Technical Classification

A practical classification is to categorize sensors according to the technical methods that are used to measure the sensed parameter (Table 5-1). 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, a tilt switch, or an inductive sensor. Each of these devices transduces motion of the sensor either directly into a voltage or indirectly into measurable changes in the electrical resistance of a piezoresistive crystal. Activity sensing is the most widely used control parameter in rate-adaptive pacing because of its ease of implementation and its compatibility with standard unipolar and bipolar pacing leads. Additionally, activity sensors can be used for either atrial or ventricular pacing modes.

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. The impedance principle has been extensively used for measuring respiratory parameters and those associated with ventricular contractility, such as relative stroke volume or the right ventricular preejection interval. Transthoracic impedance can be used to assess respiratory rate and tidal volume by measuring the continuous impedance between the pacemaker casing and an intracardiac electrode. The product of respiratory rate and tidal volume, minute ventilation (MV), is used as the sensed parameter. When using unipolar impedance from a distal electrode of a pacing lead, impedance reflects myocardial contractility, as used in the closed-loop stimulation (CLS) sensor.

The intracardiac ventricular electrogram (EGM) resulting from a suprathreshold pacing stimulus has been used to provide several parameters to guide rate modulation. The total duration of depolarization and repolarization can be estimated by the interval from the pacing stimulus to the intracardiac T wave: the QT interval, or stimulus-T interval. This parameter is sensitive to changes in heart rate and circulating catecholamines and can be derived from the paced EGM with conventional pacing electrodes. This was developed as a QT-sensing pacemaker, based on the QT interval shortening during exercise and stress.⁵

The last group of sensors is incorporated into the pacing lead itself. These specialized leads include thermistors (used to measure blood temperature), piezoelectric crystals (right ventricular pressure), optical sensors (mixed-venous oxygen level), and accelerometers at the tip of pacing leads. Some of these sensors measure highly physiologic parameters. For example, oxygen saturation is closely related to oxygen consumption during exercise. Physical activities increase cardiac output and oxygen extraction from the blood, and the tissue arteriovenous oxygen difference widens during increased tissue oxygen consumption. The fall in oxygen saturation will trigger an increase in rate that will improve cardiac output and minimize the decrease in mixed-venous oxygen saturation.⁶ The mixed-venous oxygen sensor is a true closed-loop system, although its technical complexity limits its use for rate-adaptive pacing. Sensing of changes in blood pH during exercise has also been suggested as a possible sensor, although the requirement for a specialized lead has impeded its clinical implementation.⁷

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 pressure 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 (e.g., mental stress).

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 5-2). However, only activity, MV, and CLS sensors are currently used for rate response. The activity sensor has the advantage of reliability and ease of implementation; its lack of proportionality to exercise workload has not been a major clinical disadvantage for most patients. Sensors in special leads are no longer used for rate adaptation because of their uncertain long-term reliability and difficulty during pacemaker replacement. However, these sensors are now used for hemodynamic monitoring in heart failure.

Current Activity-Sensing Devices

Medtronic (Kappa 700/900/400, Adapta, Advisa, EnRhythm)

The Medtronic Activitrax and Thera devices were the first generation of activity-sensing pacemakers to use a piezoelectric crystal bonded to the inside of the pacemaker casing to detect activity. Activity levels that exceed a programmable threshold are counted, and the number of counts that exceed a programmable threshold is used to determine the rate response. The clinician adjusts the overall rate response by programming the activity threshold and the rate-response slope. Typically, this instrumentation allows a good response from the programmed lower rate to about 100 beats per minute (bpm) during daily activities. The response to the upper rate is often less accurate and depends on the patient and the programmed settings. Despite their limitations, these piezoelectric devices were clinically highly effective when first introduced.

The current Medtronic activity-sensing device uses an accelerometer attached to the circuit board to detect movement in the anteroposterior (AP) plane. Instead of a fixed 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, as during motor vehicle travel, but allows detection of high-intensity signals during physical activity.

