Regular irregularities include paradoxical pulse and alternating pulse. It is defined as, "one that markedly decreases in amplitude during inspiration, as often occurs in constrictive pericarditis." It is usually greater than 20 mm Hg, occurs during inspiration, and often occurs in constrictive pericarditis. Alternating pulse (pulsus alternans) occurs when strong and weak beats alternate, often due to severe myocardial damage.