The FTc is not an accurate marker of left ventricular preload

Excerpt

Sir: I read the two recent papers published in Intensive Care Medicine [1, 2] utilising oesophageal Doppler to assess intravascular volume status with interest albeit a keen sense of frustration. Both assessed the corrected flow time (FTc) parameter as a measure of left ventricular preload. Being able to lay claim to having created this parameter as long ago as 1991 [3, 4], I can categorically say that both, alas, are wrong! Unfortunately, neither set of authors properly read the papers that they cited [3, 5]. Had they done so, they would have discovered that the FTc is inversely related to the systemic vascular resistance, i.e. it is a marker of left ventricular afterload. A decrease in left ventricular filling (‘preload’) is but one cause of an increase in afterload, but there are numerous other causes, for instance, excessive vasopressor doses, heart failure, and hypothermia which all produce a low FTc. As a consequence a fluid challenge produces a positive response if the Starling curve is ‘ascended’. However, a low FTc does not respond to a fluid challenge if a pathological condition prevents adequate filling of the left ventricle (e.g. pericardial tamponade, massive pulmonary embolus, tight mitral stenosis, or when haemorrhage exceeds fluid input). Similarly, there is no positive response to fluid if the low FTc (i.e. the increased systemic vascular resistance) is due to another cause of vasoconstriction. For example, in our original 1991 papers we clearly demonstrated that FTc increases in response to vasodilators in patients with heart failure and a high pulmonary wedge pressure [3] and also increases in healthy volunteers given the α-adrenergic antagonist phentolamine [4]. Given the above, it is hardly surprising that an FTc lower than 277 ms would show high specificity (94%) but a decreased sensitivity (55%) in predicting fluid responsiveness [1]. …