The sensor-driven rate is adjusted by the use of a triphasic rate curve that incorporates two rate ranges, activity of daily living (ADL) and exertion response (ER) programming (Fig. 5-4). The physician determines the lower rate limit (LRL), the ADL rate, and the sensor-driven upper rate limit (SURL). With the nominal ADL response of 3 (I-5), counts between 12 and 20 will give the ADL rate, during which the rate response will be stable. When counts are greater than 20, ER will occur to the maximum programmed SURL. The flat portion of the response curve corresponds to 25% of the total counts. Adjustment of the ADL and ER to a more sensitive setting will decrease the number of counts needed to reach between ADL rate and SURL, and shorten the ADL portion of the response curve. The actual number of counts to effect a change in pacing rate is not fixed and differs among patients. Rather, an ADL response of 3 implies that the patient will spend up to 30 minutes daily at a sensor rate at or above the ADL. This is achieved by adjusting the rate/count relationship to achieve this percentage of rate response, the ADL set point. Similarly, an ER response of 3 means that the patient will be spending about 20 minutes weekly at the higher rates, 105 to 120 bpm, the upper-rate set point.

Figure 5-4 Triphasic rate-responsive curve.

Medtronic Kappa 600/700/900, Enrhythm, and Enpulse accelerometer activity-sensing pacemakers; ADL, activity of daily living; ER, exertion response.

The acceleration signal input can be adjusted by changing the threshold (lower threshold allows detection of more acceleration signals) and slope. These two parameters affect the sensitivity to the acceleration detected. The physician can manually program these parameters to achieve a faster or slower response in both the ADL and ER ranges.

With automatic rate response, rate profile optimization will adjust these rate curves automatically, using daily comparison of the rate histogram with the target-rate histogram. The device adjusts more quickly in the first 10 days after a programming change so that the new setting will be achieved more quickly (Fig. 5-5).

Figure 5-5 Optimization using target rate histogram.

The sensor rate profile is matched against a target rate profile. The activity of daily living (ADL) range includes moderate pacing rates between the lower rate limit (LRL) and the ADL rate. The exertion rates range from the ADL rate to the upper rate limit (URL). By comparing the sensor rate profile with the target rate profile once daily, the pacemaker automatically controls how rapidly the sensor-indicated rate increases and decreases in these ranges.

(From Leung SK, Lau CP, Tang MO, et al: An integrated dual sensor system automatically optimized by target rate histogram. Pacing Clin Electrophysiol 21:1559-1566, 1998.)

The Kappa 400 is a combined activity/MV-sensing device. The general hardware and software implementation is similar to the Kappa 700/600 series. However, actual time spent in ADL and ER is longer in the Kappa 400 because a more aggressive rate response is allowed.

St. Jude Medical (Identity, Vitality, Zephyr, Accent)

These activity devices use an Omnisense accelerometer bonded to the circuit board and are sensitive to accelerations that occur in the AP axis. Activity counts above a programmable threshold (1 = most sensitive; 7 = least sensitive) are integrated and translated to a rate response using a rate-response slope (1 = least sensitive; 16 = most sensitive) (Fig. 5-6).

Figure 5-6 Rate-response and “Auto” slopes.

A, Rate-response curves from St. Jude Medical activity sensors. B, Automatic threshold (Auto) slope ensures that about 1% of the pacing rate will reach more than 23% of the heart rate reserve (HRR) over time. Note that the sensor-driven upper rate limit (SURL) may not be reached at a low slope setting. LRL, Lower rate limit.

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 be adjusted using “Auto threshold offset.” Similarly, the slope of response 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 such that almost 1% heart rate occurs beyond 23% of heart rate reserve. Typically, the slope adjusts by a factor of 2 each week. If the patient has been immobile for the previous week, as indicated by the small activity variance, the slope setting will be held constant, to avoid undue rate response when the patient resumes activity. In addition, rapid manual adjustment of the slope can be achieved by instructing the patient to perform an exercise at a prescribed slope. The Prediction Model will attempt to suggest an alternative slope based on the detected rate response. In addition, the onset and recovery kinetics are separately programmable. These devices also feature a “rest rate” that may be programmed below the LRL. The rest rate is attained when the acceleration sensor does not register significant counts (the “activity variance” level) over an extended period, implying the patient is resting or asleep.

