Marcus Mianulli

Comparison of exertion required to perform standard and active compression-decompression cardiopulmonary resuscitation

OBJECTIVE Active compression-decompression (ACD) cardiopulmonary resuscitation (CPR) utilizes a hand-held suction device with a pressure gauge that enables the operator to compress as well as actively decompress the chest. This new CPR method improves hemodynamic and ventilatory parameters when compared with standard CPR. ACD-CPR is easy to perform but may be more labor intensive. The purpose of this study was to quantify and compare the work required to perform ACD and standard CPR. METHODS Cardiopulmonary testing was performed on six basic cardiac life support- and ACD-trained St. Paul, MN fire-fighter personnel during performance of 10 min each of ACD and standard CPR on a mannequin equipped with a compression gauge. The order of CPR techniques was determined randomly with > 1 h between each study. Each CPR method was performed at 80 compressions/min (timed with a metronome), to a depth of 1.5-2 inches, and with a 50% duty cycle. RESULTS Baseline cardiopulmonary measurements were similar at rest prior to performance of both CPR methods. During standard and ACD-CPR, respectively, rate-pressure product was 18.2 +/- 3.0 vs. 23.8 +/- 1.7 (x 1000, P < 0.01); mean oxygen consumption 15.98 +/- 2.29 vs. 20.07 +/- 2.10 ml/kg/min or 4.6 +/- 0.7 vs. 5.7 +/- 0.6 METS (P < 0.01); carbon dioxide production 1115.7 +/- 110 vs. 1459.1 +/- 176 ml/min; respiratory exchange ratio 0.88 +/- 0.04 vs. 0.92 +/- 0.04 (P = NS); and minute ventilation 35.5 +/- 5.1 vs. 45.6 +/- 9.2 l/min (P < 0.01). CONCLUSIONS Approximately 25% more work is required to perform ACD-CPR compared with standard CPR. Both methods require subanaerobic energy expenditure and can therefore be sustained for a sufficient length of time by most individuals to optimize resuscitation efforts. Due to the slightly higher work requirement, ACD-CPR may be more difficult to perform compared with standard CPR for long periods of time, particularly by individuals unaccustomed to the workload requirement of CPR, in general.

