Positive-pressure ventilation alters stroke volume by transiently increasing intrathoracic pressure and thereby decreasing preload. This phasic variation in stroke volume results in a cyclic fluctuation in arterial pressure with a phase length equal to the respiratory rate. Measuring ventilation-induced arterial pressure variation allows the clinician to predict the cardiovascular response to changes in intravascular volume status. Thus, one may predict preload responsiveness because the greater the amount of ventilation-associated arterial pressure variation, the greater the patient's preload responsiveness. This variation in arterial pressure can been defined as a variation in either systolic pressure or pulse pressure. Although pulse pressure gives a clearer signal, systolic pressure variation may be easier to measure bedside without invasive hemodynamic monitoring. Newer methods of quantifying this arterial pressure variation include the respiratory systolic variation test, which can performed without an apneic baseline, and the pulse pressure variation, a potentially more accurate measure of preload responsiveness.