stroke volume, stroke work, CO)
Stroke volume
. Volume of blood ejected during a single cardiac contraction
SV = EDV-ESV
Cardiac output
. Blood flow per unit time
CO = SV X HR
Cardiac index
. CO divided by the body surface area
. Compensates that the large the individual generally the greater the CO
Stroke work
. Amount of work the heart does during a single contraction
SW = VPP X SV
. VPP: ventricular pulse pressure (peak pressure minus end-diastolic pressure)
. Under physiological conditions, the right ventricular stroke volume is equal to the left, but the RV stroke work is not equal to LV
Factors that overall performance of ventricle depends on
. Preload
. Afterload
. Contractility
. HR
Preload
. Consists of forces acting to stretch cardiac mm. Fibers prior to onset of contraction (at end of diastole)
. Determines max resting fiber length in mm.
. Preload of whole heart determined by tension in ventricular wall at end of diastole which depends on Law of LaPlace (T = Pr)
Ventricular filling time
. Time available for passive filing (both rapid and reduced) to occur
. Filling time dependent on HR
. Most filling occurs during rapid filling phase
. Normally filling time is adequate to not limit perfusion of organs
. As HR inc., time available for diastolic filling dec.
. If ventricular contractility inc. w/ inc. HR stroke volume may not dec. w/ inc. HR, but stay the same or even inc. w/ inc. HR until filling time becomes limiting at very high HRs
. Situation (inc. contractility plus inc. HR) will lead to inc. in CO, the magnitude of which can be large when exercising
What occurs when diastolic filing time decreases to less than 0.1 sec?
. Occurs due to excessively rapid HR
. Time available for completion of rapid filling phase of cardiac cycle is no longe adequate
. Primary functional implication of inadequate ventricular filing time is that it results in an inability to maintain adequate CO due to reduction in SV at very high HR
Ventricular compliance
. Property of the heart that allows it to be expanded or distended
. One large change in volume produces a relatively small change in pressure
C = DeltaV/DeltaP
. As compliance dec., a given change in volume will produce an abnormally large change in pressure
. Change affects pressure gradient required for passive and active ventricular filling to occur and continue
. Myocardial compliance is dec. in conjunction w/ ventricular hypertrophy and ischemia
. Not significantly affected by altered sympathetic or parasympathetic tone
The value of passive pressure that develops during ventricular filling depends on ____
. Compliance of ventricle
. Normally back pressure is of minor importance in terms of ventricular filling
. However, when there is abnormally low ventricular compliance (high EDV), diastolic pressure assoc. w/ ventricular filling can be more important in limiting ventricular filling
Ventricular filling pressure
. Intrathoracic pressure and central venous pressure affect venous return to RA and affect LV function by influence RV stroke volume.
. Inc. in central venous pressure/Neg. intrathoracic pressure will inc. pressure gradient and enhance filling of RV
. This affects RV stroke volume ultimately LV filling and LV stroke volume
Atrial contractility
. Atrial systole only accounts for 15-20% ventricular filling at rest
. Changes in atrial contractility normally minor factors in EDV, but importance include. When diastolic filling time becomes limited
. Atrial systole can account for up to 40% of filling at high HRs
. Lack of effective contraction is not life threatening but reduces exercise capacity
Pericardial restraint
. Pericardium protects against LV distension during volume overload
. Maintains alignment of heart w/ great vessels
. Provides lubricated surface against which ventricles move
. Accumulation of fluid or fibrosis restricts or limits ventricular filling
. At the same time ventricular compliance will be reduced
. Meaning that as ventricle fills there will be disproportionate inc. in ventricular end-diastolic pressure
Frank-Starling Mechanism
. Defines relationship between EDV and ventricular pressure development
. Intrinsic mechanism for regulation of cardiac function
. If ventricles are filled to a greater extent than normal, subsequent contraction is more forceful than normal
. Systolic inc. w/ inc. EDV
. SV also inc. w/ inc. EDV
. Creates starling curve
EDV is dependent on ___
. Body position
. When supine, there is very little diastolic reserve volume so EDV can’t be substantially inc. by inc. venous return so alterations in EDV to alter cardiac function
. When standing there is a sizeable diastolic reserve volume and EDV can be inc. significantly as venous return is enhanced
. Diastolic reserve = volume of blood that could be returned to the heart
Pressure-volume loop
. Describes function of heart when ventricular muscle is allowed to shorten and blood is ejected
. Ventricular pressure inc. to overcome pressure in the aorta and ventricular volume falls and blood is being ejected
. Intraventricular pressure is highest at peak of ejection then falls
. After aortic valve closes, intraventricular pressure continues to fall as ventricle relaxes isovolumetrically
. Filling occurs and EDV begins to inc. again
. Dimensions fo pressure-volume loop depend on EDV, afterload, and myocardial contractility
Afterload
. Resistance against which ventricle contracts
. Mean aortic pressure generally approximates afterload on intact heart
Most common cause of inc. afterload is ___
Arterial hypertension
Factors influencing afterload
. Arterial pressure: greatest effect, as it inc. afterload inc.
