H

Header. See connector block (of pulse generator).

Hemodynamics. The study of the forces involved in circulating blood through the body.

Hermetic seal. A seal that prevents penetration of water or vapor. Modern pulse generators have metallic cases which are sealed by welding, so that no fluid can enter the device. If hermeticity is lost, fluid entering the pulse generator case could potentially cause a short circuit or abrupt battery failure.

Histogram. The distribution of pacing states (PR, PV, AR, AV or PVE), sensor indicated pacing rates or other data based on data collected by event counters and displayed in a bar graph format by the programmer and based on data collected by event counters.

Housekeeping current. The amount of current used by the pulse generator even when it is not delivering an output pulse. A pulse generator uses a certain minimal current to maintain active circuits, i.e. sensing. The housekeeping current of a given unit can be obtained by interrogating the measured data before the pulse generator is connected to the lead system. Housekeeping current is measured in millionths of an ampere or microamperes. Sometimes called standby current.

Hybrid. The combined set of semiconductor chips and other electronic components prepared as a single complex circuit for the pulse generator.

Hypertrophy. Any increase in the mass of an organ or anatomical structure. Cardiac hypertrophy is the increase in mass of the myocardium resulting from either thickening of the muscle tissue without increase in the size of the cavities or increase in the size of the cavities with or without additional thickening of the muscular wall.

Hysteresis. (1) Physiology. A delay of effect. (2) Devices. A programmable feature in some demand pacemakers which allows programming of a hysteresis escape rate lower than the programmed base rate. The pulse generator will be inhibited if the intrinsic rate equals or exceeds the hysteresis escape rate; should the intrinsic rate fall below the hysteresis escape rate, there is one cycle of pacing at the escape rate followed by pacing at the programmed base rate until the pacemaker is again inhibited by a sensed event. Hysteresis is accomplished by a timing mechanism which prolongs the pacing interval following a sensed beat.

Hysteresis escape rate. The programmable rate which allows the pacemaker to accomplish hysteresis. The hysteresis escape rate is lower than the programmed base rate. The interval associated with the hysteresis escape rate (hysteresis interval) is longer than the automatic interval initiated by a sensed event and thus allows the heart more time to beat on its own before a pacing stimulus is delivered.