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 componenets of the conduction system of the heart?
- 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?
- 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?
- begins - SA node, AP spread through atria
- completes - AP reaches AV node
What is ventricular excitation started and completed by?
- begins - AV node triggered by AP
- complete - ventricles contracting
What causes pacemaker cells of SA node to trigger an AP?
- low resting membrane potential (-60 - -70mV)
- Na+ leakage leads to depolarisation
What is the mechanism underlying the AP in the pacemaker cells (SA)?
- 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?
- 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?
- 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?
- Time required before it is possible to re-stimulate muscle contraction
- 250ms in cardiac muscle
How does the ANS regulate the SA?
- 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?
- 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?
- 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?
- 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 electodes?
- RA, LA, LF
- standard limb leads
- precordial (chest) leads
What are P, Q, R, S and T in the ECG?
- P - atrial depolarisation
- PQ/PR - conduction through AV and A-V bundles - atrial contraction
- QRS - ventricular depolarisation
- QT - ventricular contraction
What does the ECG provide information about the electrical activity of the heart?
- Heart rate
- Heart rhythm
- Disturbances of rhythm and conduction
(arrhythmia, pacemaker) - Conduction velocity
- Anatomical orientation of the heart
- Relative size of chambers
- Condition of tissue within the heart
- Damage to the myocardium
- Influence of certain drugs
What are examples of ECG?
- sinus arrhythmia (normal)
- ventricular ectopic beat - heart missed a beat
- heart block - fibrosis/ischaemic heart disease
- thirs degree - complete failure of conduction from atria to ventricles
What does ventricular fibrilation result in?
- random firing of hearts
- fibrillating ventricles cannot pump blood
- fatal after a few minutes = death
What can ventricular fibrillation by caused by?
- myocardial infarction
- electrical shock
- drug intoxication
- imparied cardiac metabolism
What can be a treatment to ventricular fibrillation?
CPR followed by electronic defibrillator
