L41 - Cardiac Output 1 - Electrical Events And Heart Rate Flashcards
What is cardiac output?
The heart’s activity (pump that moves the blood (creates pressure difference)
How do you calculate cardiact output?
Cardiac output = heart rate x stoke volume
L/min = beats/min x L/beats
What is the heart rate driven by?
Waves of electrical activity that induce the cardiac muscles to contract
What are the components of the conduction system of the heart? (7)
- Sinoatrial node
- atrioventricular node
- bundle of his
- right bundle branch
- purkinje fibres (nerves)
- left bundle branch
- cardiomyocytes (contraction)
What are the pacemaker cells in the heart? (2)
- sinoatrial (SA) node
- atrioventricular (AV) node
What are the 2 sequences of cardiac excitation?
- atrial excitation
- ventricular excitation
What does the spread of APs result in?
Via the purkinje fibres results in coordinated contraction of ventricles
What is atrial excitation started and completed by? (2)
- begins - SA node, AP spread through atria
- completes - AP reaches AV node
What is ventricular excitation started and completed by? (2)
- begins - AV node triggered by AP
- complete - ventricles contracting
What causes pacemaker cells of SA node to trigger an AP? (2)
- low resting membrane potential (-60 - -70mV)
- Na+ leakage leads to depolarisation
What is the mechanism underlying the AP in the pacemaker cells (SA)? (3)
- Na+ ions leakd thru F type (funny) channels, Ca2+ ion move thru t type (transient) channels = threshold graded depolarisatoin
- rapid opening of VGCC L type = rapid depolarisation phase
- reopening of Kchannels and closing of Ca2+ channels = repolarisation phase
What are the beats/min of the intrinsic rhythms? (5)
- top to bottom
- SA - 100
- AV - 40-60
- purkinje fibres - 15-40
- slower intrinsic rhytms captures by SA node
What is a heart block?
Failure of AV conductions
What is the mechanism of contraction of ventricular cardiomyocytes? (3)
- rapid opening of VGNC = rapid depolarisation phase
- prolonged opening of VGCC and closure of K channels = prolonged plateau of depolarisation (contraction
- opening of K channels = repolarisation
How does calcium produce contraction of the cardiac muscle?
Excitation-contraction coupling in cardiac muscle
What does the prolonged refractory period allow in cardiac muscles?
Allow ventricles to fill with blood prior to pumping
What is refractory period? (2)
- Time required before it is possible to re-stimulate muscle contraction
- 250ms in cardiac muscle
How does the ANS regulate the SA? (2)
- parasympathetic - vagus nerve - reduce heartbeat
- sympathetic - sympathetic ganglia - increase heartbeat (regulates force/stoke vol)
How do the parasympathetic neurons regulate rate of depolarisation of the SA node? (5)
- release Ach
- activates m2 muscarinic receptors of SA node
- inc in K+ efflux, dec in Ca2+ influx
- hyperpolarises cells, dec rate of depolarisation
- = dec HC (bradycardia)
How do the sympathetic neurons regulate rate of depolarisation of the SA node? (5)
- release noradrenaline
- activates b1 adrenergic receptors of SA node
- = inc Na+ and Ca2+ influx
- = inc rate of depolarisation
- inc HR (tachycardia
What are both parasympathetic and sympathetic nerves like? (2)
- tonically active
- at rest - para dominates and heart rate reduced from 100 to 70
What does the ECG measure (electrocardiogram)? What is it?
- measures the elctrical activity of the heart
- a summation of the spread of APs through section of the heart
What are the ECG electrodes? (3)
- RA, LA, LF
- standard limb leads
- precordial (chest) leads
What are P, Q, R, S and T in the ECG? (4)
- P - atrial depolarisation
- PQ/PR - conduction through AV and A-V bundles - atrial contraction
- QRS - ventricular depolarisation
- QT - ventricular contraction
