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Original Article |

β-Blockade in Noncardiac Surgery:  Outcome at All Levels of Cardiac Risk FREE

Haytham M. A. Kaafarani, MD; Prasad V. Atluri, MD; John Thornby, PhD; Kamal M. F. Itani, MD
[+] Author Affiliations

Author Affiliations: Department of Surgery, University of South Florida, Tampa (Dr Kaafarani); Department of Surgery, Veterans Affairs Boston Health Care System, West Roxbury, and Boston University and Harvard Medical School, Boston, Massachusetts (Drs Kaafarani and Itani); and Department of Anesthesia, Baylor College of Medicine (Dr Atluri), and Biostatistical Laboratory, Michael E. DeBakey Veterans Affairs Medical Center (Dr Thornby), Houston, Texas.


Arch Surg. 2008;143(10):940-944. doi:10.1001/archsurg.143.10.940.
Text Size: A A A
Published online

Hypothesis  We hypothesized that the relationship among β-blocker use, heart rate control, and perioperative cardiovascular outcome would be similar in patients at all levels of cardiac risk.

Design  Retrospective cohort study.

Setting  Michael E. DeBakey Veterans Affairs Medical Center, Houston, Texas.

Patients  Among all patients who underwent various noncardiac surgical procedures in 2000, those who received perioperative β-blockers were matched and compared with a control group from the same patient population.

Main Outcome Measures  Thirty-day stroke, cardiac arrest, myocardial infarction, and mortality, as well as mortality at 1 year.

Results  Patients at all levels of cardiac risk who received β-blockers had lower preoperative and intraoperative heart rates. The β-blocker group had higher rates of 30-day myocardial infarction (2.94% vs 0.74%, P =.03) and 30-day mortality (2.52% vs 0.25%, P =.007) compared with the control group. In the β-blocker group, patients who died perioperatively had significantly higher preoperative heart rate (86 vs 70 beats/min, P =.03). None of the deaths occurred among the patients at high cardiac risk.

Conclusion  Among patients at all levels of cardiac risk undergoing noncardiac surgery, administration of β-blockers should achieve adequate heart rate control and should be carefully monitored in patients who are not at high cardiac risk.

Patients with coronary artery disease or with risk factors for coronary artery disease who undergo noncardiac surgery are at significant risk for perioperative cardiac events such as death, stroke, and myocardial infarction (MI). Prevention of these perioperative cardiac complications continues to be the goal of intense research and investigations.

Multiple mechanisms for postoperative MI have been suggested, many of which involve a hyperadrenergic state with sustained sinus tachycardia and increased myocardial oxygen demand. Landesberg et al1 noted that perioperative ischemic events in patients undergoing vascular surgery were consistently preceded by a significant rise in heart rate, usually during the immediate postoperative period. In addition, plasma catecholamine concentrations rise during the first 48 hours after surgery and are believed to be associated with increased perioperative thrombotic events such as MI.2 The combination of increased catecholamines and tachycardia subjects coronary plaque to higher intravascular shear stress, eventually leading to its rupture and the initiation of a thrombotic event.3 The potential role of tachycardia and catecholamines in the pathogenesis of perioperative myocardial events, in addition to clinical reports4 of improved outcome in patients taking β-blockers for hypertension, triggered a significant interest in perioperative β-blockade.

Results of 2 landmark clinical studies by Pasternack et al5 and by Stone et al4 in the 1980s suggested a beneficial cardioprotective role for β-blockade in noncardiac surgery. These findings were later confirmed by Mangano et al6 in a randomized, double-blinded, placebo-controlled trial that showed a 67% reduction in postdischarge cardiac death in patients receiving perioperative β-blockers. In contrast to previous studies46 that failed to detect any cardioprotective effect in the immediate perioperative period, Poldermans et al7 reported a greater than 90% decrease in perioperative mortality and nonfatal MI with the use of β-blockers in high-risk patients undergoing vascular surgery. Despite some methodological flaws, these landmark studies paved the way for extensive research about the use of perioperative β-blockade in the past 8 to 10 years. Most recently, the Canadian Metoprolol After Vascular Surgery study8 found no difference in postoperative cardiac morbidity and mortality at 1 month vs at 6 months with the use of β-blockers. Other evidence suggests that tight perioperative heart rate control is essential to decrease the incidence of perioperative cardiac morbidity and mortality.9 The most recent American College of Cardiology and American Heart Association (ACC/AHA) guidelines reviewed the available evidence and suggested a benefit to perioperative β-blockade in high-risk patients undergoing noncardiac surgery, particularly vascular surgery.10 Despite the absence of definitive evidence, the ACC/AHA guidelines recommended initiation of β-blocker therapy before surgery and titration of the resting heart rate to a target at between 50 and 60 beats/min “to assure the patient is indeed receiving the benefit of β-blockade.”10(p2667) With the recent release of preliminary results from the Perioperative Ischemic Evaluation (POISE) trial11 showing a higher incidence of total deaths and stroke in patients receiving perioperative β-blockade compared with placebo, these guidelines are now in question.