Clinical Results of Activity Sensors

Clinical studies have demonstrated that activity-based pacing systems improve exercise capacity and reduce symptoms compared with VVI pacemakers. In an early study of a piezoelectric vibration-based sensor pacemaker, Benditt et al.⁸ compared exercise tolerance during VVI and VVIR pacing using treadmill tests. Sensor-driven pacing prolonged exercise duration by 35% and led to similar increments in peak oxygen consumption and oxygen consumption at anaerobic threshold. Activity sensor–driven 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. Further, at follow-up exercise testing, reversion of the pacing system to a VVI mode resulted in prompt deterioration of both observed oxygen consumption 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, suggesting rate adaptation was the most important factor for exercise in the patient population studied.^9,10

In a study of the efficacy of automatic “rate profile optimization” over time, 11 patients with Kappa 700 pacemakers performed treadmill testing at 1 month, 1 year, and 2 years after implantation.¹¹ Based on the sinus profile at follow-up, a more aggressive slope was needed to match the sinus rate profile. This required a change in ADL response from 3 to 4 and in ER 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. Exercise capacity was maintained during the 2 years of follow-up. Variation in activity level (Activity Variance) as detected by an accelerometer is used to determine the rest rate in the St. Jude Medical devices,¹² with an overall appropriate rate response.

Limitations of Activity-Sensing Devices

Activity sensing may give inappropriate heart rate responses when subjected to environmental vibrations, such as from motor vehicle movement over rough terrain, air travel, or use of vibrating appliances or machinery. The piezoelectric vibrational sensor also responds to the application of static pressure on the pulse generator, which may cause inappropriate rate increase when the patient lies prone. This false-positive response to pressure is no longer a problem with pacemakers that incorporate an accelerometer.

The effects of various means of locomotion on pacing rate were assessed for different activity-based pacemakers.¹³ Three different activity-based pacing systems (peak counting algorithm, integration type, and accelerometers) were strapped to the chests 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. Independent of the sensor, activity-initiated rate response depends on the manner in which activity is performed rather than the exercise workload, and proportionality is generally limited.¹⁴ Activity sensors typically produce a higher heart rate walking downhill than uphill because of the greater force of heel-strike in the descending direction. Nonexercise stresses such as emotional changes are not detected.

Current Minute Ventilation–Sensing Devices

Clinical Experience of MV Sensor Pacemakers

Minute ventilation is an indirect but reliable marker of metabolic demand, and in the first generation of MV pacemakers (Meta MV, Telectronics), the rate response was proportional to the level of exertion and correlated with the normal sinus response.^20,21 Compared with VVI pacing, MV-based VVIR pacing increased exercise capacity by 33%,²¹ and maximal oxygen consumption and cardiac output were significantly improved. In one study of 10 patients with the first version of MV-driven VVIR pacing, pacing rate was highly correlated with measured MV (r = 0.89), respiratory quotient (r = 0.89), Vco₂ (r = 0.87), tidal volume (r = 0.87), Vo₂ (r = 0.84), and respiratory rate (r = 0.84). Maximum oxygen consumption also increased from 13.4 ± 3.4 to 16.3 ± 4.1 mL/kg/min (P = .0004) using MV-driven over VVI pacing.

Improvement in symptoms were also documented in the VVIR mode.²¹ The MV sensor has good long-term stability; programming of the sensor is relatively simple; and the rate response is appropriate during daily activities. Compared with activity pacing, MV is significantly better in achieving a near-normal pacing rate–workload relationship, whereas activity sensing tends to overpace at low-level exercise and underpace at peak exercise and in the recovery period.

Limitations of Minute Ventilation Sensing

As with all impedance systems, the MV sensor will likely be affected by electromagnetic interference, arm swinging, coughing, and hyperventilation.¹⁶ Artificial ventilation will induce an unphysiologic rate, so MV sensing needs to be disabled in such patients (e.g., general anesthesia, mechanical ventilation). The use of MV devices in patients with lung disease and heart failure remain controversial. Early generations of MV-sensing devices limited the maximum detectable respiratory rates to less than 48 to 60 breaths/min to minimize the cardiac effects of changes in impedance. Because of this filtering of the cardiac component of changes in impedance, MV sensors may not accurately reflect rate adaptation at very high respiratory rates, as may occur in children. A bipolar ventricular lead is needed for MV sensing in several models. The battery current for MV sensing may consume approximately 2% of the total current of a dual-chamber pacemaker.

Some amplifiers may sense the small impedance pulses unless preventive steps are taken. In some products, a blanking period of about 1 msec is applied to the amplifiers to “blind” them to these pulses. In other cases, special balancing of the pulse achieves the same objective.

