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Cardiac output (CO) is the main determinant of oxygen delivery and physical examination, and vital signs alone often fail to reflect significant derangements in CO [1]. Because of the complexity of assessment of clinical variables in unstable patients, direct measurement of CO is advisable. Pulse pressure (PP) analysis, often referred to as pulse contour (PC) analysis, is the analysis used by technologies that estimate changes in stroke volume (SV) and CO from an arterial pressure waveform signal [2, 3]. The theory behind this approach dates back to the end of the nineteenth century when the German physiologist Otto Frank proposed the Windkessel (air chamber) model of cardiovascular physiology [4]. In a system with air and fluid, different air chambers were filled with air in order to simulate compliance, resistance and impedance of the cardiovascular system. This proved to be pivotal to our understanding of the importance of the relationship between flow and pressure. In the early twentieth century Erlanger and Hooker defined a relationship between stroke volume and arterial pressure, and crucially a correlation between SV and PP [5]. It was not until the development of modern computer technology that this theory could be put into practice and SV and CO tracked continuously on a beat to beat basis [6]. Since then the number of PP analysis devices has increased continuously over the years; some of these devices require an initial calibration; some are ‘‘self-calibrating’’ based on nomograms and anthropomorphic data [7, 8]. No matter which one of these technologies one wants to use, the understanding of the physiological principles behind it is the cornerstone for its proper use. Without this, monitors may be used inappropriately and patients put at risk. So what are the important questions related to PP analysis? There are several and in this editorial we would like to focus mainly on four:

Cardiac output obtained by pulse pressure analysis: to calibrate or not to calibrate may not be the only question when used properly