Physiology of the heart 1 Flashcards
How many beats a day?
100,000 (70bpm)
Key ions in regulating heart contractions?
Na+, K+, Ca2+
Describe phase 0 of the cardiac potential
‘Rapid depolarisation’ phase
Na+ influx causes ‘ALL OR NOTHING’ depolarisation if -60mV threshold reached, MP goes from -70mV (due to high Ca2+ and Na+ outside, K+ inside) to positive
Causing a sharp upwards deflection.
Describe phase 1 of the cardiac AP
‘Partial repolarisation’ phase
Na+ channels RAPIDLY close
Na+ conductance decreases
Describe phase 2 of the cardiac AP
‘plateau’ phase
2 = Ca2+
Ca2+ conductance increases SLOWLY (more into the cell)
Initial fall in outward K+
Both of which helps extend plateau of depolarisation as more +ve inside
Why is extension of the depolarisation in phase 2 important?
Doesn’t allow successive heart beats to be fired as can’t fire another AP when still depolarising due to +ve MP
This allow time for ventricles to fill with blood and pump it out - if 100x a second couldn’t do this
I.e. prevents tetany in heart
This is called a refractory period i.e. a limit to how fast the heart can got
Which phase prevents tetany in the heart and why?
Phase 2 mostly as Ca2+ conductance increases into cardiomyocytes and K+ decreases, so more +ve intracellular membrane potential i.e. further from threshold
Describe phase 3 in the cardiac AP
‘Repolarisation’ phase
Ca2+ channels close - decrease in inwards conductance
K+ channels remain open, increasing K+ outflux
Membrane now -ve again
Describe phase 4 of the cardiac AP
'Pacemaker potential' phase Gradual depolarisation in diastole Na+ and Ca2+ increasing INFLUX K+ OUTWARD conductance decreases so RP gradually more +ve (in diastole i.e. not contracting) until reaches threshold (-60mV) and fires off another AP. Found in nodal and conducting tissue.
Which cells have pacemaker activity?
SAN, AVN, P fibres
SAN the main one
What can take over if SAN fails?
AVN and Purkinje fibres.
A patient has slow and ectopic heart beat. Which cells could be impacted?
SAN insult. AVN and P fibres take over but with a slower and ectopic beat.
During phase 4 there is gradual repolarisation
FALSE - gradual depolarisation as Na+ and Ca2+ INFLUX increase and K+ OUTFLUX decreases
During phase 3, there is an decrease in Ca2+ outflux.
FALSE - Ca2+ channels close that let Ca2+ into cell so decrease in influx.
AVN is the main pacemaker in the heart
FALSE - SAN responsible for the 70bpm
Describe how the AP spreads down cardiac conduction tissue
SAN fires AP 70bpm
This spreads down atrial walls of myocardium causing them to contract in response to depolarisation
Impulses reach AVN where there is a delay due to slow conduction, allowing full contraction of atria and time for ventricles to fill before contraction
Impulse spreads rapidly down bundle of His and Purkinje fibres to ventricles.
Name 4 unique differences of cardiac conducting tissue
1) Pacemaker activity (AVN, SAN and P fibres have but slower)
2) Rapid depolarisation (SAN and AVN dont have this phase 0 due to lack of fast Na+ currents as in nodal tissue APs governed by Ca2+)
3) Long AP and refractory period (in ventricles and P fibres only - important to put a limit on HB)
4) Influx of Ca2+ during plateau
LOOK AT LINES ON GRAPH
Name 2 types of arrhythmia’s
Abnormal generation
Abnormal propagation
Increased automaticity of heart cells is an example of abnormal propagation arrhythmia
FALSE - increased automaticity = generation error i.e. spontaneous activity causes ectopic beats.
Delayed after depolarisation is an example of abnormal generation arrhythmia
TRUE - example of triggered activity abnormal generation arrhythmia caused by e.g. too much Ca2+ from a disease.
Name the two type of abnormal propagation arrhythmia’s
Re-entry Heart Block (AVN blocked so no signals to ventricles)
Name 2 types of abnormal generation arrythmias
Triggered e.g. DAD (dad triggers me) - LOOK AT DIAGRAM IN NOTES
Spontaneous (increased automaticity i.e. ectopic beats)
What is the most common type of arrhythmia’s?
Re-entry (i.e. abnormal propagation)
Describe each of the arrythmia graphs.
Look at diagrams on slides and explanation in notes
The P wave represents ventricular depolarisation
FALSE - atrial. P wave = atrial repolarisation = sinus rhythm. Sinus tachycardia is normal duirng exercise.
Sinus tachycardia is normally the sign of a serious illness
FALSE - this happens during exercise i.e. P waves will get closer together.
Sinus bradycardia is normally the sign of a serious illness
FALSE - normal e.g. sleeping P waves get wider apart
Sinus tachycardia/bradycardia are considered arrythmia’s
FALSE - their origin is still SAN (hence Sinus). Atrial tachy/bradycardia = arrythmia’s as AVN origin.
What can cause atrial fibrillation?
Atrial thrombus (remember on atrial fibrillation graph theres a pic of CT scan pointing to atrial thrombus)
What’s the difference between atrial and ventricular fibrilation?
Atrial occurs in atria so atria can’t contract not ventricles, so defibrilation wont work on atrial.