ECG Flashcards
What is an ECG?
The ECG is a graphic representation of electrical potential difference changes against time of the myocardium throughout the cardiac cycle
How is cellular morphology that causes muscle contraction allowed?
The myocardium has electrical currents that sequentially depolarise each individual cell and result in a change in cellular morphology that allows muscle contraction.
What needs to happen to cause cardiac conduction?
- For the myocyte (cardiac cell) to contract, the potential difference across the cellular (semi-permeable) membrane and must change from negative to positive in relation to the inside of the cell.
- Changes in potential difference occur through the flow of ions through specialised in channels in the cellular membrane and also relatively freely through gap junctions.
Describe the cardiac electrical field (i.e. when is it and isn’t present)
- When a cardiac cell is depolarising or repolarising, different currents flow across the cell membrane at various points and a potential difference will occur between one part of the cell and another – a dipole.
- Flow of current along the cell surface, an external electrical field is set up around the dipole.
- When the cell is depolarised or repolarised and at resting potential, the membrane potential there is no difference in the membrane potential at difference points along the cell surface and therefore no electrical field, despite the potential difference between the inside and outside of the cell.
What are the things needed to consider for the QRS complex?
- The orientation of the recording leads
- The concept of the heart as an electrical dipole
- The change in dipole size and orientation during ventricular excitation.
Why do we have the 12 lead E CG?
- The heart is 3D – We need to see the different sides of the heart, front, back, etc.
- You can use an ECG to find out which coronary artery is causing ischaemic (lack of oxygen) heart disease.
Conduction in bundle of His/Purkinje fibres
His/Purkinje rapid conduction – Left bundle then bifurcates into 2 – the anterior and the posterior ‘vasical?’ (branches) – goes around the whole left ventricle and the right one.
Describe the different parts of a typical ECG wave
- P WAVE: atrial depolarisation
- PR SEGMENT: AV node delay
- QRS COMPLEX: Ventricular depolarisation (atria repolarising simultaneously)
- ST SEGMENT: time during which ventricles are contracting and emptying
- T WAVE: ventricular repolarisation
- TP INTERVAL: time during which ventricles are relaxing and filling
Tachycardia
A heart rate that exceeds the normal resting rate
Bradycardia
Abnormally slow heart action
What are some ways to identify a normal sinus rhythm?
• There is one positive P wave, before each QRS
• PR interval should not be greater than 200 milliseconds (0.2 s)– greater than this is an AV block.
• QRS should be less than 0.11 seconds.
QT interval should be normal- this change depending on how fast or slow the heart rate is.
What other pathological conditions produce characteristic ECG changes?
• Atrial and ventricular premature contractions (ectopics) • Atrial tachycardia • Atrial fibrillation / atrial flutter • Sinus arrest and sino-atrial block • Ventricular escape beats • Supra-ventricular arrhythmias • AV nodal re-entrant tachycardia • AV re-entrant tachycardia • Ventricular pre-excitation • Ventricular tachycardia Ventricular flutter
Effect of aortic stenosis
Aortic stenosis narrowing of the aorticvalve opening. Restriction of the blood flow from the left ventricle to theaortaand may also affect the pressure in the left atrium
– Then the heart has to pump harder and work harder to pump blood out of the left ventricle. And so, overtime the heart begins to hypertrophy.
– So, when the left ventricle muscle get thicker, the impulses has to go through more muscle mass. And so, you get a taller R wave showing the increase in muscle mass.
