Sensors for Implantable Cardiac Pacing Devices

Exercise Response in Heart Failure

In systolic heart failure, left ventricular (LV) filling pressure is elevated and the heart is operating at the relatively flat portion of the Frank-Starling curve. Without stroke volume reserve, an increase in heart rate during exercise is the primary means to increase cardiac output. In a study of 22 patients with poor LV function and implanted rate-adaptive pacemakers, the benefit of rate response for improving exercise capacity was greatest in those patients with the poorest LV function. However, the benefits of an increased pacing rate may be counteracted by a detrimental effect on LV function because of a higher percentage of right ventricular (RV) apical pacing. On the other hand, in patients with left bundle branch block (LBBB) who received cardiac resynchronization therapy (CRT), rate response can be beneficial. In these patients chronotropic incompetence is quite common. In one study, using the criterion of failure to reach at least 70% of the age-predicted maximum heart rate, CI was found in 70% of patients with heart failure. In this CI group, rate-adaptive CRT pacing improved maximal oxygen consumption and work capacity compared with CRT pacing without rate response. This improvement was independent of AV interval adaptation during exercise. These data suggest that rate response also plays a critical role in patients with impaired LV function and chronotropic incompetence, provided that the detrimental effects of RV apical pacing can be addressed.

Heart Rate Response for Nonexercise Needs

Exercise is only one of the many physiologic requirements for variation in heart rate. Other physiologic states, such as mental stress, normally lead to a rate increase. Sinus rate is also higher when a person assumes the upright posture as compared with sitting or a recumbent position. Isometric exercise also results in an increase in cardiac output and heart rate. The changes in heart rate that occur during various physiologic maneuvers (e.g., Valsalva maneuver) and baroreceptor reflex may also be important. An appropriate compensatory heart rate response may be especially important in pathophysiologic conditions such as anemia, acute blood loss, or other causes of hypovolemia, and during febrile illnesses.

Ideal Sensor Characteristics

Based on the physiology of the normal sinus node, a sensor system needs to be sensitive to both exercise and nonexercise needs ( Table 10-1 ). It should also be specific, unaffected by internal or external nonphysiologic changes that can cause an inappropriate rate change. The sensor should achieve rate response at an appropriate speed, with its response proportional to the level of exercise load. The sensor system should be technically easy to implement (preferably with standard pulse generator and lead system), should be stable in the body's internal environment, and should not significantly increase battery consumption.

Technical Classification

A practical classification is to categorize sensors according to the technical methods that are used to measure the sensed parameter ( Table 10-2 ). Body movements during exercise result in changes in acceleration forces that are transmitted to the pacemaker. Sensors that are capable of measuring the acceleration or vibration forces in the pulse generator are broadly referred to as activity sensors. Technically, detection of body movement can be achieved using a piezoelectric crystal, an accelerometer, or other mechanical devices. Each of these devices transduces motion of the sensor into an electrical signal that is computed to determine the rate response.

Impedance is a measure of all factors that oppose the flow of electric current and is derived by measuring resistivity to an injected current across a tissue. Transthoracic impedance is used to assess respiratory rate and tidal volume by measuring the continuous impedance between the pulse generator and an intracardiac electrode. The product of respiratory rate and tidal volume, the minute ventilation (MV) is used as the sensed parameter. Impedance can also measure surrogates of ventricular contractility, such as relative stroke volume or the right ventricular preejection interval. When unipolar impedance from the distal electrode of a ventricular pacing lead is measured, changes in impedance over the cardiac cycle reflect myocardial contractility, and is now used in the closed-loop stimulation (CLS) sensor.

The intracardiac ventricular electrogram resulting from a suprathreshold pacing stimulus has been used to provide several parameters to guide rate response. The total duration of depolarization and repolarization can be estimated by the interval from the pacing stimulus to the intracardiac T wave, allowing calculation of the QT interval or stimulus-T interval. This parameter is sensitive to changes in sympathetic activity such as occur with exercise or emotional stress. This was developed as a QT-sensing pacemaker, based on the normal physiologic shortening of the QT interval during exercise and emotional stress. Although this sensor has slow response kinetics to the initiation of exercise, the QT interval changes proportionately to workload. The paced vector integrated R-wave area (termed ventricular depolarization gradient) has also been used for rate response. A device has also been developed to use the paced ST segment monitoring as a surrogate marker of myocardial ischemia.

