Sensing means that if the pacer senses that the client has his or her own P wave or QRS complex, it will not fire. If the pacemaker fires, the client can go into dysrhythmias.
Here is a dangerous “failure to capture” malfunction of the pacemaker. Notice the paced beats at the beginning and the end. That’s when the pacemaker is working. When the pacemaker is not working, there is no QRS or ventricular contraction. No pulse. The client’s underlying rhythm might as well be asystole since a little atrial contraction (or P waves) is worthless.
The one below is not as bad. At least the client has more paced beats or a more life-producing rhythm.