The first wave is the P wave because the P cell is an autoregulatory cell. P waves are mainly widened and are associated with pulmonary heart disease, precordial disease or myocardial infarction. The P wave is sometimes reduced in amplitude, mainly because the impulses originate in the caudal part of the sinus node, and the rhythmic impulses of the atrioventricular node originate in the middle of the atrium, In cases of excessive obesity, hypothyroidism, or nephritis causing generalized edema or massive pericardial effusion, hyperkalemia, with a gradual increase in potassium concentration, the amplitude of the P wave will gradually decrease until it disappears.