Heart, Lung and Circulation

Calibration of Carotid Central Pressures: A Flaw in Brachial Applanation Tonometry?

      Central pressure, based on carotid and brachial applanation tonometry (AT) does not predict outcomes. This may be due to inaccuracy of brachial and carotid AT. No validation has been published.
      To test its (in)accuracy, we measured radial, brachial and carotid pressure pulse waveforms by AT in 100 subjects. Carotid systolic (SP) and pulse (PP) pressures were estimated by two techniques. First, the Pressure Equivalence (PE) technique calibrated brachial waves with brachial cuff pressure values, then the carotid wave was calibrated by assuming identical mean and diastolic pressures, with carotid SP extrapolated. Second, SphygmoCor® technique applied the Transfer Function (TF) to the radial waveform, calibrated to brachial cuff, to generate carotid SP and PP.
      Amplification was significant between carotid and brachial (8.0 mm Hg, p < 0.0001) with TF, but not with PE (1.5 mm Hg, p = NS). PE gave considerable amplification between brachial to radial (8.4 mm Hg), which was not present with the TF. Form Factors (FF = (mean pressure − diastolic pressure)/PP) for carotid and brachial waves were similar (40.2 c.f. 39.1%; p = NS), but different to the radial (34.5%; p < .0001). Amplification as brachial PP ÷ carotid PP using PE was insignificant, but with TF was positive (18%, p < 0.0001).
      The PE method for recording PP amplification in the upper limb is inaccurate. A major problem is use of AT in carotid and brachial arteries, which cannot reliably be applanated. Findings explain inability of the PE method to predict cardiovascular outcomes, and superiority of TF. “Die Methode ist Alles” (Carl Ludwig 1852).