Introduction: Pwave visibility on electrograms (EGMs) obtained by Insertable Cardiac Monitor (ICM) can enhance the diagnostic accuracy of atrial arrhythmias. Objective: To assess the optimal sensing vector for maximal Pwave amplitude obtained on sinus rhythm strips from the precordial region of patients undergoing cardiac investigation. Methods: 150 participants each had 12 separate tracings of single-lead surface EKG acquired using Northeast Holter (DR200/HE) placed to mimic ICM recordings. 3M electrodes were positioned at different vector length (75mm & 45mm) for each of three angles (vertical, oblique, horizontal) repeated in two postures (upright & supine). Impedance-based body fat percentage (BF%) was measured. Fifty patients also received ICM implant in 1:1 recruitment (Medtronic LINQ:Biotronik Biomonitor IIIm) with device EGM attained. All EKG and EGM were uploaded into DigitizeIt software for offline analysis by blinded technicians who measured calibrated Pwave amplitude. The minimum amplitude for readily visible Pwave on ICM has previously been reported as .015mV, and this threshold was used to define acceptable Pwave amplitude. Results: 150 participants [55.5% male, median age 59years (IQR 35 - 73)] each had 12 EKG strips obtained (total 1,800). Median Pwave was 45% larger for 75 vs. 45mm (.042 vs. .029mV, p<.001). Good correlation was seen for EKG and EGM, with Pwave ICC .74 (95%CI .58 - .84). In total, 79% (n=1,415) of EKG had sufficient Pwave visibility (≥.015mV), with a greater proportion seen for 75 vs. 45mm vector (84% vs 73%, p<.001). Overall, Pwave amplitude was lower for higher BF% patients. Among the top third BF% (figure), Pwaves were twice [OR 2.03 (95%CI 1.04 - 3.95)] as likely to be of visible amplitude for 75mm vector (oblique angle) compared to the best 45mm vector (vertical). Conclusion: Longer sensing vector provides superior Pwave amplitude, particularly for high BF%, which is important when factoring ICM device selection for patients.