Describe atrial and ventricular heart muscle.
Striated, elongated, grouped in irregular anastomosing columns; 1-2 centrally located nuclei
Name the specialized excitatory and conductive muscle fibers.
SA node, AV node, Purkinje fibers Side note: contract weakly and few fibrils
Define syncytium
Many acting as one
How can cardiac muscle act as a syncytium?
Intercalated discs: low resistance pathways connecting cardiac cells end to end; presence of gap junctions
What is the duration of action potentials in cardiac muscle?
.2-.3 sec
Name the channels present in cardiac muscle.
Fast Na+ channels, Slow Ca++/Na+ channels, K+ channels
What are the permeability changes of cardiac muscle during an action potential?
Na+ sharp increase at onset of depolarization Ca++ increased during the plateau K+ increased during the resting polarized state
Describe membrane physiology and permeability during cardiac depolarization and repolarization
Na+ Increase at onset of polarization; Decrease during repolarization Ca++ Increase at onset of depolarization ; Decrease during repolarization K+ Decrease at onset of depolarization ; Increase during repolarization
Tetradotoxin selectively blocks what channels?
fast Na+ channels changing a fast response into a slow response
What are 3 considerations to assess passive ion movement across cell?
Concentration gradient: high to low Electrical gradient: opposite charge attract, like repel Membrane permeability: dependent on presence and state of ion channels
When considering concentration gradient vs electrical gradient, what will an ion do?
Seek its Nernst equilibrium potential; gradient favors ion movement in one direction is offset by electrical gradient
During resting membrane potential (Er) what is the sate of the ion channels?
Fast Na+ and slow Ca++/Na+ are closed, K+ are open
The negative membrane potential is maintained by what?
Na+/K+ pump (3:2)
What binds and inhibits the Na+/K+ pump?
Digitalis
What is located in the cardiac cell membrane that exchanges Ca++ from the interior of the cell in return for Na+?
Ca++ exchange protein
Why do we have both a Na+/K+ pump and a Ca++ exchange protein?
If the Na+/K+ pump is inhibited, function is reduced and more Ca++ accumulates in the cardiac cell increasing contractile strength.
Absolute refractory period
Unable to re-stimulate cardiac cell; occurs during the plateau
Relative refractory period
Requires a supra-normal stimulus; occurs during repolarization
What protects the ventricles from supra-ventricular arrhythmias?
AV node and bundle
What is the normal pacemaker of the heart?
SA node
What are the features of the SA node?
Self- excitatory in nature, less negative Er, Leaky membrane to Na+/C++ (unstable resting Er), only slow Ca++/Na+ channels operational, no plateau, contracts feebly
What cells are under overdrive suppression by the SA node?
Cells of the AV node and Purkinje system
What is overdrive suppression?
Driving a self-excitatory cell at a rate faster than its own inherent rate, suppressing its automaticity
What is the mechanism of overdrive suppression?
Mechanism may be due to increased activity of the NA+/K+ pump creating more negative Er
What is the function of the AV node?
Delays the wave of depolarization from entering the ventricle; allows the atria to contract slightly ahead of the ventricles (.1 sec delay)
What takes over as a pacemaker in the absence of SA node?
AV node at a slower rate
What is the relationship of heart rate and cycle length?
As heart rate increases, cycle length decreases; inverse proportional
Describe systole and diastole at resting
systole > diastole
During systole, perfusion of the myocardium is restricted. Why?
The contracting cardiac muscle compresses the blood vessels- especially in the left ventricle
True or false: at a higher HR, the ventricle may not fill adequately.
True
Describe Systole.
Isovolumic contraction and ejection
Describe Diastole
Isovolumic relaxation, rapid inflow (70-75%), diastasis and atrial systole (25-30%)
Describe A to B
Ventricular filling, AV valves open, semilunar valves closed
Describe B to C
Isovolumic contraction, AV valves close, semilunar valves closed
Describe C to D
Ejection, semilunar valves open, AV valves remain closed
Describe D to A
Isovolumic relaxation, semilunar valves close, AV valves remain closed
What happens at point A?
AV valves open, ESV
What happens at point B?
AV valves close, EDV
What happens at point C?
