A, Respiratory variation of the IVC measured by using M mode. In a spontaneously breathing patient, inspiration (I) with negative inspiratory force results in inward movement of the IVC, whereas expiration (E) results in outward movement of the walls. Measurements can be made with threshold values suggestive of fluid responsiveness. B, Respiratory variation of aortic flow velocity measured by pulsed-wave Doppler is another method to identify a fluid-responsive patient.