Clin Phys Week 4 E Flashcards
What are the AP generating capabilities of the different cardiac cells?
SA: 60-100bpm
AV: 40-60bpm
Purkinje fibres: 20-40bpm
What is the order of electrical activity in the myocardium?
1) atria begin to depolarise
2) atria depolarise
3) ventricles begin to depolarise from the apex. Atria repolarise.
4) ventricles depolarise
5) ventricles begin to repolarise from the apex
.6) ventricles repolarise.
Why is there a delay in the AV node?
Size of the fibres
Less negative resting membrane potential
Fewer gap junctions connecting successive muscles
What can affect the rate of action potentials from pacemaker cells?
Nervous system
Electrolyte extracellular fluid concentration
Hormones and chemicals
Mechanical forces
What balances the heart rate?
Sympathetic cardio acceleratory centre (sympathetic nerves from the cortex)
Parasympathetic cardioinhibitory centre (vagus nerve)
What inputs come to the cardiac centre that can affect heart rate?
Higher brain centre (eg sensory and emotional stimuli)
Peripheral sensory input proprioceptor, baroreceptors, chemoreceptors, drugs (caffeine, nicotine etc) hormones
Sympathetic nervous system: noradrenaline and it’s impacts on heart rate
Distributed to all parts of the heart.
Mediates by Beta 1 receptors (SA and AV Node)
Release of noradrenaline which increases the hearts permeability to Na + and Ca+.
This results in increase of conduction of the impulse and strength of contractions called the Ionotropic effect.
Increases sinus rhythm = chronotropic effect. Can increase heart rate by up to 3x resting level.
Parasympathetic nervous system: acetylcholine and it’s impacts on heart rate
Mainly due to stimulation of SA and AV nodes.
Release of ACh increases the membrane permeability to K+.
Stops the heart beat for a few second and then beats at 40% of the resting level.
Vagal control decreases the rate of SA firing and decreases excitability of AV junctional fibres.
Mediated via M2 (muscarinic ACh receptors)
What happens to heart rate with an increase in K+ ion concentration?
Is causes the heart to dilate, become flaccid and decreases heart rate.Large quantities also block conduction of the nervous impulse through the AV bundle
What happens to the heart and heart rate with an increase in Ca2+ ion concentration?
Causes the heart to undergo a continuous spastic contraction.
What is sinus rhythm?
Set by the SA node, in adults at rest it is 70-80bpm
What is tachycardia?
SA node fires at a higher rate (eg sympathetic nervous system stimulation)
What is bradycardia?
SA node fires at a lower rate (less than 50bpm)
What is arrhythmia?
Abnormal cardiac rhythm.
Heart block: failure of conduction system. bundle branch block total heart block (damage to AV node)
What is ectopic foci?
A region of spontaneous firing (not SA node)
- nodal rhythm = rhythm set by AV node of 40-60bpm
- intrinsic ventricular rhythm = 20-40bom(Any cell in the heart outside the conduction system can generate the action potential)
What is an electrocardiogram
A recording of the electrical activity of the heart (not mechanical). The graphic record produced by an electrocardiograph.
What is an electrocardiograph?
A galvanometric device that detects, amplifies and records the minute differences in electric potential caused by the heart cells
What does each axis of the ECG mean?
The x axis is time. The y axis is mV.
One small square of 1mm by 1mm = 0.04 (40ms) seconds and 0.1m .
One large square of 5 1mm by 1mm squares = 0.2 seconds (200ms) and 0.5mV
On an ECG what is a flatline also called?
Isoelectric line
If part of an ECG record contains waves and isoelectric lines what is it called?
Interval
What is no electrical activity on an ECG called?
It is presented as an isoelectric line or flatline. Also called a segment.
On an ECG what are electrical activities presented as?
Deflections or waves
What are the 4 characteristics of waves on an ECG?
- Rate
- Rhythm
- Amplitude
- Contours
Describe the wave characteristic: rate
How often a wave is recorded per unit of time