. Peripheral vascular resistance: inc., afterload inc.
. Blood viscosity: inc., afterload inc.
. Amount of blood in aorta: mass of blood inc., greater inertia due to column of blood which has to be overcome
. Aortic compliance: reduced, afterload inc.
. Preload: inc. in preload, greater wall tension resulting from ventricular filling has to be overcome during contraction before ventricular pressure can rise
Type of relationship between level of afterload and stroke volume a a given level of preload
. Inverse
. As afterload inc., SV dec.
. Decreased SV can be compensated fro by subsequent inc. in EDV
Contractility
. Ability of heart to perform work at any given end-diastolic fiber length
. Independent of starling effect
. Influenced by metabolic condition of cells, ANS, hormones, and HR
Acute effects of a positive inotropic intervention on pressure-volume loop
. At end of diastole, ventricle is filled to same volume and pressure as before the inc. in contractility
. During systole, the ventricle ejects more blood (inc. SV) due to inc. in contractility
. Inc. SV occurs from age EDV (same preload)
Physiological modulation of ventricular contractility in normal heart is dominated by ____
ANS
. Inc. sympathetic n. Activity and E release from adrenal gland enhance contractility
. Mediated by beta-adrenergic receptors and cAMP/cAMP protein kinase system
Clinical measurements of contractility
.dP/dtmax: most accurate indirect measurement of contractility, max rate of pressure change during contraction, it is max. Positive slope of ventricular pressure curve
. Ejection fraction: ratio of stroke volume to EDV (50-75% Normal, contractile dysfunction if below 40%), difficult to measure
Ejection fraction = SV/EDV
Heart rate influences on cardiac performance
. Inc. contractility: inc. HR directly inc. contractility (Bowditch effect) which causes inc. SV
. Contribution to CO: CO doubles due to contribution of HR
Left ventricular stroke work
. Equal to the product of left ventricular pressure and stroke volume
LVSW = LVPP X SV
Minute work
. Left ventricular minute work: product of ventricular stroke work and HR (LVSW X HR)
. Index of heart function that’s provides info about dynamic contractile activity of heart over period of time rather than single beat basis
External work vs internal work
. External: area of the pressure-volume loop, work done to generate pressure and eject blood
. Internal: work done by heart prior to development of pressure, work done against non-contractile elements of the heart
Volume work vs. pressure work
. Equal amounts of external work performed by volume loading (inc. preload/EDV) or pressure loading (inc. afterload), the volume-loaded ventricle will have less O2 consumption
Cardiac efficiency
Worked performed/O2 uptake
. Normally 12-20%
. Most energy dissipated as heat
. For equal amts of external work performed by volume loading, the volume-loaded ventricle will have greater efficiency
. Pressure-loaded heart performs more internal work against non-contractile elements of heart
. Internal work does not contribute to determining efficiency
Fick Principle
. Flow = quantity consumed(or added)/arterial content-venous content
Cardiac output formula
. Expressed in L/min CO = VO2/aO2-vO2 .VO2: whole body O2 consumption .aO2: arterial blood oxygen content .vO2: pulmonary artery blood
Heart failure
. Heart fails to pump adequate CO
. Can occur on right side or left
. Left HF: failure of LV to pump adequately, leads to an inc. pulmonary venous pressure, inc. pulmonary capillary pressure
. Inc. pulmonary capillary pressure leads to inc. filtration of fluid out of capillaries causing pulmonary edema
Causes of HF
. Coronary artery disease/infarcts . Hypertension . Cardiomyopathy . Abnormal hear valves . Arrhythmias . Congenital heart defects
Coronary artery disease/infarcts
. Blockage of aa. Leads to inadequate O2 delivery to cardiac mm.
. Causes temporary or permanent damage to the muscle
. Remaining mm. Needs to work harder to pump out same CO
. If heart mm. Is inadequately oxygenated for long enough the heart muscle cells die
Hypertension
. Inc. in arterial bp (inc. in afterload) makes heart work harder to pump same amount of blood
. To maintain same CO requires ventricle to work harder, requiring more energy and O2
. Chronic hypertension can result in damage to heart mm.
Cardiomyopathy
. Heart muscle can be damaged by variety of disease processes, drugs, chronic alcohol use, or unknown causes
Effective stroke volume
. Amount of blood going out to the systemic organs
Treatment of heart failure
. Lifestyle changes
. Surgery/angioplasty: bypass, angiolasty, heart transplant
. Medications: combination of more than 1 drug
Drugs used in them treatment of heart failure
. Diuretics: eliminate excess fluid
. Cardiac inotropic drugs: inc. contractility enhancing pumping ability of the heart
. Vasodilators: reduce resistance that the heart pumps against by lowering bp
. Beta-blockers: drugs that block beta-adrenergic receptors in the heart, limits sympathetic stimulation causing lowered bp and dec. HR