Sensing of the impedance pulses by a surface electrocardiogram (ECG) monitor may occur, depending on the ECG machine’s frequency response as well as pulse width and balancing of the impedance pulse. Also, frequencies above a few kilohertz (kHz), such as generated by electrocautery and electrosurgery, are detected by the rate-response circuitry and could drive an MV-sensing pacemaker to its maximum rate. It is recommended that the rate-response function be inactivated whenever electrosurgery will be used. Respiration may also be influenced by phonation and coughing that have no direct relevance to change in cardiac output.

Sensor and Algorithm

The CLS sensor is based on unipolar impedance at the tip of a pacing lead.²² Subthreshold pulses of automatically selected outputs, ranging from 100 to 400 microamperes (µA), with biphasic duration of 46 msec are emitted 50 to 300 msec after a sensed or paced ventricular event. During late diastole immediately after a ventricular paced (Vp) or sensed (Vs) event, the blood volume of the right ventricle is highest and RV impedance is lowest. On the other hand, as contraction occurs, the walls surrounding the electrode tip draw closer, and impedance rises (Fig. 5-7, A). A baseline waveform will depend on the conduction state of the heart: AsVs, AsVp, ApVs, ApVp (As, atrial sensed; Ap, atrial paced event). Baseline CLS waveforms will be acquired only when the associated accelerometer indicates “no activity,” and a waveform will be discarded within 48 hours if not referenced. An average template of the baseline CLS waveform will take 2 to 3 days to optimize.

Figure 5-7 Closed-loop stimulation (CLS) sensor and algorithm.

A, Changes in CLS parameter during exercise. Hatched area represents the difference between baseline and exercise CLS waveforms and is converted to a rate using the exertion threshold rate (ETR). B, Impact of ETR on rate response of CLS. “Medium” ETR corresponds to a rate of 80 beats per minute (bpm; 20 bpm above lower rate limit). Programming this rate will ensure a rate response of 80% less than this rate and 20% above this rate. A higher or lower ETR will result in different rate responses.

As contractility increases during exercise, unipolar impedance will change. The time-integrated difference between the exercise and baseline impedance waveforms is converted to a pacing rate using an auto response factor, which is continually adapted and patient specific. The magnitude of rate response is then determined by a programmable exertion threshold rate (ETR; very low, low, medium, high, and very high) (Fig. 5-7, B). The ETR, acting through the auto response factor, will determine that 80% of the heart rate will occur below the ETR and 20% above the ETR. An active young individual will probably require a higher ETR than an inactive elderly patient. Again, the type of rate profile attained will be determined by the patient’s cardiac condition, physical state, and specific automatic response factor.

Although not strictly a dual-sensor pacemaker, the incorporated accelerometer performs an “on-off” function to decide on the acquisition of the baseline CLS waveform and the CLS-driven response. Full CLS-driven rate according to the programmed ETR will be allowed only if the accelerometer registers “exercise”; otherwise, only 20 bpm above the LRL is allowed to enable a nonexercise rate increase. In neurocardiogenic syncope, this rate-limiting algorithm can be inactivated for full overdrive response, as during a syncopal episode.

Current Closed-Loop Stimulation Devices: Biotronik (Inos, Protos, Cyclos, Entovis, Evia)

Acute measurements of the CLS parameter (previously known as “ventricular inotropic index”) were taken in 82 patients with chronically implanted unipolar ventricular leads at pulse generator replacement.²² A wide fluctuation of baseline impedance was observed (500-1500 Ω), whereas CLS fluctuated by about 4 to 25 ohms (Ω), with a good correlation between CLS and the baseline impedance. A clinical study of 205 patients evaluating the CLS pacemaker included a significant number of young subjects with complete AV block caused by Chagas’ disease.²³ Satisfactory rate modulation was reported in 93% of patients. In the remaining 7% of patients, rate adaptation could not be achieved because of poor exercise tolerance, severe myocardial dysfunction, and intermittent intrinsic AV conduction. In a multicenter study that included 178 VVIR (Biotronik Neos-PEP) and 84 DDDR (Biotronik Diplos-PEP, InosDR) devices, physiologic rate adaptation was possible in 93% and 96% of patients with these devices, respectively.²⁴ Apart from exercise rate response, this study also involved mental stress testing using color-word matching and the infusion of inotropic agents. A moderate level of rate response was documented in some patients with CLS pacemakers during these nonexercise conditions that was greater than during accelerometer-guided pacing. The benefit of CLS sensor pacing during mental stress and cognitive function has also been studied.²⁵

