Focused Cardiac Ultrasound: Limitations and Source of Interpretation Errors
The interesting and very comprehensive article by Haskins et al1 was thoroughly studied, and we deliberated that some comments pertaining to limitations of focused cardiac ultrasound would contribute to the further understanding of this technique.
The use of transthoracic echocardiography (TTE) in hemodynamic assessment is an attractive approach because the procedure is totally noninvasive, and a focused assessment lasts only a few minutes. However, the most significant downsides are as follows2,3: (1) It is not a continuous monitoring technique. Abrupt changes in hemodynamic status mandate a well-trained interpreting physician and immediate operation of the ultrasound machine; this might be too time consuming in urgent cases; and (2) study quality can be influenced by a plethora of factors such as patient’s position (right lateral and prone), comorbidities, mechanical ventilation, machine quality, and interobserver variability that is always an issue.
In addition, many problems arise when measurements rely on geometric assumptions and Doppler measurements. In many cases, it is attempted to perceive the function, geometry, and architecture of 3-dimensional structures, such as the left ventricular outflow tract, the left/right ventricle (LV/RV) diameters, and/or a stenotic aortic valve using a 2-dimensional technique; that premise entails risks and is sometimes a source of significant errors in the aftermath.4
Although these static indices (LV/RV diameters) have been proven to be poor predictors to assess intravascular blood volume and fluid status, many of them have been used to appraise fluid responsiveness in patients with acute hemodynamic perturbations. In clinical practice, when cardiac function is evaluated by TTE, physicians do not have preoperative baseline values of LV/RV diameters, nor do they know other specific echochardiographic measurements. Therefore, relative changes in these diameters and measurements cannot be reliably assessed. Very importantly, in a hemodynamic instability scenario, an LV with small diameter and a hyperdynamic function cannot always be hypovolemic, because clinical conditions that reduce peripheral vascular resistance can also lead to a hypercontractile LV function. In this case, other echocardiographic measurements are required, such as stroke volume assessment, in order to decide whether fluid challenges or vasoconstrictor will be given to a patient undergoing neuraxial technique.5,6
To sidestep this limitation, many dynamic indices, such as inferior vena cava (IVC) collapsibility index,5–7 which can also be assessed by echocardiography, have been put forward. Studies that exhibited good correlations between IVC collapsibility index and right atrial pressure were mostly implemented in spontaneously breathing patients.8 The correlations in mechanically ventilated patients were poor, whereas right-sided heart failure, significant tricuspid regurgitation, supine body position, constrictive pericarditis, increased intra-abdominal pressure, and fluid accumulation in the pericardium can lead to inaccurate decision making regarding the actual volume status of patients.8
Furthermore, a chief technical limitation of IVC measurements is motion artifact stemming from diaphragmatic and abdominal wall movements. Inferior vena cava is commonly displaced inferiorly by the diaphragm during inspiration: that adversely affects the accuracy of measurements especially when M-mode is used.7,8
It is also noteworthy that the main limitation of 2-dimensional echo is its poor effectiveness in distal pulmonary embolism.9 The venous ultrasonography of the lower limbs should be performed in parallel so as to allow for the exploration of the venous system. In addition, in hemodynamic instability with RV dysfunction, other pathologies such as LV inferior wall infarction implicated with RV dysfunction should be extensively ruled out.10,11 Patients with concomitant hypoxia may require saline-contrast agents for patent foramen ovale or even more so a focused lung ultrasound examination to exclude other underlying pathologies.10,11
Tissue edema and emphysema, frequently encountered in trauma patients, can also reduce or even extremely deteriorate the interpretation of ultrasound images.