The last group of sensors are incorporated into the pacing leads. These specialized leads include thermistors (used to measure blood temperature), piezoelectric crystal (right ventricular pressure), optical sensor (mixed venous oxygen level), and accelerometer at the tip of the pacing lead. Some of these sensors measure physiologic parameters. For example, oxygen saturation in the mixed venous blood is closely related to oxygen consumption during exercise. Physical activities increase cardiac output and oxygen extraction from the blood lowering the mixed venous oxygen saturation with a widening of the arteriovenous oxygen difference. The fall in mixed oxygen saturation will trigger an increase in rate that will improve cardiac output and minimize the decrease in mixed venous oxygen saturation. The mixed venous oxygen saturation is a highly proportional sensor, although its technical complexity and stability over time have limited its use for rate-adaptive pacing. Sensing of changes in blood pH during exercise has been suggested as another possible sensor, although the requirement for a specialized lead has impeded its clinical implementation. Exercise increases core temperature, and central venous temperature sensing (CVT) was used in early rate-adaptive pacemakers. The response of the RV blood temperature to the onset of exercise is quite variable from one patient to another such that these sensors required considerable adjustment. However, this sensor has recently been reintroduced in a leadless pacing pacemaker for rate response (see below).

Using an accelerometer incorporated at the tip of a unipolar ventricular electrode, the contractile state of the right ventricle can be assessed indirectly from the endocardial acceleration forces generated during isovolumic contraction of the heart, a parameter known as peak endocardial acceleration (PEA). PEA is significantly increased by exercise and other inotropic stimuli, and is now used in a CRT device to optimize interventricular (VV) and AV intervals.

Over the years, many of these sensors have been implemented in implantable devices. Significant differences in rate response were found among sensors, notably between their sensitivity and specificity ( Table 10-3 ). However, only activity, MV, CLS, and temperature sensors are currently used for rate response. Sensors in special leads have uncertain long-term reliability and present challenges for matching the lead to the pulse generator during pacemaker replacement. However, some of these sensors are now used for hemodynamic monitoring in heart failure (see Chapter 25 ). The leadless pacemaker is a form of a specialized lead such that rate-adaptive pacing can only be achieved with special lead sensors. At present, activity or temperature sensors are used for rate response in leadless devices.

Principle of Activity Sensing

Body movements, especially walking, result in vibrations that are transmitted to the upper chest or generate acceleration forces in the body. In a pacemaker, acceleration forces acting on the body during exercise are detected by a device inside the pacemaker case. With triaxially mounted accelerometers placed on the surface of an externally attached pacemaker, acceleration signals during a variety of exercises were measured. The axes include the anteroposterior ( x -axis), lateral ( y -axis), and vertical ( z -axis). Because of the sloping of the chest and the swaying of the body (and hence the pulse generator) during walking, these axes are not true horizontal ( x - or y -) or true vertical ( z -) axes. The frequency content of the acceleration signals are fast-Fourier transformed. The root mean square value of acceleration signal is used to quantify the acceleration forces. The following findings have been reported :

• 1.

  Axes most relevant to detect walking: The recording of acceleration signals in a typical subject during walking is shown in Figure 10-3 . It is clear that either the x -axis or z -axis can be used to detect the acceleration forces during walking. On the other hand, the y -axis is useful only to detect body swaying during walking. In an implanted pacemaker, the x -axis would be more practical than the z -axis because the top of the pacemaker can vary with implantation or change with pacemaker rotation in the pocket, whereas the anteroposterior axis remains relatively fixed. The choice of an appropriate accelerometer axis is critical to ensure an appropriate rate response in a leadless pacemaker as the orientation of the accelerometer in such a device is highly variable.

  

• 2.

  Effects of walking speed and gradient on the acceleration signals: Acceleration forces are represented by the integrated root mean square value of accelerations. Walking at a higher speed will induce significant increase in acceleration signal ( Fig. 10-4 ). Although walking up a slope also increases the acceleration forces, the increase is less than that induced by walking faster. Thus activity sensors are less sensitive to this form of exercise.

  

• 3.

  Frequency range of acceleration forces during walking: During walking, the fast-Fourier transformed acceleration shows that the majority of the signal is less than 4 Hz. Low-pass filtering at 4 Hz can therefore improve the specificity and proportionality of the sensor.

• 4.

  Other forms of exercise: Appropriate increase in acceleration force occurs during running. However, the acceleration forces during upper limb movement and cycling are limited. Thus activity sensors underdetect these forms of exercise.