Considerable interest surrounds the use of the CLS sensor to detect changes in posture. Passive head-up tilt, which depletes intravascular volume, increases the inotropic state of the heart. In a multicenter study, Inotropy Controlled Pacing in Vasovagal Syncope (INVASY), 50 patients with severe vasovagal syncope and positive head-up tilt test were randomized between DDD-CLS and DDI mode at 40 bpm.²⁶ Whereas 7 of 9 patients in the DDI arm experienced syncope within 1 year, only 4 of 41 patients in the DDD-CLS arm had presyncope. The authors suggested the efficacy of this approach, although a placebo effect of pacing was suspected in 22% of patients. In 131 patients with chronotropic incompetence, CLS pacing resulted in a higher pacing rate compared with accelerometer sensor, but no difference in 6-minute walking distance (6MWD) test.²⁷ However, twice as many patients preferred the CLS mode. The use of CLS in the left ventricle, as in a CRT device for monitoring, may be a useful application of this technology (see later discussion).

Advantages and Limitations

Closed-loop stimulation appears to be a good sensor to measure cardiac contractility using a conventional ventricular pacing electrode. The demand on pacing energy is acceptable. As a contractility sensor, CLS is sensitive not only to exercise but also to nonexercise requirements, and it may therefore be used for monitoring cardiac contractility for non–rate augmentation purposes. CLS can be used only in a pacing mode that incorporates a ventricular lead. Ventricular ischemia and cardioactive medications likely influence CLS rate adaptation, although automatic adjustment may permit long-term function. A small proportion of patients are not suitable for CLS sensors because of severely impaired RV function. Such patients may be identified preoperatively, and a backup activity sensor is always available.

Sensor and Algorithm

The contractile state of the heart can be identified by the maximal velocity of shortening of unloaded myocardial contractile elements, which can be measured with a catheter tip accelerometer attached to the ventricular wall. The “peak endocardial acceleration” represents the endocardial vibration measured by the accelerometer in the right ventricle during isovolumetric contraction phase of the ventricles. This signal is closely associated with the intensity of the first heart sound. The ELA-Sorin BEST (Biomechanical Endocardial Sorin Transducer) sensor is a microaccelerometer consisting of an acceleration sensor built into a nondeformable capsule on the tip of a standard unipolar ventricular pacing lead. The lead is placed against the RV wall so as to be sensitive to its acceleration and insensitive to the pressure of blood and myocardium (Fig. 5-8). This system has a frequency response up to 1 kHz and a sensitivity of 5 mV/G (1 G = 9.8 m/sec/sec).

Figure 5-8 Biomechanical Endocardial Sorin Transducer.

The BEST sensor (Sorin Biomedica Cardio) consists of a microaccelerometer located inside a rigid capsule at the tip of a standard ventricular pacing lead, which is connected to a triple header. PEA, Peak endocardial acceleration.

In early animal experience using an external system and an implantable radiotelemetry system, the PEA was not affected by heart rate but significantly increased by emotional stress, exercise stress testing, and inotropic stimulation.²⁸ The PEA signal changes in parallel to the maximal LV dP/dt and appears to measure the global LV contractile performance rather than the regional mechanical function. The PEA signal that occurs 150 msec after the R wave corresponds to the isovolumetric contraction phase of the left ventricle (PEA-I)²⁹ (Fig. 5-9). A smaller signal also occurs in the 100-msec period after the T wave, the so-called PEA-II, which corresponds to the isovolumetric LV relaxation. PEA-II is related to peak negative dP/dt (r = 0.92) and aortic diastolic pressure (r = 0.91).³⁰ To enable the PEA sensor to be used in patients requiring an ICD lead, the sensor has been incorporated into a right atrial (RA) lead (SonR sensor). Preliminary results suggest that the SonR sensor is similar to the RV PEA signal and that it might be useful in patients receiving cardiac resynchronization therapy with defibrillation (CRT-D).²⁹ This offers the potential to measure cardiac hemodynamics in heart failure patients with an ICD.

Figure 5-9 BEST tracings.

Electrocardiogram (ECG), left ventricular (LV) dP/dt, aortic pressure (Ao Press), LV pressure, and endocardial accelerometer tracing (EA) recordings in a sheep with the BEST sensor (see Fig. 5-8). The peak of the accelerometer tracing during the isovolumic diastole (PEA-II) occurs in a 100-msec period after the T wave on the ECG.

