6 Cardiology Research and Practice Estimation of cardiac output Doppler echocardiography Main limitations: - Poor image quality - Doppler angle error - Measurement error - Heart plane motion Hemodynamic assessments Estimation of RAP IVC collapsibility index Main limitations: - Body position - Poor image quality - Measurement error - Motion artefacts - Right heart failure - Severe triscuspid regurgitation Assessment of ﬂuid responsiveness Volume expansion: • Volume challenge PLR Change in CO or SV Main limitations: - Same as CO - Increased IAP Respiratory variations SV variation IVC variability index IVC distensibility index Main limitations: - Same as CO and IVC - Mechanical ventilation - Increased IAP - High respiratory rate Figure 6: Summary of hemodynamic assessment by transthoracic echocardiography. Respiratory variation in right atrial pressure has also been used to predict ﬂuid responsiveness [23, 24]. As a surrogate for RAP, the changes in IVC diameter with respiratory varia- tion has also been shown to be a satisfactory predictor for ﬂuid responsiveness in mechanically ventilated patients. Barbier et al. demonstrated a percentage distensibility of IVC, deﬁned as (IVCmax− IVCmin)IVCmin, of 18% oﬀers 90% sensitivity and speciﬁcity in discriminating ﬂuid responders from non-responders [25]. On the other hand, Feissel et al. found that a percentage variation in IVC, deﬁned as (IVCmax− IVCmin)IVCmean, of 12% can predict ﬂuid respon- der with >90% sensitivity and speciﬁcity [26]. Of note, IVC distensibility and variability should not be confused with IVC collapsibility (see above). While the former two predict ﬂuid responsiveness, the latter predicts RAP. The use of superior vena cava (SVC) collapsibility index has also been used, but transesophageal echocardiography is necessary to visualize the SVC [27]. 4.3. Limitations. One of the main concerns of using res- piratory variation is its applicability in spontaneously breath- ing patients or in those not in mandatory controlled mode [28]. In one study, SVV was found to be a poor predictor for ﬂuid responder in patients with septic shock on pressure support mode [29]. Breath-by-breath changes in inspiratory eﬀort may alter the intrathoracic pressure hence the regular SVV or IVC variations required for such analysis. Further, variations in tidal volume and duration of respiratory cycle in noncontrolled mode may negate the use of SVV or IVC variations. Charron et al., however, showed that VTIaortic was still a satisfactory predictor for ﬂuid responsiveness even when the tidal volume was increased [30]. However, no follow-up study was done in this regard. Elevated intraabdominal pressure is another confound- ing factor for assessment of ﬂuid responsiveness. Intra- abdominal hypertension has been shown to render the use of PLR useless in predicting ﬂuid responsiveness [31]. Animal studies also demonstrated that the normal cutoﬀs for SVV are not helpful in predicting ﬂuid responsiveness [32, 33]. Increase in respiratory rate in mechanically ventilated pa- tients reduces the VTIaortic variations rendering the use of SVV useless [34]. Intra-abdominal hypertension can also aﬀect the size of IVC and its response to respiration. 5. Conclusion TTE has proven itself to be an indispensible critical care tool in recent years. Although its main uses are for exploring the cardiac function, its applications in hemodynamic assess- ment are increasingly popular. Estimations of cardiac output and right atrial pressure, and ascertaining the ﬂuid status of the patients, are common uses of TTE (Figure 6). Hemodynamic assessment by TTE is confronted by two major limitations: internal and external validities. Internal validity stems mainly from technical limitations including image quality and measurement errors. Some of these can be minimized by quality control and skill improvement over time. External validity concerns with the applicability and appropriateness of using a particular TTE measurement to answer a speciﬁc hemodynamic question. In face of the issue of external validity, the operator should constantly ask him- self or herself whether a particular TTE measurement is valid (suitable) for the patient in question. The power of echocardiography can only be unleashed through the under- standing of its strength and limitations more fully.