Successful use of emergency transthoracic pacing in bradyasystolic cardiac arrest

We present the document successful resuscitation of six patients using emergency transthoracic pacing. Two patients were resuscitated from asystole, one had a slow supraventricular bradycardia following head trauma and spinal shock, and three patients had cardiovascular collapse secondary to complete A-V dissociation. One patient developed a non-fatal pericardial tamponade, but there were no cases of pneumothorax. All patients failed to respond to standard medical therapy. We believe that the initially successful resuscitation of these patients was related directly to the pacing procedure. Three patients had underlying pathology that did not allow longterm survival. Three patients were discharged from the hospital without neurologic sequelae. Although emergency transthoracic pacing has a relatively low success rate in bradyasystolic cardiac arrest and may be associated with serious complications, the procedure may be life-saving in selected cases.     

Prehospital trial of emergency transcutaneous cardiac pacing

A prospective alternate-day controlled trial of prehospital transcutaneous cardiac pacing (PACE) of hemodynamically significant bradycardia and asystole was undertaken. All patients had a Glasgow coma scale score of 12 or less. Patients in the control group (n = 101) received standard advanced cardiac life support (ACLS) care. Patients in the pacing group (n = 101) were to receive PACE in addition to standard ACLS treatment; 89 patients were actually paced. The two groups were comparable in terms of age, sex, presenting rhythm, and mean times to cardiopulmonary resuscitation (CPR) and ACLS. For the 144 patients in whom the time of arrest could be estimated, the mean times to CPR and ACLS were 5.3 +/- 4.0 and 10.9 +/- 7.1 min, respectively. For the 65 paced patients in whom the time of arrest could be estimated, the mean time from arrest to pacing was 21.8 +/- 8.8 min (range 2 to 43). Multivariate analysis of outcome variables (presentation to emergency department with a pulse, admission to the hospital, and discharge from the hospital) revealed that an initial rhythm of ventricular tachycardia or fibrillation and a short time to ACLS were correlated with a favorable outcome (p less than .05; logistic regression analysis). A short time to PACE was associated with admission to the hospital (p = .20; logistic regression analysis). The use of a stand-alone transcutaneous pacing device in the prehospital arrest setting was associated with generally long times until pacing and did not appreciably improve outcome. Use of PACE in patients demonstrating prehospital bradycardia without neurologic impairment remains to be evaluated.     

Immediate emergency department external cardiac pacing for prehospital bradyasystolic arrest

Approximately 25% of patients in prehospital cardiac arrest present in bradyasystolic rhythms, and their long-term prognosis is very poor. Our study was undertaken to determine the utility of immediate emergency department (ED) external cardiac pacing in this situation. Twenty patients presenting with bradyasystolic prehospital cardiac arrest were entered in the study. All received the usual advanced cardiac life support therapy, but also were externally paced immediately using an automated external defibrillator and pacemaker (AEDP). Only two of 20 patients showed evidence of electrical capture, and none developed pulses with pacing. Four of the 20 patients developed a sinus rhythm and blood pressure during resuscitation. Three survived to leave the ED, but none survived to leave the hospital. An increase in the rate of bradycardia and pulseless idioventricular rhythms that was independent of electrical capture or pharmacologic therapy was noted occasionally. Although survival was not enhanced using the AEDP, the device was reliable, easy to use, and free of complications. External cardiac pacing warrants further investigation in the prehospital setting.     

Prehospital external cardiac pacing: a prospective, controlled clinical trial

This prehospital prospective, controlled study was conducted to determine if prehospital cardiac pacing affects survival. The study involved 239 patients, 226 pulseless, nonbreathing patients (rhythms of asystole and electromechanical dissociation with heart rates less than 70) and 13 patients with hemodynamically significant bradycardia (heart rate less than 60; blood pressure less than 90 mm Hg; not responding to atropine). Patients were assigned to treatment or control groups on an every-other-day basis. One hundred three patients were treated with an external cardiac pacing device; 22 (21.4%) were resuscitated (arrival at admitting hospital with pulse and blood pressure) and seven (6.8%) were saved (survival to hospital discharge). One hundred thirty-six patients were not paced and served as controls; 28 (20.6%) were resuscitated (P = .90) and six (4.4%) were saved (P = .71). Analysis of pacing times showed increased resuscitation in patients paced early. All surviving paced patients were paced in 17 minutes or less. Analysis of rhythm subgroups showed no significant difference in the resuscitation or survival rates of paced and control groups for primary asystole, primary electromechanical dissociation, and secondary asystole and electromechanical dissociation occurring after countershock treatment of ventricular fibrillation when compared respectively. However, among patients with hypotensive bradycardia, six of six paced patients were resuscitated and five were saved, while only two of seven controls were resuscitated (P = .01) and one was saved (P = .01). Interpretation of the bradycardic patient data is limited by inequalities noted between control and treatment groups with regard to the administration of isoproterenol.(ABSTRACT TRUNCATED AT 250 WORDS)     