The present study was undertaken among patients at all levels of cardiac risk. We sought to determine the effect of β-blockers on perioperative heart rate and on perioperative cardiac morbidity and mortality in noncardiac surgery.

PATIENT POPULATION

Between January 1, 2000, and December 31, 2000, patients undergoing general anesthesia for various noncardiac surgical procedures at Michael E. DeBakey Veterans Affairs Medical Center, Houston, Texas, were screened and classified before surgery by the operating surgeon as at high, intermediate, low, or negligible cardiac risk based on ACC/AHA guidelines. Because the patients were evaluated in 2000, they were classified according to the ACC/AHA guidelines published in 1996.12 These were slightly modified in subsequent ACC/AHA guidelines.10

Patients included in the study underwent a noncardiac surgical procedure using general anesthesia within 6 months of the preoperative risk classification. If a patient underwent more than 1 noncardiac surgery using general anesthesia, only the first procedure was included in the analysis. The surgical procedures performed included plastic, vascular, urologic, abdominal, orthopedic, head and neck, ophthalmologic, skin and soft-tissue, and hernia repair surgery. Each procedure was also classified as high, intermediate, or low in risk as defined by the ACC/AHA guidelines.12

USE OF β-BLOCKERS

Patients' medical records were reviewed for the use of β-blockers at the time of surgery. Note was made of patients for whom β-blockers were started in the perioperative phase and of patients who were already receiving β-blockers before the operation for various reasons such as hypertension, congestive heart failure, or coronary artery disease. Specific β-blockers were recorded such as metoprolol succinate or metoprolol tartrate, atenolol, and propranolol hydrochloride.

HEART RATE TRACING

For each patient, a preoperative heart rate (PreopHR) reading was documented before the induction of general anesthesia and the start of the operation. Intraoperative heart rate was recorded every 5 minutes. From these tracings, the mean heart rate (MeanHR), maximum heart rate (MaxHR), and minimum heart rate (MinHR) values were extracted for each patient.

FREQUENCY MATCHING

Patients who received perioperative β-blockers or were already taking (and had received) β-blockers at the time of surgery constituted the β-blocker group. This group was matched to a control group from the same patient population that did not receive β-blockers at the time of surgery. The 2 groups were matched for age, sex, smoking status, cardiac risk (as per the ACC/AHA classification12), procedure risk (as per the ACC/AHA classification), and renal insufficiency (defined as a preoperative creatinine value >2.0 mg/dL [to convert creatinine level to micromoles per liter, multiply by 88.4]).

OUTCOME VARIABLES

Thirty-day postoperative cardiovascular morbidity was measured for both groups. These included stroke, nonfatal MI, and cardiac arrest with successful resuscitation. Mortality at 30 days and mortality at 1 year were documented as well. To determine the occurrence of these outcomes, a review of all medical records was performed by one of us (H.M.A.K., P.V.A., and K.M.F.I.). In particular, the diagnosis of MI involved a combination of clinical symptoms, electrocardiographic changes, elevation of troponin or creatine kinase MB level, and medical records documentation by surgeons, internists, or cardiologists.

