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

We determined whether early changes in central haemodynamics, as determined by transthoracic impedance, induced by a 70° 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 (T₁), slow ejection time (T₂) and dZ/dt_max (where Z is thoracic impedance), assessed by the transthoracic impedance technique], were recorded during supine rest and during a 45min 70° 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° HUT (fainters) and 30 (39±2 years) had a negative outcome (non-fainters). When measured between 5 and 10min of 70° HUT, T₂ had increased significantly only in the fainters, and a change in T₂ of >40ms from baseline predicted a positive outcome with a sensitivity of 68% and a specificity of 70%. During supine rest prior to 70° HUT, the fainters exhibited a shorter T₂ than non-fainters (183±10 compared with 233±14ms; P<0.01), and a T₂ of <199ms predicted a positive outcome to 70° HUT with a sensitivity of 68% and a specificity of 63%. Incorporation of the changes that occurred from rest to 70° 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° HUT with a consistent sensitivity and specificity when combined with peripheral haemodynamic variables.