Electrical activity of the heart Flashcards
What do the autorythmic cardiac cells do and where are they found?
Capable of depolarising without input from nervous system (myogenic)
Account for 1% of cardiac cellls
Found in the : SAN node, AVN, bundle of His and the bundle branches, purkinje fibres
Fastest firing cellls found in the SAN ; they drive the heart
Describe how an AP is generated in a pacemaker cell
When -60mv is reached, ↑ Na+ influx (if) and ↑ Ca2+ influx (iCa)
↓ K+ efflux = membrane potential increases until threshold
At threshold potential = opening of more Ca2+ = further increase in membrane potential
at 0mv = K+ channels open = decreases membrane potential until -60mv
whole process takes 1s in SAN (longer in AVN to allow delay for emptying of atria)
Rapid spread of excitation through the heart is facilitated by the presence of __________ _______ between fibres
Intercalated discs
How are cardiomyocytes adapted to :
achieve a low resistance system
function as a syncytium (single unit)
gap junction reduce the resistance
intercalated discs connect all myocytes, allowing cardiac muscle to function. As a syncytium
How is the heart adapted to allow the ventricles to contract as co-ordinated units and hence expel blood effectively?
ventricular excitation occurs synchronously (due to rapid spread down septum and through Purkinje fibres)
Describe the AP in a ventricular contractile cell
Takes much longer (~3s)
-90mv resting potential
when AP from pacemaker cells arrives, Na+ channels in contractile cell open = increase in membrane potential
when potential becomes positive, K+ efflux and Ca2+ influx = causes plateau that maintains depolarised state for a few ms until enough K+ channels open that potential decrease again
Explain excitation-contraction
Influx of Ca2+ during action potential
Triggers release of further Ca2+ from sarcoplasmic reticulum
Free Ca2+ activates contraction of myocardial fibres (SYSTOLE)
Explain how diastole occurs
Uptake of Ca2+ by sarcoplasmic reticulum and extrusion of Ca2+ by Na+/Ca2+ exchange and outward Ca2+ pump
Lowers free Ca2+ allowing relaxation (DIASTOLE)
What determines cross-bridge cycling & force of contraction?
Amount of Ca2+ released into myoacardial fibres
explain the importance of the plateau phase of the AP of a ventricular cell
provides a long refractory period and therefore protects the heart from tetanus (sustained, fused contraction)
Ensures heart contracts inside to out (endocardium to epicardium) and bottom to top (apex to base)
Describe the neuronal modulation of heart rate
effect of sympathetic system :
increases heart rate by activation of β1-adrenoceptors in SA node via noradrenaline
increases slope of pacemaker by increasing if & iCa2+
effect of parasympathetic nerves :
decreases heart rate by activation of M2 muscarinic receptors in SA node
increases K+ permeability to hyperpolarise membrane potential
and decreases slope of pacemaker potential by decreasing if & iCa2+
REPOLARISATION TOWARDS A POSITIVE ELECTRODE PRODUCES A ______ DEFLECTION
REPOLARISATION AWAY FROM A POSITIVE ELECTRODE GIVES AN ________ DEFLECTION
Downward
upward
depolarisation moving away from the positive electrode gives a _____ deflection
depolarisation moving towards the positive electrode gives an _____ deflection
no recording on ECP if positive electrode is ____ to the direction of the wave of excitation
Downward
upward
Perpendicular
label this ECG to describe how a normal ECG looks like
what part of the heart do the 3 bipolar limb leads measure the electrical activity of
explain where they are positioned
all three leads produce the typical looking ECG
Wave terminology
what do the following terms mean?
P wave
QRS complex
T wave
ST length
T-P interval
example is for classical ECG recorded with +ve electrode at apex
P = atrial depolarisation
QRS = ventricular depolarisation
T = ventricular repolarisation
ST length/position ejection
T-P interval filling
What is right axis deviation
In healthy individuals wave of depolarisation is in the direction of lead II
In right axis deviation, This causes the deflection in lead I to become negative and the deflection in lead aVF/III to be more positive. due to hypertrophy of RV
Associated with conditions where it is heart is pumping more blood to lungs such as COPD and pulmonary hypertension
describe left axis deviation
Wave of depolarisation is towards limb lead I instead of limb lead II ; Leads I and aVL are positive; leads II and aVF are negative (wave of depolarisation is away from axis of these lead)
due to hypertrophy of LV
Associated with hypertension or valvular heart disease
Describe heart block ; what are the different degrees ; typical symptoms
Symptoms : bradycardia and dizziness due to delay until pacemaker cells surrounding AVN are activated
1st degree - long P-R ; no symptoms
2nd degree - some P with no QRS
3rd degree - complete block ; no A-V conduction
describe atrial fibrillation ; what is it, how does it present itself on an ECG ; what are the problems associated with it
occurs when AP is initiated in the atria from pacemaker cells other than the SAN node = uncoordinated cardiac cycle = stasis of blood in atria = increased risk of clotting = big risk factor for stroke
Characterised by unidentifiable p waves
Describe the ECGs produce by the 3 unipolar limb leads and the part of the heart they measure they electrical activity of
avR lead is the opp of normal ecg as its viewing the heart from the same plane but opposite direction ; avF views heart from inferior wall
Describe the 6 chest leads - what plane do they measure the electrical activity of the heart in ?
where should they be placed on the chest
how do the ECGs produced look like
As you transition from V1 to V6, R waves get progressively bigger and S waves get progressively smaller
V1 - R ics4 parasternally ; shows activity of septum
V2 - L ics4 parasternally ; shows activity of septum
V3 - between V3 and V4 ; shows activity of RV /anterior wall of heart
V4 -mid clavicular line of Lics5 ; anterior wall of heart
V5 - mid axillary line of Lics5 ; lateral wall of LV
V6 - in armpit via Lics5 ; lat wall of LV
what are ectopic heart beats ?
Extra ventricular contraction that is not meant to occur (no preceding p wave) or skipped heart beat (when electrical impulse arrives at the AVN too quickly
happens spontaneously ; not usually a cause for concern
What is an atrial flutter ?
Type of tachycardic arrhythmias ; Occurs when atria contract too quickly due to an overactive SAN - only some impulses are conducted to the AVN
regular QRS complex with multiple P waves inbetween
loss of flat lines of ECG (isoelectric potins)