(From Plicchi G, Mercelli E, Parlapiano M, et al: PEA-I and PEA-II based implantable haemodynamic monitor: preclinical studies in sheep. Europace 4:49, 2002.)

Current PEA Devices: BEST Sensor Rate–Adaptive Pacemakers (Sorin Best-Living Systems: Miniliving D and S)

These PEA devices use a dedicated unipolar ventricular pace/sense lead, which is IS-1 compatible. Together with the two electrodes for PEA signals, a tripolar connector is necessary. These devices also incorporate an activity sensor detecting the vibration (gravitational sensor). In the PEA mode, continuous PEA signal collected over time is used as a reference, against which the change in PEA is compared. A linear pacing rate response curve is used to relate the change in PEA signal and the rate response.

In the dual-sensor mode, the combined output from both sensors is used to drive the rate to an intermediate level, and further rate increases are guided by the PEA sensor alone. In addition, prolonged absence (about 45 minutes) of signals from both the gravitational and the PEA sensor will allow the basic rate to decrease to the programmable rest rate.

Clinical Results

Clinical studies show good correlation between the sinus rate and PEA sensor–indicated rate during daily life activities and submaximal stress testing.^31,32 Similar results were obtained in patients tested during electrophysiologic studies using an external system; changes in PEA were linearly related to the RV dp/dt during dobutamine infusion. PEA signals have been used to monitor hemodynamic function and program the A-V interval. In 13 patients with end-stage heart failure implanted with a DDD-PEA device with a custom lead arrangement, PEA level during RV, LV, and biventricular (BiV) pacing were compared.³² Both LV and BiV pacing increased stroke volume (21% and 37%, respectively) compared with RV pacing, and mean PEA changes over 15 minutes were also higher (43% and 38%). In addition, an apparent minimum PEA level at the optimal A-V interval has shown promise in automatically detecting the optimal A-V interval in a dual-chamber device.³³ The algorithm is now automatic. An increase in PEA during head-up tilt-table testing has been observed, and the use of PEA-driven overdrive pacing in patients with vasovagal syncope has been reported.³⁴ Patients randomized to DDDR have a reduced frequency of syncope compared with DDI pacing. In 15 patients with a CRT pacemaker, the PEA correctly indentified the optimal AV interval in 75% of patients.³⁵ These data suggest the potential use of PEA sensor for hemodynamic monitoring (see later).

Advantages and Limitations

PEA is a proportional sensor that shows good correlation to cardiac workload, especially at higher ranges. The PEA sensor is limited by the need for a specialized lead, leading to concerns about its long-term stability and its use at pacemaker replacement (when a standard header pacemaker is used). Development of the PEA sensor in an atrial lead allows its use in patients requiring an ICD lead.

Medtronic Pacemakers

Medtronic marketed two dual-sensor pacemakers: the combined activity and QT sensors (Vitatron) and the combined activity and MV sensors (Kappa 400). Both are no longer manufactured. In the activity/QT device, a “faster win” algorithm was used, with highest rate of two sensors used to determine actual pacing rate. Since the QT sensor is more specific for physiologic changes, an increase in activity sensor level in the absence of QT documenting exercise was used as a cross-check, resulting only in a brief and limited rate response. Using an adaptive sensor level to the SURL, the QT sensor level was continuously scaled down if, at the SURL, QT continued to shorten, suggesting the sensor level had reached the upper rate too soon. The converse was also operative. The combined sensor rate was found to be closer to the sinus rate during daily activities.³⁶

In the Kappa 400, a piezoelectric sensor is used for activity sensing, and MV is sensed from a bipolar ventricular lead. Differential sensor blending is used. Up to the ADL rate, activity input predominates, whereas MV-driven pacing will predominate at the SURL. Activity and MV sensors were checked against one another. In the absence of piezoelectric sensor indicating exercise, MV pacing only reached the ADL rate, and vice versa. Only when both MV and activity signified exercise would pacing occur above the ADL rate.

