Electrical and mechanical events of the heart Flashcards
Why does the heart contain the SAN?
Initiates aps due to the unstable membrane potential that is continuously drifting towards threshold; can initiate cardiac contraction in absence of external (hormonal/neural) control
What is the intrinsic rate of the SAN?
~100bpm
What is the effect of sympathetic stimulation on the SAN?
Increases in sodium funny channel current, causing further depolarisation and increase in slope drifts towards threshold
What is the effect of parasympathetic stimulation on the SAN?
Increase K+ permeability, causing repolarisation/hyperpolarisation (overall decrease of the inwards current) and decrease in slope towards threshold
What does the rate of firing of SAN depend on?
Initial value of membrane potential, and slope of drift towards threshold
How is stimulation conducted through the cardiac muscle?
SAN originates stimulus; stimulus spreads through muscle fibres through gap junctions; stimulates atria almost simultaneously; signal delayed at AVN to allow atria to empty before ventricular contraction begins; AVN and bundle of His are only pathway of the electric stimulus to travel from atria to ventricles; ventricular stimulation spreads via network of conduits from AVN to Purkinje fibres; signal travels down interventricular septum via the two bundle branches; ventricles begin to contract at apex, depolarisation spreading upwards via Purkinje fibres
How is the recording of an ECG influenced?
By size and direction of current
What type of inflection would a large current from -ve to +ve pole create?
Large, positive inflection
What type of current would a small, negative inflection indicate?
Small current, from +ve pole to -ve pole
What might no inflection indicate?
No electrical activity, or a perpendicular current
What type of inflection would an angled current create?
Smaller, positive inflection
What does the ECG represent?
Composite of all change in electrical potentials of all cells at any moment in time
What does the size and shape of ECG waves depend on?
Position of heart relative to placement of electrode; direction of excitement; mass of tissue being excited at each given time
What is the Einthoven triangle?
Most common way to measure HR and rhythm
What is the most complex way to measure heart electrical activity?
12 lead ECG; provides a 3D perspective of what happens in heart at any given time, and can indicate if a specific part of the heart isn’t operating as it should be
What is the typical way to measure heart electrical activity?
3 lead ECG; can seen normal rhythm ad rate
What does the P wave indicate?
Atrial contraction; initiation and completion of atrial excitation
What does the Q wave indicate?
Initiation of ventricular excitation
What does the RS complex indicate?
Completion of ventricular excitation
What does the T wave indicate?
Ventricular relaxation
What are the two phases of the mechanical events of cardiac cycle?
Systolic phase and diastolic phase
What happens during the systolic phase?
Isovolumetric ventricular contraction: pressure generated by ventricles as start to contract; pressure large enough to close atrioventricular valves, but not large enough to open semilunar valves; no blood leaves the heart. Ventricular ejection: pressure inside ventricular > pressure outsie heart; pressure sufficient to open semilunar valves and blood leaves heart
What happens during the diastolic phase?
Isovolumetric ventricular relaxation: pressure falls as ventricles start relaxing; pressure low enough to shut semilunar, but not low enough to open atrioventricular valves; no blood entering/leaving. Ventricular filling: pressure in atria > pressure in ventricles; atrioventricular valves open; blood into ventricles (passive initially as by gravity). Atrial contraction: boosts filling of ventricles; normally is not that important, but becomes to fill ventricles with correct amount of blood during exercise etc
What does the pressure-volume curve show?
Shows what happens throughout heart cycle as far as pressure changes and what changes in pressure does to volume