Mechanical emergency stimulation in asystole and extreme bradycardia

Repetitive precordial thumping is the simplest method of temporary cardiac pacing. In 90 patients out of 100 with witnessed cardiac arrest because of asystole or marked bradycardia the critical situation could be effectively bridged over by this until a sufficient spontaneous rhythm returned or electrostimulation was ready to function. 69 patients were conscious during the stimulation. During the mechanical pacing only in 2 patients there occurred ventricular flutter or fibrillation, which was stopped by electric defibrillation. The existence of the myocardial contractility is the presupposition for effective mechanical and electrical pacing.     

Outcome of resuscitation from bradyarrhythmic or asystolic prehospital cardiac arrest

Previous studies of outcome as a function of the initial electrophysiologic mechanisms recorded at the scene of prehospital cardiac arrest have demonstrated that bradyarrhythmias and asystole have the worst prognosis. In this report, our observations in bradyarrhythmic and asystolic arrests occurring from 1980 to 1982 are compared with those from 1975 to 1978. From 1980 to 1982, 61 (27%) of 225 cardiac arrest events meeting entry criteria for the study were bradyarrhythmic or asystolic. Only 2 (8%) of 24 patients with asystole and 1 (20%) of 5 patients with sinus bradycardia survived prehospital intervention. Only 1 of these 29 patients was discharged from the hospital alive. In contrast, 15 (47%) of 32 patients who presented with idioventricular rhythm at initial contact survived prehospital intervention and were hospitalized, and 8 (25%) of these 32 were ultimately discharged alive. When compared with the 1975 to 1978 patients with bradyarrhythmia and asystole, both prehospital survival (8 versus 30%, p less than 0.001) and survival after hospitalization (0 versus 15%, p less than 0.05) significantly improved, but the improvement occurred predominantly in the subgroup with idioventricular rhythm. Survivors within this subgroup tended to have a prompt response to prehospital pharmacologic interventions that were not available to the 1975 to 1978 group. The response was manifested by return to a sinus mechanism or increase in the rate of idioventricular rhythm. In conclusion, outcome has improved for a specific subgroup of victims of prehospital cardiac arrest with bradyarrhythmia or asystole; the improved outcome may relate to field interventions by rescue personnel at the scene of arrest but the mortality rate is still high.     

Prehospital brady-asystolic cardiac arrest.

Of 133 persons with spontaneous cardiac arrest attended by paramedics within 10 minutes, 100 (75%) had ventricular fibrillation as the initial rhythm and 33 (25%) had extreme bradycardia or asystole. The latter group of arrhythmias was characterized by sinus arrest or severe sinus bradycardia (90%) and complete A-V block (10%). Junctional escape rhythm was also absent or markedly retarded. Despite cardiopulmonary resuscitation and the administration of epinephrine, atropine, isoproterenol, and sodium bicarbonate, recovery of the sinus and junctional tissues was infrequent. Ventricular fibrillation developed in 11 cases (33%). One patient lived 12 days, but all others were dead on arrival or died in the emergency room. Among the 13 coronary causes of death proved at autopsy, 10 (77%) were due to a fresh thrombus and seven (54%) to an occluded proximal right coronary artery, suggesting a causal relation to this type of arrest.     

Recurrent cardiac arrest due to electro-mechanical dissociation in a patient with variant angina -- a case report

We present a case of a 44-year-old male with recurrent episodes of cardiac arrest in the course of Prinzmetal's angina. Episodes of variant angina can be life threatening due to episodes of advanced atrioventricular block, asystole, ventricular tachycardia or ventricular fibrillation. It has been suggested to implant an ICD in all patients with variant angina after cardiac arrest. This patient received an ICD, however, he died suddenly 6 months later. The possible mechanism of cardiac arrest was an electromechanical dissociation.     

