What are the AP generating capabilities of the different cardiac cells?
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?
Electrolyte extracellular fluid concentration
Hormones and chemicals
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?
If part of an ECG record contains waves and isoelectric lines what is it called?
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?
Describe the wave characteristic: rate
How often a wave is recorded per unit of time
Describe the wave characteristic: rhythm
Is the recording of the same wave in the same intervals
Describe the wave characteristic: amplitude
Are all the waves the same height?It depends on the strength of electrical activity.Is measured in millivolts.
Describe the wave characteristic: contours
Are the waves originating from the same source?Do they present a similar shape?
In an ECG the P wave represents what?
Depolarisation of the atria
In ECG the PQ interval represents what?
Time of atrial contraction
In an ECG the PR Interval is what?
The time it takes for the impulse to reach the ventricles (0.12-0.2s)
In an ECG the QRS complex is what?
Depolarisation of the ventricles
In an ECG the ST segment is what?
Time of ventricular contraction
On an ECG the T-wave is what?
Re-polarisation of the ventricles
What are the two types of ECG recordings?
Describe bipolar recordings
Biopotentials are recorded between two active leads
Described unipolar recordings
Bio potentials are recorded between one active electrode and a zero potential (earth) on the skin.
Three lead ECG
Looks at the heart in vertical plane between two points.Limb leads look at the heart from the sides and feet.Right wrist (anterior)Left wrist (anterior)Left leg (medial surface just above muscles)
What is Einthovens triangle?
An imaginary triangle formed by active electrocutes and flow of current.
What is the order of leads on a 3 lead ECG and where are the positive and negative charges placed?
Lead 1:+ve left arm -ve right arm
Lead 2:+ve left leg -ve right arm
Lead 3+ve left leg -ve left arm