University:
Western Sydney University
Course:
MEDI 2003 | Clinical Sciences 2
Academic year:

2024
Views:

442

Pages:

5
Author:

Mariam A. A.

Cardiac pacemaker • • • • • • • Normally the sinoatrial node in the right atrium o Specialised muscle cells o Unstable/pacemaker potential Phase 0: upstroke o Calcium influx is slow (cf ventricular) o Absence of classical fast sodium cannel Phase 1: rapid repolarisation o Inactivation of calcium channels o Transient potassium efflux Phase 2: plateau phase is poorly sustained Phase 3: repolarisation due to K+ efflux Phase 4: diastolic potential o Unstable → pacemaker potential o Slow decay of potassium efflux o Depolarisation activated by calcium influx o Hyperpolarisation activates funny (If) current (mixed Na and K permeability) If pacemaker activity in SA/AV nodes and Purkinje fibres o Mixed Na and K current o Voltage and ligand modulated ▪ Activated by hyperpolarisation (<70-90mV) ▪ cAMP binds to channels to increase their opening rate and probability ▪ SNS stimulation increases the number of cAMP molecules Cardiac rhythms • Sinus rhythm o Any cardiac rhythm where depolarisation of the cardiac muscle begins at the sinus node → vagal tone via vagus nerve o Set by SA at 60-100 bpm o Adult rest = 70-80 bpm • AV node o 40-50 pbm o Transient K+ efflux • Purkinje fibres o 20-40 bpm o Poorly sustained Sequential activation • SA node → atria • SA node → atrioventricular (AV) node • AV-node → Bundle of His, right and left bundle branches, Purkinje fibres o Coordinate depolarisation and contraction Electrocardiogram (ECG) • Recording of differences in surface electrical voltages • Magnitude of voltages depend on the mass of tissue involved e.g. ventricles > atria • Direction of voltages are used to average the sum of depolarisation and repolarising waves over time Recorded potential • Voltage proportional to depolarisation (and subsequently repolarised) mass o -70mV at cellular membrane X surface area • Distance from generator (~1.5mV amplitude at body surface) • Orientation: o Maximum when electrodes are parallel (a-b) to ionic wave o Zero when perpendicular to dipole (c-d) Lead configuration Types of recordings • Intracellular o Single cell o Measures membrane potential o DC coupled • Extracellular o Sum of all cells o Records changes o AC coupled Phases of recording Einthoven’s triangle ECG