External cardiac programmed stimulation for noninvasive termination of sustained supraventricular and ventricular tachycardia

To examine the feasibility of using a noninvasive temporary pacemaker for termination of well-tolerated supraventricular (SVT) and ventricular tachycardia (VT), a standard external demand pacemaker was modified to allow stimulation with single or multiple extrastimuli and overdrive pacing. To evaluate the efficacy, safety and tolerance of external cardiac programmed stimulation, a standard arrhythmia termination protocol was used in 223 tachycardias in 22 patients. The technique of external cardiac programmed stimulation was used in 209 episodes of SVT in 13 patients. It terminated 95% of the episodes with success in 19 of 20 episodes of atrioventricular nodal reentrant tachycardia and 179 of 189 episodes of atrioventricular reciprocating tachycardia. Of 198 episodes of SVT terminated by the technique 168 (85%) were terminated by a single extrastimulus and 28 (14%) by double extrastimuli. Only 2 episodes of SVT required overdrive pacing for termination. External cardiac programmed stimulation did not result in atrial fibrillation or arrhythmia acceleration. Of 14 episodes of sustained monomorphic VT 5 were terminated by external cardiac programmed stimulation. One tachycardia was terminated by a single extrastimulus, 1 by double extrastimuli and 3 by overdrive pacing. Arrhythmia acceleration occurred once and was terminated by endocardial pacing. On 27 separate occasions patient evaluation of maximal discomfort included 4 ratings of mild, 10 of moderate, 11 of severe and 2 of intolerable discomfort. External cardiac programmed stimulation is effective and safe in patients with well-tolerated sustained supraventricular or ventricular arrhythmias.     

Implications of mechanism of bradycardia on response to pacing in patients with unexplained syncope.

AIM: Asystole >3 s or sinus bradycardia with a ventricular rate <40 in association with complete heart block or sinus node dysfunction are considered to be Class 1 indications for permanent cardiac pacing. Nevertheless, these phenomena may be observed in symptomatic patients with neurocardiogenic syncope, who may not respond to pacing therapy. We hypothesized that the pattern of spontaneous bradycardia in symptomatic patients would distinguish patients with sinus node dysfunction or conduction system disease who would benefit from pacing from patients with neurally-mediated syncope who would derive lesser benefit. METHODS AND RESULTS: Patients with symptomatic spontaneous bradycardia during long-term monitoring for unexplained syncope who underwent pacemaker implantation were classified according to the ISSUE classification system and followed for recurrent syncope. Follow-up included review of medical records, pacemaker clinic visits, and telephone interviews. Loop recorder tracings were reviewed to identify characteristics potentially predicting a favourable response to pacing. Thirty-three patients (21 male; age, 70 +/- 14) were followed for 3.56 +/- 1.71 years. Six patients had a recurrence of syncope during the follow-up. All patients with recurrent syncope despite pacing demonstrated a Type 1A (n = 5) or 1B (n = 1) pattern with gradual onset of bradycardia at baseline, suggesting a neurocardiogenic mechanism. There was no difference in the severity of bradycardia or duration of asystole in baseline loop recorded events in responding and non-responding patients. Multivariate analysis using stepwise logistic regression revealed that the ISSUE classification and the absence of structural heart disease were the only independent predictors of treatment failure of cardiac pacing in patients with spontaneous symptomatic bradycardia. CONCLUSION: Patients with syncope associated with abrupt bradycardia displayed a better response to cardiac pacing therapy than those with gradual onset bradycardia.     

Transcutaneous and transvenous cardiac pacing for early bradyasystolic cardiac arrest