Technical Aspects of Activity Sensors

There are three different types of activity sensors: a piezoelectric sensor, and an accelerometer using either piezoelectric or piezoresistive materials ( Fig. 10-5 ). Activity sensors were first introduced as piezoelectric crystals attached to the inside of the pulse generator. Pressure waves initiated in the skeleton and soft body tissues during physical activities result in a physical deformation of the piezoelectric element (see Fig. 10-5A ). Because deformation of the piezoelectric sensor induces a voltage that is proportional to the amount of deformation, measurement of these induced voltages permits estimation of the level of physical activity. Because the piezoelectric element is usually attached to the posterior surface of the pulse generator casing during manufacturing, it is typically positioned directly against the pectoralis major muscle to ensure good physical contact with the skeletal muscles, but programming of rate response parameters can compensate if the sensor is in the opposite position. Generally, the piezoelectric element produces potentials in the range of 5 to 50 mV during rest and as much as 200 mV during vigorous activity. The frequency to which these systems are most sensitive is between 10 to 15 Hz, close to the typical resonant frequency of the human body ( Fig. 10-6 ). Vibrations sensed by a piezoelectric sensor are dependent on the body tissue coupling mass, which is made up of the connective tissue and muscles surrounding the pacemaker. Because this mass is variable among patients and changes with pacemaker pocket maturation, variation in rate response from patient to patient for the same level of activity is observed with the piezoelectric crystal sensor.

An accelerometer measures the rate of change in velocity. Accelerometers can be made of piezoelectric crystals mounted on the circuit board with a cantilever (see Fig. 10-5B ) or as part of an integrated circuit using silicon wafers sandwiching a suspended mass, the piezoresistive accelerometer (see Fig. 10-5C ). These accelerometers have a narrow range of frequencies, typically between 1 and 8 Hz, close to acceleration forces induced during physical activities, and below those produced by environmental vibration (see Fig. 10-6 ). When an accelerometer device is implanted, no special orientation of the pulse generator is needed. It may be flipped over or rotated in the pocket, and excess lead may be coiled beneath it without affecting its performance.

In accelerometers, the coupling mass is a small seismic mass that is constant between patients and is mechanically insulated from the pulse generator casing. During activities this mass deflects the lever by an amount that is proportional to the change in velocity and the direction of acceleration. Equal acceleration forces induce equal sensor signal output independent of the tissue mass surrounding the pacemaker and the physical characteristics of the patient, such as weight and height. Thus accelerometers allow a more predictable rate response. The characteristic properties of the piezoelectric crystal, the piezoelectric accelerometer, and the piezoresistive accelerometer are summarized in Table 10-4 . The ability of the accelerometers to detect physiologic vibrations rather than noise significantly improves the specificity of these devices.

Algorithms

The Medtronic Activitrax (Medtronic, Minneapolis, MN) was the first activity-sensing pacemaker available and used a piezoelectric sensor. After filtering and amplification of the raw signal, the signal was passed through a threshold discriminator. The number of piezoelectric signals that exceeded a programmable threshold (activity threshold), rather than the intensity of signal, determined the rate response ( Fig. 10-7A ). This resulted in a rate response that occurred in an “on” or “off” manner. In clinical studies with this sensor, treadmill exercise testing with increasing speed (increasing step frequency) resulted in an incremental rise in the pacing rate. On the other hand, tests of patients on a treadmill at a constant speed with a changing incline from 0% to 15% did not result in further rise in pacing rate despite the increase in workload. Thus this sensor had limited correlation between changes in pacing rate and changes in metabolic exercise load. The increase in pacing rate at vigorous exercise was usually limited as the algorithm used a plateauing slope that was designed to give a stable rate during daily activities. However, this activity-sensing pacemaker gave a prompt pacing response at the onset of physical activities, and improvement in cardiopulmonary exercise testing and symptoms were documented with this simple system.

A better correlation of workload with pacing rate was achieved with signal integration. Signals from the accelerometers are rectified and integrated to determine the pacing rate through a number of algorithms (see Fig. 10-7B ). A linear relationship between acceleration signals is used in some devices allowing a direct increase from the LRL to SURL. On the other hand, to enhance rate stability, some accelerometer devices utilize a triphasic rate-response curve so that accelerometer signal levels during daily activities are mapped to the flat portion of the curve, resulting in an increase to a stable rate response known as the ADLR (activities of daily living rate). A linear slope from the ADLR to SURL is used when the sensor indicates more strenuous exercise.

