What is ‘Square Wave Response’ to Valsalva Maneuver?

Normal response to Valsalva maneuver consists of four phases. In Phase I, blood pressure rises due to rise in intrathoracic pressure being transmitted to the aorta. In phase II, arterial pressure comes down due to decreased left ventricular preload as a result of sustained increase in intrathoracic pressure impeding pulmonary venous drainage. There is decrease in stroke volume and pulse pressure. Intrathoracic pressure drops on releasing the strain in Phase III leading to transient decrease in systolic blood pressure below baseline value. This is followed by a Valsalva overshoot in blood pressure due to increased venous return and leads to reflex bradycardia. Absence of this typical four phase response of blood pressure to Valsalva strain in congestive heart failure is known as square wave response. The decrease in Phase II does not occur as the ventricular preload is high in congestive heart failure. Blood pressure rises with strain and remains elevated throughout the strain. It returns to baseline level on release of strain.