心血管课件-起搏时间间期PPT.ppt

If the upper tracking rate interval is longer than the TARP, the pacemaker will exhibit Wenckebach behavior for some period of time before it goes into a 2:1 block pattern as the atrial rate increases. If the upper rate interval is shorter than the TARP, the pacemaker will exhibit 2:1 block behavior first and will never be able to achieve the upper tracking rate as the atrial rate increases. This situation is not as desirable as the situation in which there is a period of Wenckebach before 2:1 block because patients can tolerate the gradual ventricular rate drop of Wenckebach better than the precipitous ventricular rate drop caused by 2:1 block. This picture is meant to provide some cheap laughs. Actually, the physician is real as is the device. The physician is Dr. Ake Senning from Sweden. The device is the first implantable pacemaker (circa 1958). The battery needed to be recharged every few days! Compare the P-P interval at which the pacemaker will exhibit Wenckebach to the P-P interval at which the pacemaker will go into 2:1 block. Upper tracking interval = 60,000/120 = 500 msec 2:1 block interval = TARP = 200 msec + 350 msec = 550 msec As the P-P interval shortens from the lower rate interval, the 2:1 block interval (550 msec) will be met before the upper tracking interval (500 msec). Therefore, the pacemaker will go into 2:1 block as the atrial rate increases and will never exhibit Wenckebach behavior. In this example, the TARP is shorter than the upper tracking rate interval. Therefore, the pacemaker will exhibit Wenckebach operation as the P-P interval exceeds the upper tracking rate interval. Going to 2:1 block first without a Wenckebach period may not be the optimal situation because many patients do not tolerate a precipitous drop in ventricular rate well. What can we do to make Wenckebach occur first? Shorten PVARP (Note: Sensor varied PVARP will be addressed later on in the module with mode switching, but it can be discussed as a solution