Current Activity-Sensing Devices

Medtronic Activity-Sensing Devices

Current Medtronic activity-sensing devices use an accelerometer attached to the circuit board to detect accelerations in the anteroposterior plane. Instead of a fixed activity threshold, a rolling threshold is used to provide a count of activity that depends not only on the frequency of counts that cross a threshold value, but also on the amplitude (strength) of these counts. This rolling threshold minimizes the detection of low-intensity but high-frequency accelerations, such as occur when traveling in motor vehicles, but allows detection of high-intensity signals during physical activities. The sensor threshold can be programmed to “Low,” “Medium/Low,” “Medium/High,” and “High.” With a “Low” threshold programmed, the sensor will detect the low movement/activity that occurs at minimal exertion and may be suitable for a relatively sedentary individual. Conversely, at a “High” programmed threshold, the sensor will only react to vigorous body activities, which is a more suitable threshold for an active individual. An LRL (termed lower rate ) and SURL (termed upper sensor rate ) will determine the maximum range at which the sensor will adjust the rate.

Two types of rate-response curves are used in Medtronic devices ( Fig. 10-8 ). In some models a linear (termed single zone ) rate-response curve is used, so that increasing body activity results in a higher accelerometer signal that will linearly increase the pacing rate (see Fig. 10-8A ). Programming a higher rate-response slope will result in a higher rate for the same amount of activity, and vice versa. On the other hand, a triphasic curve (termed dual zone ) is used in most Medtronic devices (see Fig. 10-8B ). In addition to the LRL and SURL, an ADLR needs to be programmed (commonly between 90 and 110 beats per minute [bpm]). This represents the clinician-desired pacing rate during most daily activities for that patient. The rate response between LRL and ADLR will be in the so-called ADL zone. An exertion zone occurs between the ADLR and SURL. The rate-response parameters in these two zones are termed ADL response and exertion response, respectively. Programming to a higher response will increase the percentage of pacing rates that occur closer to the ADLR and SURL, respectively. In effect, this changes the rate profile of the patients according to their clinical profile. This will determine, indirectly, the rate-response slopes in these zones through the ADL and upper sensor rate set points. A lower set point number in each zone represents a more aggressive rate-response slope. Finally a reaction time (termed activity acceleration parameter ) determines how quickly the pacing rate increases to the target rate indicated by the sensor during exercise, and the recovery time (termed activity deceleration parameter ) determines how quickly the pacing rate returns to the target rate of the sensor when an activity stops or reduces in intensity.

The programming parameters of the Medtronic rate-adaptive devices are summarized in Table 10-5 . In the single zone devices, programming of the sensor response involves setting the LRL, SURL, activity threshold, and rate-response slope. Either a walking exercise in the clinic or a formal exercise test can be used to set these rate-response parameters. In the activity devices with dual zone rate response, the physician can separately adjust the rate-response profile in the ADL and exertion zones. However, the simplest approach is to activate automatic sensor programming known as the “Rate Profile Optimization” (RPO). After programming the LRL, SURL, and ADLR, the device automatically adjusts the sensor indicated rate profile to a physician-set target rate profile ( Fig. 10-9 ). By adjusting the ADL response and/or exertional response parameters , the physician can determine the target rate profile of the patient. The physician can choose one of the five response levels in either ADL or exertion zone categories, depending on his or her assessment of how active the patient is (1 = least active, 5 = most active). This allows a patient-specific target rate profile. The RPO then automatically adjusts the ADL and/or upper sensor rate set points, which will determine directly the rate response of an activity in the ADL and exertion zones, respectively. As the set point values decrease, rate increase will be greater for the same amount of activity, thus making the patient spend more time in the corresponding rate zone. The set points are the actual “rate-response slope” in the standard rate-adaptive system. Based on the SIR profile, RPO will adjust, if necessary on a daily basis, the set points to attain the programmed target rate profile. Alternatively, the physician can directly change the set points. This will set the rate response in the ADL and exertion response zones immediately (in some devices only after an exercise test). The physician can turn off RPO at this point and fix these set points, or allow RPO to continue adjustment based on the programmed target rate profile.

TABLE 10-5

Programmable Rate-Response Parameters of the Medtronic Activity-Sensing Devices

Single Zone Devices
Parameters	Options
Rate response	On or Off
Lower rate	Nominally 60 bpm
Upper sensor rate	Nominally 130 bpm
Activity threshold	Low, Medium/Low, Medium/High, High
Rate response	1-10
Activity acceleration	Nominally 30 sec
Activity deceleration	Nominally “Exercise”
Dual Zone Devices
Parameters	Options
Rate response	On or Off
Lower rate	Nominally 60 bpm
Upper sensor rate	Nominally 130 bpm
ADL rate	Nominally 95 bpm
Activity threshold	Low, Medium/Low, Medium/High, High
ADL response	Nominally 3
Exertion response	Nominally 3
Activity acceleration	Nominally 30 sec
Activity deceleration	Nominally “Exercise”
Rate profile optimization	On/Off
ADL rate set point	Amount of the time spent in ADL zone
Upper rate set point	Amount of time spent in exertion Zone

ADL, Activity of daily living.