Semilunar valves open
What happens at point D?
Semilunar valves close
Area enclosed be volume pressure loop is a measure of what?
Work or external work
End Diastolic Volume (EDV)
Volume in ventricles at the end of filling
End Systolic Volume (ESV)
Volume in ventricles at the end of ejection
Stroke Volume
SV=EDV-ESV; volume ejected by ventricles
Ejection fraction
percent of EDV ejected (SV/EDV x 100); normal is 50-60%
Preload
Stretch on the wall prior to contraction (proportional to the EDV)
Afterload
Changing resistance (impedance) that the heart has to pump against as blood is ejected; Changing aortic BP during ejection of blood from LV
A wave
Atrial contraction
C wave
Ventricular contraction; bulging of AV valves and tugging on atrial muscle
V wave
Associated with atrial filling
When left ventricle pressure is greater than aortic pressure, what is the state of the aortic valve?
Open; valves open with a forward pressure gradient
When aortic pressure is greater than left ventricle pressure, what is the state of the aortic valve?
Closed; valves close with a backward pressure gradient
Name the AV valves.
Mitral and Tricuspid
What are characteristics of AV valves?
Thin and filmy, chorda tendineae act as check lines to prevent prolapse, papillary muscles increase tension on chordae tendinae
Name the semilunar valves.
Aortic and Pulmonic
Heart Murmurs associated with systole have which pathologies?
Aortic and pulmonary stenosis, mitral and tricuspid insufficiency
Heart murmurs associated with diastole have which pathologies?
Aortic and pulmonary insufficiency, mitral and tricuspid stenosis
Heart murmurs with both systole and diastole have which pathologies?
Patent ductus arteriosus, combined vulvar defect
What is the Law of Laplace?
At a given operating pressure, as ventricular radius increases developed wall tension also increases Wall tension = (pressure x radius) / 2 Increased tension = increased force of ventricular contraction
Define Chronotropic
Anything that affects the heart rate; i.e. caffeine would be a positive
Define Dromotropic
Anything that affects conduction velocity
Define Inotropic
Anything that affects strength of contraction
Describe Frank-Starling Law of the Heart
Within physiological limits, the heart will pump all the blood that returns to it without allowing excessive damming of blood in veins
What are mechanisms of Frank - Starling?
Increased venous return causes increased stretch of cardiac muscle fibers: increased cross bridge formation and increased CA++ influx which both increase force of contraction, and increased stretch on SA node which increases HR
Describe hetero-metric auto-regulation
Within limits, as cardiac fibers are stretched the force of contraction is increased
Describe homeo-metric auto-regulation
Ability to increase strength of contraction independent of a length change; flow induced
What is the result of direct stretch on SA node?
Increase Ca++ and/or Na+ permeability which will increase HR
What are extrinsic influences of SA node stretch?
Autnomic nervous system, hormonal, ionic and temperature
What are the results of sympathetic innervation of the heart?
Increased HR, increased strength of contraction and increased conduction velocity
What are the results of parasympathetic innervation of the heart?
Decreased heart rate, decreased strength of contraction, decreased conduction velocity
Atropine blocks parasympathetic effects via which receptors and what is the result?
Blocks Beta receptors and muscarinic receptors; HR will increase and strength of contraction will decrease
Propanolol block SNS effects on the heart via which receptors?
Beta receptors
Stimulation of the left stellate ganglion effects?
Decreased ventricular fibrillation threshold and prolongation of QT interval
Stimulation of the right stellate ganglion effects?
Increased ventricular fibrillation threshold
Describe the cardio-accelerator reflex
Stretch on right atrial wall induces stretch receptors which in turn send signals to MO to increase outflow to heart; helps prevent damming of blood in heart and central veins
Neuro-cardiogenic syncope, a baroreceptor reflex in the ventricles is stimulated by?
Occlusion of circumflex artery (inferior wall infarct) and increase in left ventricle pressure and volume (aortic stenosis)
Neuro-cardiogenic syncope (baroreceptor reflex) results in?
Hypotension and bradycardia
When does left coronary flow peak?
Onset of diastole
When does right coronary flow peak?
Mid systole