Multiple-Sensor Systems for Physiologic Cardiac Pacing

The first implantable cardiac pacemakers were designed to pace at a fixed rate, with no attempt to reproduce normal heart rate changes. However, technologically advanced pacemakers were introduced that adjusted the pacing rate by sensing the patients native atrial activity and adjusting the ventricular pacing rate accordingly. By the mid-1980s, artificial sensors were introduced into pacing systems and provided another means for pacemakers to adjust their rates appropriately. Incorporation of artificial sensors in implantable pacing systems revolutionized the technology of cardiac pacing and its clinical practice [1-6]. Initially, artificial sensors were viewed exclusively as a means to modify the pacing rate in response to changing levels of physical exertion. In that rate-adaptive role, sensor-based pacemakers have been well received by physicians and patients Figure 1, Table 1. More recently, the role of artificial sensors expanded to include automatic adjustment of certain programmable pacemaker settings, such as the interval between atrial and ventricular pacing stimuli (that is, the atrioventricular interval). In addition, sensors are also being used to inform the pacing system of the development of certain arrhythmias, particularly atrial flutter or fibrillation. This latter capability permits those pacemakers that use the patients atrium to set the pacing rate (atrial tracking) to change their mode of operation to one in which the pacing rate is determined by the sensor. This mode-switching feature prevents the undesirable high-rate pacing that can occur if the pacemaker continues to track the atrial rate during a pathologic atrial tachycardia. Soon, sensors will allow pacemakers to adjust many other operating characteristics automatically (such as the level of energy output, electrical polarity of the intracardiac electrodes, pacing mode, and so on) [7], thereby optimizing efficiency of operation while minimizing the need for specialized medical follow-up. Table 1. International Generic Pacemaker Code* Figure 1. Changes in pacing mode use in North America in the past decade (see ). In the past decade, physicians have gained considerable clinical experience with rate-adaptive pacing systems that incorporate a single artificial sensor [8-16]. Although this experience has been largely favorable, no single artificial sensor is suitable for all potential pacing system applications [6, 17]. Consequently, pacemakers using combinations of artificial sensors that can operate in a complementary manner are being developed and clinically evaluated (Table 2). This review surveys the role of artificial sensors in cardiac pacing systems and examines the rationale for and the clinical status of pacemakers that use multiple artificial sensors. Table 2. Current Multiple Artificial Sensor Pacing Systems* Role of Sensors in Cardiac Pacing Systems Rationale for Using Sensors To Adjust Pacing Rate Although prevention of symptoms such as syncope or dizziness resulting from severe bradycardia is the primary goal of pacemaker therapy, careful selection of the pacing system and its mode operation also provide the opportunity to improve exercise tolerance [8-13, 18], to diminish the risk for development of atrial fibrillation [19-24], and to decrease overall mortality rates [18, 21-26]. In terms of exercise tolerance, appropriate heart rate responsiveness contributes the most to increased cardiac output during exertion. This applies to healthy persons and to most patients with pacemakers, whether they are vigorously active or must simply manage the physical demands associated with activities of daily living (such as climbing stairs, walking, carrying groceries, and doing the laundry) [1, 4, 8-18]. For example, studies by Karlof [27], Fananapazir and coworkers [28], Ausubel and colleagues [29], and Ryden and associates [30] clearly show that exercise capacity, stroke volume changes with exercise, and maximum oxygen consumption (a measure of the bodys ability to provide essential fuel for vigorous exertion) depend primarily on heart rate change in most patients. In addition, Ryden and associates [30] also showed that over a wide range of atrioventricular intervals, there was no difference between maximum oxygen uptake in patients with pacemakers during rate-adaptive, single- and dual-chamber pacing. Furthermore, in terms of the hearts own energy requirements during exercise, Nordlander and colleagues [31] found that increasing heart rate, such as occurs with rate-adaptive pacing, does not necessarily adversely affect myocardial oxygen consumption compared with fixed-rate pacing. In fact, provision of an appropriate heart rate change during exercise may prevent ventricular dilation and reduce the need for premature encroachment on cardiac compensatory responses. In principle, providing heart rate responsiveness in many patients with pacemakers can be accomplished using the patients own native atrial or sinus node rate to determine the appropriate moment-to-moment ventricular paced rate (that is, atrial tracking, such as in the VD or DDD pacing modes shown in Table 1. Indeed, if a healthy sinus node is present, such an approach not only provides the most physiologic heart rate response but also maintains a relatively normal relation between atrial and ventricular contractions. However, symptomatic disturbances of sinoatrial function [that is, the sick sinus syndrome] are common in patients with pacemakers [1, 19-24]. In fact, they are the primary indication for a pacemaker in more than 50% of patients who have them in Western countries [18]. Consequently, tracking the native atrial rate often does not provide the optimal physiologic heart rate response. Consequently, other techniques, such as the incorporation of artificial sensors into implantable pacing systems, are essential in many patients with pacemakers to ensure reliable heart rate responsiveness [1-6, 18]. Current Sensor Systems The sensing of atrial electrical potentials by an intra-atrial electrode with subsequent adjustment of ventricular pacing rate (that is, atrial tracking) was the first attempt to develop a sensor-triggered rate-adaptive pacemaker [32, 33]. In that case, the sensor is the patients own atrial rate. However, for the reasons previously noted, tracking to adjust the pacing rate may not be suitable in many patients. The earliest efforts to develop artificial sensors to adjust the rate of implantable cardiac pacemakers were reported in the mid-1960s [34]. However, not until the mid-1970s did Cammilli and coworkers [35], using central venous pH sensing, provide the first implantable device. Theoretically, reduced central venous pH associated with exercise would signal the pacemaker to increase its rate. The resulting additional cardiac output would then not only tend to normalize the pH but ultimately would also cause the pacemaker to slow down again (a closed-loop sensor system). Unfortunately, the pH sensor was not particularly stable in the long term, and the relation between pH and heart rate was complex. For the most part, researchers have abandoned the pH sensor concept. The first commercially successful sensor-based pacing system used a specialized ceramic element with piezoelectric characteristics (that is, they produce an electrical potential when physically deformed, such as by vibration) bonded to the inside surface of the pacemaker shield (Activitrax; Medtronic Inc., Minneapolis, Minnesota) [3, 4, 10-12, 36-39]. With this device and its more recent derivatives, the metal of the pacemaker shield may be thought of as a drum head. The ceramic activity sensor emits electrical signals (the piezoelectric effect) approximately in proportion to the vibrations transmitted through the pacemaker from the patients working muscle and skeleton. These piezoelectric signals are an indirect but useful reflection of the patients physical activity (thus, the widely used term activity sensor) and can be used to change the pacing rate appropriately [11, 12, 36-41]. The rapid clinical acceptance of the first activity-based, rate-adaptive pacing system encouraged development of similar pacemakers. Some manufacturers adopted the activity detection concept, either by using the drum head design described previously (such as Synchrony devices, Siemens-Pacesetter Systems, Sylmar, California) or an accelerometer modification (such as Relay; Intermedics Inc., Freeport, Texas). Other manufacturers devised unique rate-adaptive sensor solutions [42-52]. For example, measurement of the interval from the pacing stimulus to the peak of the T wave (the so-called stim-T interval, an estimate of the QT interval for paced heart beats), has proved effective (for example, Rhythmyx; Vitatron Medical b.v., Dieren, the Netherlands) [43]. This concept is based on the fact that the stim-T interval (like the QT interval on the standard electrocardiogram) is modified by changes in circulating or locally released catecholamines. Thus, for a given heart rate, a change in myocardial catecholamine exposure (for example, in association with physical exertion) will modify the stim-T interval and provide a marker for altered levels of physical activity. Similarly, estimation of exercise-induced change in minute ventilation by transthoracic plethysmography (such as Meta; Telectronics Inc., Sydney, Australia), or in central venous temperature using a thermistor on the pacing lead (such as Kelvin; Cook Pacemakers Inc., Leechburg, Pennsylvania) have yielded useful rate-adaptive pacing techniques [14, 44-47]. Additional promising concepts for rate-adaptive sensors include monitoring exercise-induced changes in central venous oxygen saturation using an intravascular oxygen sensor, and measuring exercise-related changes in right ventricular pressure characteristics (using a small pressure sensor built into the ventricular pacing lead) [50-52]. Conversely, some other interesting sensor proposals, s

Clinical Evaluation of Chronotropic Incompetence: Implications for Rate-Adaptive Pacing

The introduction of sensor-based rate-adaptive pacemakers in the 1980s revolutionised the practice of cardiac pacing 1. In particular, the availability of a means to restore heart rate responsiveness focused attention on the physiological importance of providing symptomatic patients with as normal a chronotropic response as possible. Thus, the recognition of chronotropic incompetence became a true clinical concern, and ultimately led to a new indication for implantation of cardiac pacemakers.