Figure 53-13 Electrocardiographic (ECG) configurations observed at various locations when a central venous catheter is used as an intravascular ECG electrode. The configurations in the figure will be observed when "lead II" is monitored and the positive electrode (the leg electrode) is connected to the catheter. P indicates the sinoatrial node. The heavy black arrow indicates the P-wave vector. Note the equi-biphasic P wave when the catheter tip is in the mid-right atrial position.^[111]