The dual-sensor rate response has been reported to be reliable for both maximal and submaximal activities, as well as resistant to nonphysiologic interference.³⁷ Compared with MV sensor alone, dual-sensor mode reduces oxygen deficit acquired during exercise by enhancing the initial rate response.³⁸ “Quantitative rate adaptation” is also superior in the dual-sensor mode.^39,40 “Rate profile optimization” was found to be a useful method for rate-adaptive programming, comparable to manual programming,⁴¹ and may be superior to accelerometer alone for rate optimization.¹¹

Boston Scientific (Insignia, Pulsar Max, Altrua)

An accelerometer activity sensor is integrated with the MV sensor, using differential sensor blending. At low heart rates, the blended sensor rate is approximately 80% accelerometer and 20% MV sensor. This ratio changes to 40%/60% near the SURL. In addition, if the MV rate is higher than the accelerometer rate, the dual-sensor rate follows the MV level. In a study involving 120 patients with Insignia, the programmed sensor modes were randomized to the accelerometer sensor alone, the MV sensor alone, or the dual-sensor mode, each for a 3-month period.⁴² Using the implanted Activity Log to determine the mean percentage and intensity of activity, quality of life (QOL), and New York Heart Association (NYHA) classes were assessed at the end of each period. Overall, either single-sensor DDDR mode improved Activity Log, QOL, and NYHA scores compared with DDD pacing, but with no difference between the two sensors. The dual-sensor mode did not improve these measurements. This study may have been limited by the prolonged, triple-crossover design but does suggest that clinical differences are likely minimal between sensors and their combinations.

The Limiting Chronotropic Incompetence for Pacemaker Recipients (LIFE) study of 1256 patients with chronotropic incompetence found that the blended sensor improved metabolic-chronotropic slope compared with the activity sensor alone⁴³ (Fig. 5-12). However, besides improved physical activity time/energy expenditure, there was no measurable difference in QOL between the blended sensor and the activity sensor alone. A prospective randomized parallel study on activity and MV with the primary outcome of Vo₂ is planned (APPROPRIATE study).⁴⁴ This study will randomize 1000 patients who will be screened for CI using a 6MWD and treadmill heart rate at 1 month. Patients with confirmed CI will have their sensors optimized with a brief walking test. Apart from Vo₂ max, rate changes during daily activities will be assessed. This study will shed light in the role of different sensors on exercise oxygen uptake. (Unfortunately, the study was prematurely terminated because of suboptimal recruitment.)

Figure 5-12 Sensors in patients with chronotropic incompetence (CI).

Overall sensor response (blended sensor [BS] and activity sensor [XL]) from 1 to 6 months in CI patients with adequate exercise tests. Blue lines indicate paired BS responses from 1 month (dashed line) to 6 months (solid line). Orange lines indicate paired XL responses from 1 month (dashed line) to 6 months (solid line). The black solid line indicates a normal metabolic-chronotropic relationship (MCR) slope; black dashed lines, 96% confidence interval CI (0.8-1.3).

(From Coman J, Freedman R, Koplan BA, et al: A blended sensor restores chronotropic response more favorably than an accelerometer alone in pacemaker patients: the LIFE study results. Pacing Clin Electrophysiol 31:1433-1442, 2008.)

ELA-Sorin (Chorus, Talent, Symphony, Rhapsody, Reply)

An accelerometer and a MV sensor are combined in these devices. Sensor blending and cross-checking are both operative to effect rate adaptation during exercise (Fig. 5-13). When the accelerometer is active but MV has not increased, as may occur at the beginning of exercise, rate response occurs according to a fixed activity response curve to a limited rate. When MV increases, rate response follows the MV sensor–driven rate. Persistent absence of accelerometer signal is considered the cessation of exercise, and this allows a decrease in the pacing rate using a recovery curve to the LRL, even though MV remains higher than baseline.

Figure 5-13 Algorithm of dual-sensor device (ELA-Sorin).

A, Combination of activity (ACT) and minute ventilation (MV) inputs during exercise. B, Autocalibration of the sensor. If the maximum sensor level is reached before the sensor-driven upper rate limit (SURL; indicated by X), then the slope is adjusted in steps to Y to match the SURL to the maximum sensor level. LRL, Lower rate limit.

A multicenter study involved 81 patients with the ELA dual-sensor pacemakers.¹⁹ In patients who underwent exercise stress testing at 1-month follow up, sensor-driven rate had good correlation with the sinus rate (r = 0.92 ± 0.07; P < .001), with the slope of linearity at 1.0 ± 0.2. Using metabolic reserve to relate to heart rate reserve, a slope of 1.1 ± 0.2 was obtained, suggesting a close relation between the dual-sensor rate and metabolic workload.