Cut off from every nerve, a heart still beats, because the SA node holds cells that fire on their own. The faster the rate, from 40 up to 160 a minute, the tighter the ECG waves crowd together: P is the atria contracting, QRS the ventricles, T the repolarisation. In atrial fibrillation the P wave vanishes, replaced by rippling f waves and a completely irregular ventricular rhythm; in mitral stenosis the left atrium swells as blood dams up, unable to cross into the ventricle during diastole.
Cut off from every nerve, a heart still beats, because the SA node holds cells that fire on their own. The faster the rate, from 40 up to 160 a minute, the tighter the ECG waves crowd together: P is the atria contracting, QRS the ventricles, T the repolarisation. In atrial fibrillation the P wave vanishes, replaced by rippling f waves and a completely irregular ventricular rhythm; in mitral stenosis the left atrium swells as blood dams up, unable to cross into the ventricle during diastole.
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