Physiology - Cardiac Flashcards
Describe the normal cardiac conduction pathway
- SA node acts as the pacemaker, connected to the AV node by anterior, middle and posterior internodal tracts
- AV node delays passage of impulse from atria to ventricles
- bundle of his connects AV node to right and left bundle branches (anterior and posterior fasicles on the left)
- purkinje fibers conduct impulses from bundle branches
- ventricular muscle conducts impulse from left side of IV septum to right and down the apex then up to AV grooves
What is the normal ECG complex and what does each wave represent
waves:
- p wave = atrial depolarisation
- q wave = normal left to right depolarisation of the IV septum
- r wave = sodium influx
- t wave = ventricular repolarisation
intervals:
- pr interval = reflexes conduction through the AV node
- st interval = ventricular repolarisation
- qt interval = ventricular depolarisation and repolarisation
How does sympathetic and parasympathetic stimulation change the prepotential
sympathetic:
- NA stimulation of beta 1 receptors causes increased Na+/Ca+2 permeability, making the RMP less negative
- this increases the slope of the pre-potential and firing rate
parasympathetic:
- ACh stimulation of M2 receptors causes increased K+ conductance and slows opening of Ca+2 channels
- this causes hyperpolarisation and decreases the slope of the pre-potential and decreases the firing rate
Describe the difference between a ventricular muscle action potential and a pacemaker cell potential
- ventricular muscle has a greater negative RMP (-90 compared to -60)
- ventricular muscle depolarisation is due to Na+ influx, Ca+2 plays no role
- ventricular muscle does not have a prepotential and no automaticity
Why does tetany not occur in cardiac muscle
cardiac muscle contraction lasts 1.5 times as long as the action potential
What mechanisms cause abnormalities of cardiac conduction
- abnormal pacemaker: lead to ectopic beats, pacemaker failure, fibrillation
- re-entry circuits: lead to tachyarrhythmias
- conduction delays: lead to heart block and bundle branch blocks
- prolonged repolarisation: lead to long QTc
- accessory pathways: lead to WPW
- electrolyte disturbance: cause arrhythmia or arrest
What conditions may predispose to increased automaticity
IHD
scar tissue from previous heart operation
structural heart disease
channelopathies
electrolyte imbalance
Describe how the kidney handles K+
- K+ is filtered at the glomerulus
- most filtered K+ is actively reabsorbed at the proximal tubules
- K+ is then secreted into the fluid by the distal tubules (induced by aldosterone)
What are the ECG findings associated with hypokalaemia
long PR interval
ST depression
T wave inversion
U waves
Explain the electrophysiology causing STE in MI
1) rapid repolarisation of infarcted muscle due to accelerated K+ ch opening, current flow out of infarct (sec-min)
2) decreased resting membrane potential due to loss of intracellular K+, current flow into infarct (min)
3) delayed depolarisation, current flow out of infarct (30 min)
What are the causes and complications of AF
causes:
overall due to multiple re-entry circuits in atria or foci in pulmonary vein
-IHD, valvular disease, HTN, cardiomyopathy, thyrotoxicosis, pulmonary embolism, sepsis, electrolyte disturbance
complications:
- reduction in cardiac output due to loss of atrial kick causing haemodynamic instability
- embolic events such as stroke
Describe how the waveforms of an ECG relate to the cardiac cycle
- atrial systole starts just after the p wave
- ventricular systole starts near the end of the r wave and ends just after the t wave
What are the phases of the cardiac cycle
1) Atrial systole = phase 1
- contraction of atria propels some additional blood into the ventricles
2) Isovolumetric ventricular contraction = phase 2
- mitral valve closes with increase in ventricular pressure without change in muscle length or volume
3) Ventricular ejection = phase 3
- aortic and pulmonary valves open, 70-90ml blood is ejected from each ventricle, 50ml remain in each ventricle
4) Isovolumetric ventricular relaxation = phase 4
- aortic, pulmonary and AV valves are closed
- ends when ventricular pressure falls below atrial pressure and AV valves open and ventricles begin to fill
5) Ventricular filling = phase 5
- mitral and tricuspid valves open, aortic and pulmonary valves are closed
- blood enters ventricles (70% of ventricular filling)
When do the heart sounds occur in the cardiac cycle
- first = lub, closure of AV valve at beginning of ventricular systole
- second = dub, closure of pulmonary and aortic valves at end of ventricular systole
- third = 1/3 of the way through diastole due to rapid ventricular filling
- forth = due to ventricular filling in patients with ventricular hypertrophy, never heard normally
What are the 2 factors that determine cardiac output
Cardiac Output = heart rate X stroke volume
s
troke volume is determined by preload, afterload and contractility
heart rate is determined by sympathetic and parasympathetic stimulation
What is the stroke volume in a normal adult at rest
70-90ml
What is the definition of ejection fraction
the percentage of end diastolic ventricular volume ejected with each stroke, normally 65%
What is cardiac preload and what factors effect it
- the degree to which ventricles are stretched prior to contracting, equivalent to end diastolic volume
- increases force of contraction by the frank starling law
determined by: blood volume, venous return, sympathetic tone, venous compression (muscle pump)
What methods can be used to measure cardiac output
- ficks principle: output from LV = oxygen consumption / AO2-VO2
- indicator dilution method = flow = amount of indicator injected / concentration of indicator in arterial blood
What are causes of decreased cardiac output
arrhythmia
reduced preload (venodilation, volume loss)
increased afterload
reduced contractility (ischaemia)
What are the determinants of myocardial oxygen consumption
heart rate
myocardial contractility
wall tension
What are the changes in cardiac function with exercise and how are these mediated
- oxygen extraction can increase by 100%
- cardiac output can increase by 700%: mostly by increased HR from adrenaline and sympathetic discharge
- systolic blood pressure increases, diastolic blood pressure decreases: by sympathetic discharge
- stroke volume can increase by less than 200%: contributed by increased venous return
What physical laws are involved in the alteration of cardiac function in exercise
Starling Law = determines the energy of contraction
energy of contraction is proportional to initial fiber length
Leplace Law = determines left ventricular wall stress
wall stress is directly proportional to LV pressure and radius and indirectly proportional to wall thickness
What factors influence contractility
hypoxia
drugs (inotropes)
pH
sympathetic tone
hypercapnea
myocardial damage