Emergency transcutaneous cardiac pacing was studied prospectively in 19 patients presenting to the emergency department with a bradyasystolic cardiopulmonary arrest of 20 minutes duration or less. Pacing was initiated when conventional advanced cardiac life support (including atropine administration) and a fluid challenge failed to restore a pulse. Seventeen patients also had placement of transvenous pacemaker electrodes for cardiac pacing. Transcutaneous cardiac pacing rapidly established a blood pressure in the two patients who for clinical reasons did not receive a transvenous pacemaker. Five patients were transcutaneously paced within five minutes of cardiac arrest (Group 1) and the remaining 14 were paced between five and 20 minutes following cardiac arrest (Group 2). Two of the Group 1 patients were admitted and subsequently recovered full neurological and prearrest cardiac function. Fewer Group 2 patients developed a blood pressure (P = .04), and there were no patients with full neurologic recovery in this group (P = .06). Similar results were found for transvenous cardiac pacing; there was a greater incidence of a palpable pulse and measurable blood pressure (P = .05 for both) in the Group 1 patients than in the Group 2 patients. No difference in clinical outcome was noted between the two pacing techniques. These results support the concept that cardiac pacing must be initiated early if the outcome of bradyasystolic cardiac arrest is to be altered.     

Noninvasive transcutaneous cardiac pacing in prehospital cardiac arrest

This study evaluated the efficacy of prehospital external cardiac pacing in cardiac arrest patients. From October 1984 to June 1985, 91 patients were paced. Mean time from cardiac arrest to advanced life support (ALS) intervention in this metropolitan-rural ALS system was 14.5 minutes. Electrical capture occurred in 85 (93%), mechanical capture (pulses) occurred in ten (11%), and a measurable blood pressure occurred in three (3%) of the 91 patients. Despite a high rate of electrical capture, palpable pulses were produced only in 11%, and no patients survived to be discharged from the hospital. There was no difference in the frequency of electrical capture, palpable pulses, or outcome for patients receiving pharmacologic intervention before or after pacing. Likewise there was no difference in the frequency of electrical capture, palpable pulses, or outcome for patients receiving ALS therapy within or after ten minutes of their arrest. Although we found that external cardiac pacing was easily used in the prehospital setting, pacing did not result in any increase in survival in cardiac arrest patients.     

Transcutaneous cardiac pacing in the treatment of out-of-hospital pediatric cardiac arrests.

STUDY OBJECTIVE: To evaluate the effectiveness of transcutaneous cardiac pacing in out-of-hospital treatment of cardiac arrests in pediatric patients. DESIGN: We describe the outcome of patients treated during a prospective trial of transcutaneous cardiac pacing in the field. We compare their outcome with that of out-of-hospital arrests in submersion patients who were not paced. We identified patients from Seattle and King County Emergency Medical Services reports, hospitals, and medical examiner's registries. MEASUREMENTS AND MAIN RESULTS: Nine patients in cardiac arrest caused by drowning (six) and sudden infant death syndrome (three) were paced in the field. All were less than 6 years old. The one survivor was severely neurologically impaired and died six months later. Transcutaneous cardiac pacing produced electrical capture in two patients but no detectable pulse or blood pressure. Ten submersion patients less than 6 years old in cardiac arrest were not paced. One survived, with mild neurologic impairment at hospital discharge. CONCLUSION: Transcutaneous cardiac pacing was not effective and was not associated with improved survival.     

急诊介入治疗合并院前心脏骤停急性心肌梗死疗效观察

目的 评价急诊经皮冠状动脉介入治疗(PCI)合并院前心脏骤停急性ST段抬高型心肌梗死(STEMI)的临床疗效.方法 入选2004年9月至2008年11月接受急诊PCI的STEMI患者1446例,其中合并院前心脏骤停患者(心脏骤停组)49例,无院前心脏骤停患者(无心脏骤停组)1397例.分析患者住院期间和出院后1年的临床情况,包括总病死率、心脏不良事件、卒中及出血事件等.结果 与无心脏骤停组比较,心脏骤停组急诊PCI成功率差异无统计学意义(88.8%比85.7%,P=0.497),住院期间心原性休克(3.0%比22.4%,P＜0.001)和心脏骤停(5.9%比44.9%,P＜0.001)的发生率较高,住院期间总病死率较高(2.0%比36.7%,P＜0.001).发病至院外抢救时间、心脏骤停时心律为心室停顿、入院时Glasgow昏迷评分≤7分和人院时心原性休克是心脏骤停组患者住院期间死亡的独立危险因素.随访1年显示,无心脏骤停组与心脏骤停组总病死率(6.5%比6.9%)、再次心肌梗死(1.4%比3.4%)、再次血运重建(3.4%比6.9%)和卒中发生率(6.4%比6.9%)差异均无统计学意义.结论 与无院前心脏骤停STEMI患者比较,合并院前心脏骤停STEMI患者住院期间病死率较高,但是急诊PCI后1年的疗效相似.     

