What is the function of the resistance vessels (small arteries and arterioles)?
Thick muscular wall regulates blood flow
Divert blood flow
What is the function of the capacitance vessels (Veins)?
- Provide a reservoir of blood
- Can collapse via the smooth muscle in their walls to provide extra blood when needed.
How do myocytes communicate with one another?
Through specialised structures called intercalated discs, consisting of:
- Gap junctions; allow action potential to pass from 1 cell to the other
- Desmosomes; holds the cells together during contraction
- Connexons; ion channels that allow communication between cells.
What causes the plateau phase?
Opening of slowly opening Ca2+ channels, which triggers further Ca2+ release from the sarcoplasmic reticulum through voltage gated Ca2+ channels
Where do chronotropic agents such as adrenaline act and what do they alter?
They act on the SA node by either speeding up (positive chronotropes) or slowing down (negative chronotropes) the rate of decay of the membrane potential.
Which vessels have the greatest overall cross sectional area?
The movement of what ion causes the initial depolarisation in a myocyte?
Na+ influx due to fast opening Na+ channels
How long should the QRS complex last?
Normally between 80-120ms
What is the typical duration of the ST segment?
Make sure measured from the end of the QRS complex and beginning of the T wave.
What ECG changes are seen in left bundle branch block?
- Wide QRS complex
- Slow rising, tall R waves with flat/notched tops in I, aVL and V5/6
- Broad S wave in V1 and V2ST depression and T wave inversion in V5 and V6
What is syncope?
It is a transient loss of consciousness due to transient global cerebral hypoperfusion characterised by rapid onset, short duration and spontaneous recovery
What is 3rd degree heart block?
- It is where there is complete dissociation of the atria and the ventricles electrically
- The P-P intervals are regular and the R-R intervals are regular, but are not associated with one another
- The atria beat at normal sinus rhythm of 60-100bpm, whereas the ventricles generate 'escape beats' of 30-45bpm, with wide QRS complexes
What does Darcy's law describe?
It describes the flow of a fluid
What causes the partial repolarisation phase?
The inactivation of the Na+ channels
Where is the block in Mobitz type II heart block?
It is in the Bundle of His; usually signifies serious heart disease.
What is the function of the conduit/muscular arteries (large and medium arteries)?
The muscle prevents the collapse of the arteries when a joint bends etc.
Which leads best show the P wave?
- Leads II
What does the P wave show?
The depolarisation of the atria.
What causes the QRS complex?
The depolarisation of the ventricles.
What causes the T wave?
The repolarisation of the ventricles
Why is the absolute refractory period important in myocytes?
It allows adequate filling of the ventricles
What is the cause of Mobitz type I (Wenkebach) heart blocks?
A block in the AV node; usually transient.
Which vessels are the most compliant in the body?
What are the functions of the elastic arteries (Aorta and large arteries)?
Windkessel Effect:- Stores elastic energy-
Dampen pulse wave
Where do dromotropes such as Verapamil act?
They act on the AV node, altering the conductance speed through it.
Which blood vessels contain the largest proportion of blood in the body?
The systemic veins
What are the 3 lead types found in a 12 lead ECG?
- Bipolar limb leads; I, II and III
- Unipolar augmented limb leads; aVR, aVL and aVF
- Unipolar precordial leads; (V1-V6)
Which ion is responsible for the repolarisation of the myocytes membrane?
K+ moving out of the cell through multiple types of K+ channels
What are the 3 layers of blood vessels, and what does each 1 contain?
Tunica intima; endothelium, basal lamina, connective tissue and interal elastic lamina
Tunica media; smooth muscle and external elastic lamina
Tunica adventitia; connective tissue, nerves and vasa vasorum in larger vessels.
What are the 5 phases of a myocyte action potential?
1. Partial repolarisation
2. Plateau phase
What is seen on a ECG in a right bundle branch block?
- Wide QRS complex
- RSR configuration in V1 and V2
- ST depression and T wave inversion in V1 and V2
What a 1st degree heart block?
- It is where the PR interval is longer than 0.22s
- There is a 1:1 conduction from the atria to the ventricles.
What are the difference between the SA and AV nodal potentials and the myocyte potentials?
Nodal potentials are initiated by Ca2+, no Na+ channels.
- They have no plateau phase
- They are slower at depolarising
- They have decaying resting membrane potentials due to slow K+ efflux
- There is slower cell-cell conductance in the nodes, esp, AV node
- Nodal potentials are initiated at a more positive threshold (-40mV compared to -65mV)
What are the 2 different types of 2nd degree heart block?
Mobitz type I; Wenkebach; PR interval becomes progressively longer until a P wave fails to conduct through to the ventricles
Mobitz type II; Normal PR interval, but occasionally not conducted through to ventricles so no QRS complex.
What is the normal duration of the PR interval?
- Make sure it is measured from the beginning of the P wave to the beginning of the QRS complex
What briefly does Starling's law of the heart state?
It states that strength of the heart contraction is greater if the muscle is stretched before contraction begins
Which phase of the myocyte action potential is associated with heart contraction?
The plateau phase
What are the 3 layers of the heart?