ANALYSIS

After adequate matching was ensured, PreopHR, MeanHR, MaxHR, and MinHR were compared between the β-blocker group and the control group. The incidences of postoperative cardiovascular events and of all-cause mortality at 30 days and at 1 year were compared between the 2 groups. Finally, within each group, the PreopHR, MeanHR, MaxHR, and MinHR were compared between those patients who had a postoperative event (including mortality) and those who did not. t, Fisher exact, and χ2 tests were used as appropriate. Confidence intervals were set at the 95th percentile, and statistical significance was set at P ≤ .05. Institutional review board approval was obtained for the conduct of this study.

PATIENT POPULATION AND MATCHING

A total of 1238 patients underwent noncardiac surgery and were classified before surgery into high, intermediate, low, or negligible cardiac risk categories based on the ACC/AHA classification.12 Two hundred thirty-eight patients received β-blockers perioperatively. These patients constituted the β-blocker group and were matched to a control group of 408 patients who underwent noncardiac surgery at the same medical center during the same year but who did not receive perioperative β-blockade. The matching criteria used were age, sex, cardiac risk, procedure risk, smoking status, and renal insufficiency.

BASELINE CHARACTERISTICS

Table 1 gives the baseline characteristics in the β-blocker and control groups. The 2 groups were comparable for sex, smoking status, renal insufficiency, cardiac risk classification, and procedure risk classification. In addition, no statistical difference was noted between the ages of the 2 groups (mean [SD], 61.7 [9.8] years for the β-blocker group and 60.9 [12.0] years for the control group; P =.34).

Table Graphic Jump LocationTable 1. Comparison of Baseline Characteristics Between the β-Blocker and Control Groups
HEART RATE CONTROL

The mean values for the PreopHR, MeanHR, MinHR, and MaxHR were calculated for both groups. Table 2 summarizes the results. The β-blocker group, having received perioperative β-blockers, had lower PreopHR (70 vs 74 beats/min, P =.001), MeanHR (71 vs 78 beats/min, P < .001), MinHR (60 vs 63 beats/min, P =.02), and MaxHR (86 vs 91 beats/min, P < .001).

Table Graphic Jump LocationTable 2. Comparison of Heart Rate (HR) Control Between the β-Blocker and Control Groups
OUTCOMES

When outcomes in the 30-day postoperative period were evaluated and compared, patients in the β-blocker group had statistically higher incidences of nonfatal MI (2.94% vs 0.74%, P =.03), total cardiovascular morbidity (5.04% vs 1.47%, P =.003), and all-cause mortality (2.52% vs 0.25%, P =.007) (Table 3). All-cause mortality rates at 1 year were similar between the 2 groups.

Table Graphic Jump LocationTable 3. Analysis of Outcomes Between the β-Blocker and Control Groups
PreopHR AND 30-DAY MORTALITY

In the β-blocker group only, the mean PreopHR of the patients who died within 30 days of their surgery was significantly higher than that of the patients who survived (86 vs 70 beats/min, P =.03). In addition, the patient who died within 30 postoperative days in the control group had a PreopHR of 110 beats/min compared with a mean PreopHR of 74 beats/min in the rest of the control group. There were no statistical differences in the MeanHR, MinHR, or MaxHR when the patients who died were compared with the patients who survived the 30-day postoperative phase in both the β-blocker and the control groups.

None of the patients who died within 30 days of surgery in the β-blocker group were classified as at high risk; 3 were at intermediate risk; and 3 were at low or negligible risk. All of them underwent intermediate-risk to high-risk procedures.

Despite many studies in the past 3 decades, controversy continues regarding the indications and proper use of β-blockers in noncardiac surgery. According to the ACC/AHA 2006 update on perioperative β-blockade,10 class I recommendations (ie, conditions in which there is evidence or general agreement that the treatment is beneficial, useful, and effective) exist only for the following 2 patient populations: (1) patients who are already taking β-blockers before surgery for other indications such as angina, dysrhythmias, or hypertension and (2) patients undergoing vascular surgery with high cardiac risk as evidenced by ischemia on preoperative cardiac testing.10 Most studies have not evaluated the effect of β-blockers in patients at intermediate or low cardiac risk. In addition, few studies of perioperative β-blocker use have evaluated the optimal type, dose, regimen, or duration of therapy required.10 No definite consensus has been reached regarding the population at intermediate cardiac risk and the target heart rate needed for effect. The ACC/AHA recommendation for a resting heart rate at between 50 and 60 beats/min along with maintenance of a heart rate below 80 beats/min through the intraoperative and postoperative periods is the only guideline that is available.