Chronic bundle branch block and use of temporary transvenous pacemakers during coronary arteriography

A retrospective analysis of 217 consecutive patients with chronic bundle branch blocks undergoing cardiac catheterization was done to evaluate the need for temporary transvenous pacing during coronary arteriography. In patients without temporary right ventricular pacemakers (n = 185), only one episode of high-grade atrioventricular block occurred during coronary arteriography which required the urgent use of temporary pacing. All other bradyarrhythmias, including five episodes of transient asystole (greater than 3-sec pause) and four episodes of atrioventricular block (second degree or higher) were successfully managed without pacemaker utilization. Patients with prophylactic right ventricular pacemakers (n = 32) had a greater prevalence of ventricular fibrillation than those without pacing electrodes located in the right ventricle (2% vs. 9% respectively; P less than 0.05). These findings suggest that routing prophylactic pacemaker insertion during coronary arteriography in patients with chronic bundle branch block is not warranted and may place the patient at risk for developing iatrogenic ventricular arrhythmias.     

Management and therapy of vasovagal syncope: A review

Vasovagal syncope is a common cause of recurrent syncope. Clinically, these episodes may present as an isolated event with an identifiable trigger, or manifest as a cluster of recurrent episodes warranting intensive evaluation. The mechanism of vasovagal syncope is incompletely understood. Diagnostic tools such as implantable loop recorders may facilitate the identification of patients with arrhythmia mimicking benign vasovagal syncope. This review focuses on the management of vasovagal syncope and discusses the non-pharmacological and pharmacological treatment options, especially the use of midodrine and selective serotonin reuptake inhibitors. The role of cardiac pacing may be meaningful for a subgroup of patients who manifest severe bradycardia or asystole but this still remains controversial.     

Out-of-hospital cardiac arrests in Helsinki: Utstein style reporting.

OBJECTIVE: To determine the epidemiology of out-of-hospital cardiac arrests and survival after resuscitation and to apply the Utstein style of reporting to data collection. DESIGN: Prospective cohort study. SETTING: A middle-sized urban city (population 516,000) served by a single emergency medical services system. PATIENTS: Consecutive prehospital cardiac arrests occurring between 1 January and 31 December 1994. INTERVENTION: Advanced cardiac life support according to the recommendations of American Heart Association. MAIN OUTCOME MEASURES: Survival from cardiac arrest to hospital discharge, and factors associated with survival. RESULTS: Four hundred and twelve patients were considered for resuscitation. The overall incidence of out-of-hospital cardiac arrest was 79.8/100,000 inhabitants/year. Fifty seven patients (16.6%) survived to discharge when resuscitation was attempted. 32.5% survived when cardiac arrest was bystander witnessed and was of cardiac origin with ventricular fibrillation as the initial rhythm. When asystole or pulseless electrical activity was the first rhythm recorded, discharge rates were 6.2 and 2.7% respectively. The cause of cardiac arrest was cardiac in 66.5%, and ventricular fibrillation was the initial rhythm in 65.0% of bystander witnessed cardiac arrests of cardiac origin. 22.1% of patients received bystander initiated cardiopulmonary resuscitation. The mean time intervals from the receipt of the call to the arrival of a first response advanced life support unit and mobile intensive care unit at the patient's side and to the return of spontaneous circulation were 7.0 and 10.3 and 12.6 and 16.7 min respectively. In the logistic regression model bystander witnessed arrest, age, ventricular fibrillation as initial rhythm, and the call-to-arrival interval of the first response unit were independent factors relating to survival. Utstein style reporting with modification of time zero was found to be an appropriate form of data collection in this emergency medical services system. CONCLUSIONS: After implementation of major changes in the emergency medical services system during the 1980s survival from out-of-hospital cardiac arrest markedly increased. However, early access, which has turned out to be the weakest link in the chain of survival, should receive major attention in the near future. Utstein style reporting with a modified time zero was found to be appropriate, although laborious, protocol for data collection.     