St. Jude Medical Activity-Sensing Devices

A ball sensor was introduced in the Microny device (St. Jude Medical, St. Paul, MN). This utilized a ball in a cage arrangement (termed Casino ball ) that had two functions: the position of the ball within the cage would indicate the patient's posture, and the frequency at which the ball hit the walls of the cage was an indication of activity level. However, all current activity-sensing devices of the St. Jude Medical use an accelerometer sensor bonded to the circuit board. Collection of sensor signal can be either in the active or passive mode, and occurs at a sampling interval of 26 seconds. The activity counts over time results in an activity variance from which sensor signals that represent rest and activity levels can be discriminated. Activity counts above a programmable threshold (1 = most sensitive; 7 = least sensitive) are integrated and translated into a pacing rate using a rate-response slope (1 = least sensitive; 16 = most sensitive) ( Fig. 10-10 ). The pacing rate will then change between the programmed LRL and SURL (termed maximum sensor rate ), depending on the patient's level of activities.

The threshold level can be adjusted manually or automatically (Auto). In the Auto setting, the device measures the sensor activity level over the preceding 18 hours to determine the threshold parameter. The Auto threshold can further be adjusted manually using “Auto threshold offset,” which is ±2.0 in steps of 0.5 from the zero point (termed 0.0 ) determined automatically. Similarly, the slope of rate-response curve can be calibrated manually or automatically. In the slope Auto adjustment, the device records the activity variance occurring above the threshold over the last 7 days, and the slope is adjusted to achieve a rate response so that about 1% of pacing heart rate occurs beyond 23% of heart rate reserve (the difference between the SURL and the LRL). The rate-response slope is adjusted a maximum of two units each week to achieve this target. If the patient has been immobile for the previous week, as indicated by a small activity variance, the slope setting will be held constant to avoid overly aggressive rate response once the patient resumes activity. The threshold has to be in Auto if Auto slope is used. Alternatively manual adjustment of the slope value can be rapidly achieved by instructing the patient to perform an exercise at a prescribed slope, and according to the physician's assessment of rate response, the slope is then adjusted. When programmed in this manner, the slope will be fixed. In addition, the reaction and recovery times are separately programmable. The St. Jude Medical devices also incorporate a rest rate that may be programmed below the LRL. This rest rate is activated when the accelerometer sensor does not register significant activity counts over an extended period, implying the patient is at rest or asleep. This conserves pacing energy and mimics circadian heart rate rhythm. In 25 patients with simultaneously recorded sinus rate and activity variance histogram, the rest rate as driven by the accelerometer sensor provided a circadian lower rate mimicking sinus rhythm.

Boston Scientific Activity-Sensing Devices

In the Boston Scientific devices, a choice of activity, MV, or both can be selected. Activity sensing utilizes an accelerometer mounted on the integrated circuit board to detect acceleration during physical activity. It has a frequency range of 1 to 10 Hz. Four parameters are used to determine the rate response: rate-response slope ( response factor; 1 = least sensitive, 16 = most sensitive), activity threshold (very low, low, medium-low, medium, medium-high, high, very high), reaction time, and recovery time. Similar to most rate-adaptive pacemakers, rate response occurs in between the LRL and SURL (termed maximum sensor rate ). After programming an activity threshold, rate response will be determined by the rate-response slope ( Fig. 10-11 ). In Boston Scientific (Marlborough, MA) devices, programming of the activity sensor can be done manually in the activity-only mode and by automatic adjustment of both sensors in the dual-sensor mode. The reaction and recovery times then determine the rapidity that the pacing rate reaches the target rate determined by the sensor.

