Formation of the P-R segment.

Variance of opinion among authors regarding the cause and duration of each succeeding portion of the P-R interval points out that knowledge regarding the physiologic basis for these waves is incomplete. Data regarding the P-R segment are scant in electro-cardiographic literature. The necessity for precision in measurement and the need for better visualization of electrocardiographic waves to enhance their diagnostic value are illustrated by study of the generally ignored region between the P and QRS. T HE purpose of this report is to present data and tracings that define the region of the P-R segment more precisely than usual in clinical practice. By more precise measurement and better visualization of that portion of the electrocardiographic wave, factors of significance to its formation and of importance to the interpretation of the entire elec-trocardiogram are demonstrated. MATERIALS AND METHODS The electrocardiograms studied were from normal male subjects between the ages of 26 and 46 years. During the initial phase of the study electrocardiograms recorded on direct-writing elec-trocardiographs at 25 mm. per second with a standardization sensitivity of 10 mm. per millivolt were used, so that measurements could be related to those used for clinical interpretation. These electrocardiograms were optically magnified 10 times for better visualization of the beginning and end of waves to obtain more precise measurements .' The P-R segment was measured in all 12 leads of 20 of these electrocardiograms. The P-R segment was defined as that portion of the electrocardiogram inscribed as an isoelectric line between the discernible termination of the P wave and the onset of the first deflection attributable to the ventricular complex. Difficulty was experienced in measurement due to the usual thick baseline of tracings with the standard sensitivity of 10 mm. per millivolt. These 2 factors, regardless of instruments used, hinder detection and accurate interpretation of minor potential changes. To sharpen the points of potential change on the X axis of the electro-cardiogram and for better visualization of the region of the P-R segment, a paper speed of 75 mm. sensitivity was increased to a range of 250 to 500 mm. per millivolt. This could be accomplished only with cathode-ray tube electrocardiographs. The latter instruments were used during the remainder of the study for verification of the findings suggested by the optically magnified directly written electrocardiograms.