Session 7 Flashcards
Describe and draw a diagram of the electrical conducting system of the heart and describe how excitation normally spreads through the ventricular myocardium
First the atria depolarise, then after a delay of about 120ms at the AV node, activity spreads through the interventricular septum to excite the ventricular myocardium from endocardial to epicardial (inside to outside) surface at the apex of the heart. Finally excitation spreads up towards the base.
Describe in general terms the pattern of spread of excitation over the normal heart from the SA node to the AV node to the ventricles
SA node –> AV node (delay 120 ms) –> septum –> Purkinje fibres
Describe the signal recorded by an extracellular electrode placed near a myocardial cell during systole
At each systole each cell fires an AP, including a depolarisation and repolarisation. Extracellular electrodes only record a change in membrane potential, therefore skin electrodes see two signals with each systole.
Be able to state rules governing the sign of the signal recorded by a positive recording electrode when depolarisation and repolarisation spreads towards and away from that electrode
Depolarisation moving towards an electrode - upward
Depolarisation moving away from an electrode - downward
Repolarisation moving towards an electrode - downward
Repolarisation moving away from an electrode - upward
Describe the form of signal recorded by a single electrode ‘viewing’ the heart from the apex. Label the waves PQRST and identify the signals associated with atrial depolarisation, ventricular depolarisation and ventricular repolarisation
Atrial depolarisation - small upward deflection
Excitation down septum - small downward deflection
Depolarisation if ventricular muscle - large upward deflection
Depolarisation upwards towards base of ventricles - small downward deflection
Repolarisation of ventricular muscle - medium upward deflection
P wave - atrial depolarisation
Q wave - septal depolarisation
R wave - main ventricular depolarisation
S wave - end of ventricular depolarisation
T wave - ventricular repolarisation
Describe how the QRS complex will change if the viewing electrode if moved around a circle with the heart at its centre
Viewing head on - large upward deflection
Viewing sideways on - no signal
Viewing end of - large downward deflection
Be able to place electrodes correctly to record from ECG leads I,II,III, aVR, aVL and aVF
Limb leads:
Red - right upper
Yellow - left upper
Green - left lower
Blue - right lower
Chest leads:
V1 (red) - right of sternum, 4th intercostal space
V2 (yellow) - left of sternum, 4th intercostal space
V3 (green) - between V2+V4
V4 (brown) - midclavicular line, 5th intercostal space
V5 (black) - left anterior axillary line, 5th intercostal space
V6 (purple) - midaxillary line, 5th intercostal space
State the equivalent single electrode view of leads I,II,III, aVR, aVL and aVF
I - downwards II - diagonally upwards III - diagonally upwards aVR - left aVL - right aVF - upwards
Calculate the heart rate from a rhythm strip for a regular and irregular heart rhythm
ECG machines run at 300 squares per minute.
Regular - heart rate = 300/squares from R-R
Irregular - use a larger interval, multiply the number of beats in 10 seconds by 6
Identify the following abnormalities in ECG traces:
Ventricular ectopic beats, atrial fibrillation, ventricular fibrillation, types of heart block
Ventricular ectopic beats - wider, taller trace
Atrial fibrillation - no P wave
Ventricular fibrillation - quivering on trace
Heart block 1 - widened PR interval
Heart block 2a - widening of PR interval, erratic
Heart block 2b - normal PR, missed QRS complex
Heart block 3 - no association of PR and QRS, slow beats
Describe in outline the ECG changes associated with the acute phase of MI, myocardial ischaemia during exercise
Acute phase of MI - pathological Q wave, elevated ST
Ischaemia - depression of ST wave, tented T wave