Coronary pacing during percutaneous transluminal coronary angioplasty

To avoid venous puncture, a new concept for standby cardiac pacing during percutaneous transluminal coronary angioplasty (PTCA) and diagnostic cardiac catheterization was developed. It uses an arterial guidewire as a unipolar pacing electrode with the second electrode attached to the skin. The system was tested in 25 coronary arteries of 22 patients undergoing PTCA and in the left ventricles of 10 patients undergoing diagnostic cardiac catheterization. Coronary pacing via the guidewire used for directing the balloon catheter was possible in all patients and in 24 of the 25 coronary arteries attempted. Maximum duration of pacing was 8 min. Threshold currents ranged from 1 to 15 mA (mean 5.7). Left ventricular pacing via the same wires or standard wires used for introduction of diagnostic or guiding catheters was possible in all patients and was maintained for up to 10 min. Threshold currents ranged from 1 to 7 mA (mean 3.9). Neither method for pacing produced adverse effects during these short applications. The setup for coronary pacing also allowed recording of an intracoronary electrocardiogram during PTCA. The presented system provides backup for the rare event of sustained bradycardia during PTCA or diagnostic cardiac catheterization. If applied cautiously, it may safely and reliably replace the standby of a conventional transvenous pacing catheter.     

Transcutaneous pacemaker in cardiovascular emergencies]

OBJECTIVES AND DESIGN OF THE STUDY: Retrospective study to evaluate the efficacy and tolerance of the transcutaneous cardiac pacemaker in the urgent treatment of asystole or severe bradycardia. SETTING: Coronary Care Unit (CCU) and emergency area of the central reference Hospital. PATIENTS: 24 patients, 20 males and four females, aged between 57 and 84 years (mean 70.4 +/- 7.9). Five pts were in asystole and 19 in severe bradycardia. INTERVENTIONS: The transcutaneous pacemaker used, was the "Cardio Aid Zoll NTP" model. The intensity of the electrical stimulation was increased progressively, until electrical capture or intolerable discomfort by the patients was achieved. We defined by electric efficacy, the visualization of pacing spike followed by a deflection due to ventricular depolarization; and by hemodynamic efficacy, the evidence of myocardial contraction, defined as a palpable pulse, synchronous with the pacing artefact. MAIN RESULTS: Stimulation threshold ranged from 30 to 140 mA (mean 67.7 +/- 23.4). The duration of pacing was from 15 minutes to 13 hours, being more than one hour in only four situations. From the 20 conscious patients, or the ones who got conscious, 15 (75%) tolerated well the stimulation. It was intolerable in five pts (25%). No significative side effects due to the use of transcutaneous pacemaker were observed. CONCLUSIONS: The transcutaneous pacemaker was efficient in the electric and hemodynamic stabilization in the majority of patients. It was generally well tolerated and without important side effects. We think that it may be a valid alternative to transvenous pacing technics in the treatment of asystole and severe bradycardia situations.     

Pacemaker therapy in sleep apnea syndrome

Summary Sleep apnea syndrome (SAS) is a serious health problem that particularly afflicts patients with cardiovascular disease. Pathophysiologically, an obstructive form of SAS with loss of air flow despite thorax movements and a central form of SAS with simultaneous cessation of air flow and respiratory movements are distinguished. Central SAS is present in almost 50% of patients with severe heart failure (HF). It induces alternating bradycardia and tachycardia related to fluctuations of the vago-sympathetic tone. Suppression or reduction of heart rate fluctuations by pacing may mitigate sleep apnea. In HF with marked nocturnal bradycardia, increasing the heart rate may improve cardiac output, shorten circulation time and decrease pulmonary congestion, thus diminishing the apneic threshold. Potentially, stimulation of vagal and sympathetic nerve fibers behind the superior vena cava/right atrial junction by pacing may directly influence cardiac vagal or sympathetic afferent neurons reducing central sleep apnea episodes. In obstructive SAS, polysomnography characterizes a specific pattern of sleep apnea where bradycardia precedes the onset of apnea. This could result from hypervagotonia first inducing bradycardia and then apnea. Atrial overdrive pacing may counteract hypervagotonia by maintaining sympathetic activity. Pacing might not benefit obstructive SAS due to excessive adiposity, anatomical obstacles or unrelated to bradycardia. In the future, cardiac pacing might be considered as an ancillary therapy for SAS in patients with bradycardia and/or hypervagotonia during sleep. Larger studies need to confirm this hypothesis in patients without a conventional indication for pacing. Device recording of sleep apnea could become a powerful tool to guide SAS therapy.