In our study of patients at all levels of cardiac risk, including higher proportions of low-risk and intermediate-risk patients, β-blockers failed to provide perioperative cardioprotection. Unexpectedly, patients receiving β-blockers were at higher risk of 30-day cardiovascular events and overall mortality. This mortality difference disappeared at 1 year. A recent observational study by Lindenauer et al13 found that β-blockade decreases in-hospital mortality only in the surgical population with the highest cardiovascular risk and “might be harmful” in the subset of patients with the lowest risk. More recently, the controversy about the use of perioperative β-blockade was further fueled by results of the POISE trial.11 Similar to our study, this large, double-blinded, placebo-controlled trial showed an increase in overall 30-day mortality with the use of metoprolol, despite a lower incidence of postoperative MI, raising new concerns about the safety of perioperative β-blockade in noncardiac surgery.11

Our study analysis demonstrates that the patients who died within 30 days of their operation were not classified as high-risk patients and raises questions about the safety of β-blockers in low-risk or intermediate-risk patients. In addition, the patients who died within 30 days had a clinically and statistically higher PreopHR than that of their counterparts. As subtle as it may be, this finding suggests that a low target preoperative rather than intraoperative heart rate is essential for the protective effect of β-blockers. The relationship between preoperative (rather than intraoperative or postoperative) heart rate and perioperative mortality stresses the importance of not only initiating but also titrating the effect of β-blockers to an acceptable target heart rate before surgery. The biologic basis for such a finding is intriguing and could be explained by evidence in animal investigations showing that treating aged hearts with metoprolol improves the β-adrenergic receptor density and its signaling pathways.14

Our study has some limitations. First, it is a retrospective study, and some additional confounding variables might exist, despite our meticulous attempt at matching the β-blocker and control groups. Second, our population is a predominantly male veteran population, and one might argue whether our findings are applicable to the general population undergoing noncardiac surgery. Third, there was no uniform regimen for β-blockade used in our population with regard to the type, dose, or duration of therapy, and data regarding the concomitant use of statins, α-blockers, or antiplatelet therapy were unavailable.

In summary, our study adds to the controversy regarding the optimal use of perioperative β-blockers in patient populations at various levels of cardiac risk. Overall, our data found worse perioperative cardiovascular outcome and worse overall mortality associated with the use of β-blockers. Patients who died were at less than high cardiac risk and had worse PreopHR control. Further investigations in this field with standardizing of β-blockade regimen and with monitoring of heart rate in populations at various levels of cardiac risk should be pursued.

Accepted for Publication: January 5, 2008.

Correspondence: Kamal M. F. Itani, MD, Department of Surgery, Veterans Affairs Boston Healthcare System (112), 1400 VFW Pkwy, West Roxbury, MA 02132 (kitani@med.va.gov).

Author Contributions:Study concept and design: Kaafarani and Itani. Acquisition of data: Kaafarani, Atluri, and Itani. Analysis and interpretation of data: Kaafarani, Thornby, and Itani. Drafting of the manuscript: Kaafarani and Itani. Critical revision of the manuscript for important intellectual content: Kaafarani, Atluri, Thornby, and Itani. Statistical analysis: Thornby. Administrative, technical, and material support: Kaafarani and Itani.

Financial Disclosure: None reported.