Biotronik Activity-Sensing Devices

Biotronik devices can be dual sensor using CLS and activity (termed VVI-CLS or DDD-CLS ), or activity only (termed VVIR or DDDR). The activity sensor is only used to set the CLS waveform at rest in a dual-sensor device (see below). In the single accelerometer sensor VVIR or DDDR modes, rate response is determined by the LRL, SURL, activity threshold, and rate-response slopes. The slopes are self-programmed based on a set target relationship between the sensor activity and rate. This automatic sensor gain dictates that 90% of the SURL has to be reached for 90 sec/day. If this sensor rate is reached, the gain is scaled down one slope. If the SURL is not reached in 7 days, the gain is scaled up by one slope. Reaction (termed rate increase ) and recovery (termed rate decrease ) times are programmable. Manual adjustment of the target may be accomplished by using the Preview feature, which can profile the SIR for a brief exercise at different rate-response slopes, without repeating the exercise itself ( Fig. 10-12 ). Once programmed, the sensor setting can be fixed or programmed to automatic sensor gain to allow for further automatic adjustments.

Clinical Experience with Activity Sensors

Clinical studies have demonstrated that activity-sensing, rate-adaptive pacing improves exercise capacity and reduces symptoms compared with VVI pacing. In an early study of a piezoelectric activity sensor pacemaker, Benditt et al compared exercise tolerance during VVI and VVIR pacing using treadmill testing. VVIR pacing prolonged exercise duration by 35% and resulted in a similar increase in peak oxygen consumption (V o _{2 max} ) and V o ₂ at anaerobic threshold compared with VVI pacing. Activity-sensing pacing also reduced the patient's perception of exertion at comparable exercise levels, and the benefit was sustained when exercise testing was repeated after an average 5 months of follow-up. Furthermore, at follow-up exercise testing, programming of the pacing system back to the VVI mode resulted in deterioration of both observed V o _{2 max} and exercise duration. Crossover studies comparing VVIR mode to DDD pacing found no significant difference in symptom scores, maximal exercise performance (treadmill), or plasma concentrations of epinephrine, norepinephrine, and atrial natriuretic peptide, confirming that rate adaptation was the most important determinant of exercise capacity in the patient populations studied.

In a study of the efficacy of automatic RPO over time, 11 patients with Medtronic Kappa 700 pacemakers performed treadmill testing at 1 month, 1 year, and 2 years after pacemaker implantation. Based on the sinus profile at follow-up, it was necessary to program a more aggressive slope to match the sinus rate profile in some patients. This required a change in ADL and exertion response parameters from 3 to 4 at 1 year, and a change in activity threshold from medium-low to low at 2 years. These adjustments enabled better approximation of pacing rate to sinus rate during treadmill exercise. This and other studies highlight the importance of repeated assessment of rate-response parameters despite automatic programming of sensors. Appropriate automatic adjustment to the rest rate using activity sensor to mimic circadian rate-response were reported for St. Jude Medical devices and Sorin pacemakers.

Limitations of Activity-Sensing Devices

Activity sensing may give inappropriate heart rate responses when the patient is exposed to environmental vibration, such as travelling in a motor vehicle or a train over a rough terrain, during air travel, or the use of vibrating appliances or machinery. The effects of various modes of locomotion on pacing rate have been assessed for different activity-based pacemakers. Three different activity-based pacing systems (peak counting algorithm, integration algorithm, and accelerometers) were strapped to the chest walls of volunteers. Bicycling on the street resulted in higher pacing rates than did stationary bicycling for each type of pacemaker, although none of the pacemakers reached the heart rate achieved by the normal sinus node. During driving, the pacemakers increased the pacing rate, although the intrinsic sinus rate continued to be higher. In passively riding passengers, the pacemakers tended to produce a higher pacing rate than that of the normal sinus node. The piezoelectric sensor also responds to static pressure on the pulse generator, which may cause inappropriate rate increases when the patient lies prone. This false positive response to pressure is no longer a problem with the accelerometer sensor.

Proportionality and sensitivity are the additional issues with activity sensing (see Table 10-3 ). Activity-sensing rate response depends on the manner in which activity is performed rather than on the exercise workload, and proportionality is generally only moderately correlated to exercise workload. Activity sensors typically produce a higher heart rate walking upstairs than downstairs because of the greater force of heel-strike on descent. Nonexercise stresses such as emotional changes are not detected.

Physiologic Principle

Relationship Between Heart Rate and Respiratory Parameters During Exercise

Changes in heart rate during exercise are closely related to changes in V o ₂ at all levels of exertion. At metabolic workloads less than anaerobic threshold, both V o ₂ and heart rate are directly proportional to MV, with correlation coefficients greater than 0.9 in most studies. The correlation between respiratory rate and V o ₂ during submaximal exercise is less than 0.54 as the normal ventilatory response at the onset of exercise is predominantly the result of a change in tidal volume rather than in respiratory rate ( Fig. 10-13 ).