4 to obtain the maximal velocity. Foreshortening apical-5- chamber window should be avoided; otherwise CO should not be measured. Of note, motion artefact due to respiration can also lead to angle error by tilting the heart plane up and down. Sometimes, it is diﬃcult to diﬀerentiate this motion artefact from the SV variation (see below) in a ﬂuid-depleted but responsive patient. The operator may see alternating 4- and 5-chamber views in concert with the respiratory phase if it is due to motion artefact. Arrhythmias is another factor that can lead to an measurement error. For patients with atrial ﬁbrillation, the VTILVOT measurement should be averaged over at least 5 consecutive cardiac cycles. The HR should also be averaged. Ectopic beats should be avoided. 3.2. Estimation of Right Atrial Pressure. In the classical Guy- ton’s theory, cardiac output not only depends on the cardiac mechanics, but also on the venous return which determines the right atrial pressure (RAP) which can be approximated by central venous pressure (Figure 3). The inferior vena cava (IVC) has long been used to predict RAP in nonventilated patients [6]. In response to a change in intrathoracic pres- sure during respiration the IVC diameter changes. The dia- meter is the largest during expiration, and is the smallest during inspiration or sniﬃng. The IVC collapsibility index, deﬁned as the diﬀerence in IVC diameters (during expiration and inspiration) divided by the maximal diameter (or (Dexpiration − Dinspiration)/Dexpiration where D is the diameter), has been shown to be correlated to RAP [6, 7]. Various cutoﬀs have been used to predict RAP [6, 8]. The IVC collapsibility index is obtained from the sub- costal window. The patients are asked to perform a brief rapid inspiration or a sniﬀ. The maximum diameter of the IVC is obtained during expiration, and minimum diameter is obtained during an inspiratory or a sniﬀ maneuver. Limitations. Contrary to traditional belief, RAP and central venous pressure have been proven to be poor predictors for ﬂuid status (or blood volume) in critically ill patients [9, 10]. Since IVC diameter and collapsibility index are used as surrogates to estimate RAP, it follows that such information will not be very useful in predicting ﬂuid status in critically ill patients despite the fact that IVC collapsibility index cor- relates with RAP. The main limitation of the applicability of IVC col- lapsibility index is its applicability in mechanically venti- lated patients. Studies that demonstrated good correlations between IVC collapsibility index and RAP were mostly per- formed in spontaneously breathing patients [6, 7]. The cor- relations in mechanical ventilated patients were poor [11, 12]. Compounded to the mechanical ventilation eﬀects are a number of confounding factors that also aﬀect the IVC dia- meter and collapsibility in critically ill patients. For example, right heart failure, signiﬁcant tricuspid regurgitation, and supine body position, which are common in critically ill patients, are known to dilate the IVC [12–15]. Despite these, an IVC diameter of ≤12 mm has 100% speciﬁcity, with only Cardiac output Cardiology Research and Practice Cardiac function curve Venous return curve CO 0 Preload Right atrial pressure Figure 3: The relationship of venous return and cardiac function in determining cardiac output. As depicted in the venous return curve, venous return reduces with increasing right atrial pressure. The cardiac function curve illustrates the eﬀect of increasing right atrial pressure on cardiac output (CO). Increasing right atrial pressure causes as increase in CO until a plateau (ﬂat portion) is reached. At equilibrium, CO is determined by the point where two curves cross each other. The right atrial pressure at this point is the preload. The cardiac function curve is also known as the Frank-Starling curve. 25% sensitivity, of predicting an RAP of 10 mmHg or less in mechanically ventilated patients [12]. The main technical limitation results from motion ar- tefact are due to diaphragmatic and abdominal wall move- ments. IVC is commonly displaced inferiorly by the dia- phragm during inspiration or sniﬃng, and aﬀects the meas- urements especially when M-mode is used. Abdominal wall motion during sniﬃng can displace the transducer, hence the ultrasound plane, during the measurements. Care must therefore be taken to minimize such displacements while maintaining the ability to capture the changes in diameter during inspiration or sniﬃng. Measurements are best done in 2D mode with high frame rate to minimize such measure- ment errors. 4. Determination of Fluid Responsiveness It is a well-known fact that administering ﬂuid to patients whose hearts are operating at the ﬂat (top) portion of the Frank-Starling (preload versus SV) curve is harmful to pa- tients [16]. Various techniques have been developed to iden- tify those “ﬂuid responsive” patients who will beneﬁt from ﬂuid administration, that is, those whose ventricles are operating at the steep (ascending) part of the Frank-Starling curve (Figure 4). In a systematic review and meta-analy- sis of clinical study, it was found that approximately half (52.9%) of the study population were responders [17]. Two approaches have been adopted to predict ﬂuid responsive- ness: (a) volume expansion and (b) respiratory variation in SV or its surrogates. 4.1. Volume Expansion. The volume expansion approach uti- lizes the fact that if preload is increased acutely in those who will beneﬁt from ﬂuid administration, an accompanying in- crease in SV (or CO) should be observed (Figure 4) [18]. Volume expansion can be achieved via (a) passive leg raising