Landesberg  GMosseri  MZahger  D  et al.  Myocardial infarction after vascular surgery: the role of prolonged stress-induced, ST depression–type ischemia. J Am Coll Cardiol 2001;37 (7) 1839- 1845
PubMed Link to Article
Parker  SDBreslow  MJFrank  SM  et al. Perioperative Ischemia Randomized Anesthesia Trial Study Group, Catecholamine and cortisol responses to lower extremity revascularization: correlation with outcome variables. Crit Care Med 1995;23 (12) 1954- 1961
PubMed Link to Article
London  MJ Multilead precordial ST-segment monitoring: “the next generation?” Anesthesiology 2002;96 (2) 259- 261
PubMed Link to Article
Stone  JGFoëx  PSear  JW  et al.  Myocardial ischemia in untreated hypertensive patients: effect of a single small oral dose of a β-adrenergic blocking agent. Anesthesiology 1988;68 (4) 495- 500
PubMed Link to Article
Pasternack  PFImparato  AMBaumann  FG  et al.  The hemodynamics of β-blockade in patients undergoing abdominal aortic aneurysm repair. Circulation 1987;76 (3, pt 2) III1- III7
PubMed Link to Article
Mangano  DTLayug  ELWallace  A  et al. Multicenter Study of Perioperative Ischemia Research Group, Effect of atenolol on mortality and cardiovascular morbidity after noncardiac surgery. N Engl J Med 1996;335 (23) 1713- 1720
PubMed Link to Article
Poldermans  DBoersma  EBax  JJ  et al. Dutch Echocardiographic Cardiac Risk Evaluation Applying Stress Echocardiography Study Group, The effect of bisoprolol on perioperative mortality and myocardial infarction in high-risk patients undergoing vascular surgery. N Engl J Med 1999;341 (24) 1789- 1794
PubMed Link to Article
Yang  HRaymer  KButler  R  et al.  The effects of perioperative β-blockade: results of the Metoprolol After Vascular Surgery (MaVS) study, a randomized controlled trial. Am Heart J 2006;152 (5) 983- 990
PubMed Link to Article
Feringa  HHBax  JJBoersma  E  et al.  High-dose β-blockers and tight heart rate control reduce myocardial ischemia and troponin T release in vascular surgery patients. Circulation 2006;114 (1) ((suppl)) I344- I349
PubMed Link to Article
Fleisher  LABeckman  JABrown  KA  et al. American College of Cardiology/American Heart Association Task Force on Practice Guidelines Writing Committee to Update the 2002 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery; American Society of Echocardiography; American Society of Nuclear Cardiology; Heart Rhythm Society; Society of Cardiovascular Anesthesiologists; Society for Cardiovascular Angiography and Interventions; Society for Vascular Medicine and Biology, ACC/AHA 2006 guideline update on perioperative cardiovascular evaluation for noncardiac surgery: focused update on perioperative β-blocker therapy: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Update the 2002 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery): developed in collaboration with the American Society of Echocardiography, American Society of Nuclear Cardiology, Heart Rhythm Society, Society of Cardiovascular Anesthesiologists, Society for Cardiovascular Angiography and Interventions, and Society for Vascular Medicine and Biology. Circulation 2006;113 (22) 2662- 2674
PubMed Link to Article
POISE Study Group,Devereaux  PJYang  HYusuf  S  et al.  Effects of extended-release metoprolol succinate in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled trial. Lancet 2008;371 (9627) 1839- 1847
PubMed Link to Article
Eagle  KABrundage  BHChaitman  BR  et al. Committee on Perioperative Cardiovascular Evaluation for Noncardiac Surgery, Guidelines for perioperative cardiovascular evaluation for noncardiac surgery: report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines: Committee on Perioperative Cardiovascular Evaluation for Noncardiac Surgery. Circulation 1996;93 (6) 1278- 1317
PubMed
Lindenauer  PKPekow  PWang  K  et al.  Perioperative β-blocker therapy and mortality after major noncardiac surgery. N Engl J Med 2005;353 (4) 349- 361
PubMed Link to Article
Leosco  DRengo  GIaccarino  G  et al.  Exercise training and β-blocker treatment ameliorate age-dependent impairment of β-adrenergic receptor signaling and enhance cardiac responsiveness to adrenergic stimulation. Am J Physiol Heart Circ Physiol 2007;293 (3) H1596- H1603
PubMed Link to Article

Figures

Tables

Table Graphic Jump LocationTable 1. Comparison of Baseline Characteristics Between the β-Blocker and Control Groups
Table Graphic Jump LocationTable 2. Comparison of Heart Rate (HR) Control Between the β-Blocker and Control Groups
Table Graphic Jump LocationTable 3. Analysis of Outcomes Between the β-Blocker and Control Groups

