How Do You Flip the R Wave?

Since March 2014, the U.S. Food and Drug Administration has approved the novel intracoronary physiologic measurement modality, iFR (instantaneous wave-free ratio). The iFR has shown to be non-inferior to FFR (fractional flow reserve) in the accuracy of measuring the physiologic significance of indeterminate coronary lesions.^1-3 In addition, it has other advantages: it is faster due to not needing to wait for adenosine to induce maximal hyperemia, there is subsequent elimination of side effects from adenosine such as dyspnea, chest pain, and bronchospasm, and it has lower costs also attributed to not using costly adenosine. However, we still need to use a hybrid approach of iFR and FFR if initial iFR results fall into the indeterminate zone (between 0.86-0.93). 

In cardiac catheterization laboratories using the Mac-Lab (GE Healthcare) in which the EKG analog output I is used for iFR, EKG lead II is the only lead available for iFR use. However, to obtain an iFR value, the R wave must be positive. In patients with paced rhythms, Q wave inferior infarct, and/or left axis deviation, the iFR is unable to be measured, as the R wave in lead II may be negative.
We innovated a methodology to flip the R wave in lead II so that the iFR modality can still be utilized in these patients, avoiding administration of the unnecessary coronary vasodilators. Proper five-lead wire system placement is necessary for obtaining the expected positive and negative voltage deflections of leads I, II, III, aVR, aVL, and aVF, demonstrated by use of opposing positive and negative poles and the ground lead seen most commonly in Einthoven’s Triangle. We reversed the left arm lead with the left leg lead and place the right arm lead behind the right shoulder. The reversal of the left arm and left leg leads from the usual fashion, in conjunction with placing the right arm lead posterior on the right shoulder, generates a circuit that creates the positive voltage deflection that is required in lead II for basic functioning of the iFR modality (Figures 1-3). We have had success in using iFR by using this switching maneuver and have been able to avoid defaulting to FFR as a result. 

The authors can be contacted via Wah Wah Htun, MD, at drhtun80@gmail.com.

References

1. Berry C, van‘t Veer M, Witt N, Kala P, Bocek O, Pyxaras SA, et al. VERIFY (Verification of Instantaneous Wave-Free Ratio and Fractional Flow Reserve for the Assessment of Coronary Artery Stenosis Severity in EverydaY Practice): a multicenter study in consecutive patients. J Am Coll Cardiol. 2013 Apr 2; 61(13): 1421-1427.
2. Jeremias A, Maehara A, Genereux P, Asrress KN, Berry C, De Bruyne B, et al. Multicenter core laboratory comparison of the instantaneous wave-free ratio and resting Pd/Pa with fractional flow reverse: the RESOLVE study. J Am Coll Cardiol. 2014 Apr 8; 63(13): 1253-1261.
3. Sen S, Escaned J, Malik IS, Mikhail GW, Foale RA, Mila R, et al. Development and validation of a new adenosine-independent index of stenosis severity from coronary wave-intensity analysis: results of the the ADVISE (Adenosine Vasodilator independent Stenosis Evaluation) study. J Am Coll Cardiol. 2012 Apr 10; 59(15): 1392-1402.