Changes in the transthoracic impedance signal predict the outcome of a 70° head-up tilt test

Abstract

We determined whether early changes in central haemodynamics, as determined by transthoracic impedance, induced by a 70 degrees head-up tilt (HUT) test could predict syncope. Heart rate, arterial blood pressure and central haemodynamics [pre-ejection period and rapid left ventricular ejection time ( T (1)), slow ejection time ( T (2)) and d Z /d t (max) (where Z is thoracic impedance), assessed by the transthoracic impedance technique], were recorded during supine rest and during a 45 min 70 degrees HUT test in 68 patients (40+/-2 years) with a history of unexplained recurrent syncope. We found that 38 patients (42+/-3 years) had a symptomatic outcome to 70 degrees HUT (fainters) and 30 (39+/-2 years) had a negative outcome (non-fainters). When measured between 5 and 10 min of 70 degrees HUT, T (2) had increased significantly only in the fainters, and a change in T (2) of >40 ms from baseline predicted a positive outcome with a sensitivity of 68% and a specificity of 70%. During supine rest prior to 70 degrees HUT, the fainters exhibited a shorter T (2) than non-fainters (183+/-10 compared with 233+/-14 ms; P <0.01), and a T (2) of <199 ms predicted a positive outcome to 70 degrees HUT with a sensitivity of 68% and a specificity of 63%. Incorporation of the changes that occurred from rest to 70 degrees HUT in other haemodynamic variables (heart rate >11 beats/min, systolic pressure <2 mmHg, diastolic pressure <7 mmHg and pulse pressure <-3 mmHg) increased the specificity to 97% and the positive predictive value to 93%. Thus transthoracic impedance could detect differences in central haemodynamics between fainters and non-fainters during supine rest and during the initial period of 70 degrees HUT with a consistent sensitivity and specificity when combined with peripheral haemodynamic variables.

Entropy in Investigation of Vasovagal Syndrome in Passive Head Up Tilt Test

“…We also observed a statistically significant decrease in stroke volume (SV) just after the tilt and before the loss of consciousness. The decrease in SV after tilting was also reported by other researchers [44][45][46]. The decrease in SV before the loss of consciousness directly preceded syncope.…”

Assessment of Systolic Ejection Time as a Hemodynamic Marker of Incipient Bradycardic Vasovagal Syncope. A Pilot Study

“…At present, there are no implantable devices equipped with hardware and software capabilities to measure and monitor LVET. Nevertheless, LVET can be estimated by means of accelerometers that detect heart sounds, 7,8 subcutaneous or endocardial impedance, 10,11,13 and other systems could be designed to detect the pulse pressure in peripheral arteries or peripheral perfusion. Nevertheless, additional data are needed to demonstrate the reliability of LVET measurement and tracking through an implantable system.…”

“…We chose an HR decrease ≥10% to be very fast in detecting HR changes. Then, we tested four thresholds for ET% decrease (2.5%, 5%, 10%, 15%) and five numbers of consecutive beats 5,8,10,12,15 ≥10%. Then, we analyzed sensitivity and specificity of each of the 20 combinations.…”

“…Sensitivity and specificity of the algorithm for alarm generation. Different combinations of ET% decrease (2.5%, 5%, 10%, 15%) and number of consecutive beats (5,8,10,12,15) with an HR decrease ≥10% are shown in the table and in the corresponding receiver operating characteristic curve.…”

“…However, several observations have shown that hypotension and bradycardia occur during both spontaneous and tilt‐induced VVS, 1,2,12 even if bradycardia is not always present. On the other hand, measuring systolic time intervals such as LVET or preejection time 7,9,10 might be much easier and less invasive than measuring blood pressure (BP) by an implantable device for long‐term applications. A recent study on subcutaneous impedance also showed clear waveforms corresponding to the aortic pressure waveforms 13 and also this approach could be used to estimate LVET.…”