References

Landesberg  GMosseri  MZahger  D  et al.  Myocardial infarction after vascular surgery: the role of prolonged stress-induced, ST depression–type ischemia. J Am Coll Cardiol 2001;37 (7) 1839- 1845
PubMed Link to Article
Parker  SDBreslow  MJFrank  SM  et al. Perioperative Ischemia Randomized Anesthesia Trial Study Group, Catecholamine and cortisol responses to lower extremity revascularization: correlation with outcome variables. Crit Care Med 1995;23 (12) 1954- 1961
PubMed Link to Article
London  MJ Multilead precordial ST-segment monitoring: “the next generation?” Anesthesiology 2002;96 (2) 259- 261
PubMed Link to Article
Stone  JGFoëx  PSear  JW  et al.  Myocardial ischemia in untreated hypertensive patients: effect of a single small oral dose of a β-adrenergic blocking agent. Anesthesiology 1988;68 (4) 495- 500
PubMed Link to Article
Pasternack  PFImparato  AMBaumann  FG  et al.  The hemodynamics of β-blockade in patients undergoing abdominal aortic aneurysm repair. Circulation 1987;76 (3, pt 2) III1- III7
PubMed Link to Article
Mangano  DTLayug  ELWallace  A  et al. Multicenter Study of Perioperative Ischemia Research Group, Effect of atenolol on mortality and cardiovascular morbidity after noncardiac surgery. N Engl J Med 1996;335 (23) 1713- 1720
PubMed Link to Article
Poldermans  DBoersma  EBax  JJ  et al. Dutch Echocardiographic Cardiac Risk Evaluation Applying Stress Echocardiography Study Group, The effect of bisoprolol on perioperative mortality and myocardial infarction in high-risk patients undergoing vascular surgery. N Engl J Med 1999;341 (24) 1789- 1794
PubMed Link to Article
Yang  HRaymer  KButler  R  et al.  The effects of perioperative β-blockade: results of the Metoprolol After Vascular Surgery (MaVS) study, a randomized controlled trial. Am Heart J 2006;152 (5) 983- 990
PubMed Link to Article
Feringa  HHBax  JJBoersma  E  et al.  High-dose β-blockers and tight heart rate control reduce myocardial ischemia and troponin T release in vascular surgery patients. Circulation 2006;114 (1) ((suppl)) I344- I349
PubMed Link to Article
Fleisher  LABeckman  JABrown  KA  et al. American College of Cardiology/American Heart Association Task Force on Practice Guidelines Writing Committee to Update the 2002 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery; American Society of Echocardiography; American Society of Nuclear Cardiology; Heart Rhythm Society; Society of Cardiovascular Anesthesiologists; Society for Cardiovascular Angiography and Interventions; Society for Vascular Medicine and Biology, ACC/AHA 2006 guideline update on perioperative cardiovascular evaluation for noncardiac surgery: focused update on perioperative β-blocker therapy: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Update the 2002 Guidelines on Perioperative Cardiovascular Evaluation for Noncardiac Surgery): developed in collaboration with the American Society of Echocardiography, American Society of Nuclear Cardiology, Heart Rhythm Society, Society of Cardiovascular Anesthesiologists, Society for Cardiovascular Angiography and Interventions, and Society for Vascular Medicine and Biology. Circulation 2006;113 (22) 2662- 2674
PubMed Link to Article
POISE Study Group,Devereaux  PJYang  HYusuf  S  et al.  Effects of extended-release metoprolol succinate in patients undergoing non-cardiac surgery (POISE trial): a randomised controlled trial. Lancet 2008;371 (9627) 1839- 1847
PubMed Link to Article
Eagle  KABrundage  BHChaitman  BR  et al. Committee on Perioperative Cardiovascular Evaluation for Noncardiac Surgery, Guidelines for perioperative cardiovascular evaluation for noncardiac surgery: report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines: Committee on Perioperative Cardiovascular Evaluation for Noncardiac Surgery. Circulation 1996;93 (6) 1278- 1317
PubMed
Lindenauer  PKPekow  PWang  K  et al.  Perioperative β-blocker therapy and mortality after major noncardiac surgery. N Engl J Med 2005;353 (4) 349- 361
PubMed Link to Article
Leosco  DRengo  GIaccarino  G  et al.  Exercise training and β-blocker treatment ameliorate age-dependent impairment of β-adrenergic receptor signaling and enhance cardiac responsiveness to adrenergic stimulation. Am J Physiol Heart Circ Physiol 2007;293 (3) H1596- H1603
PubMed Link to Article

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