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Biomedical subjects

P C Austin

Publications and source records attributed to P C Austin.

At least 19 recordsLinked to original sources
PubMed2002

Biology or bias: practice patterns and long-term outcomes for men and women with acute myocardial infarction.

OBJECTIVES: The goal of our study was to examine how age and gender affect the use of coronary angiography and the intensity of cardiac follow-up care within the first year after acute myocardial infarction (AMI). Another objective was to evaluate the association of age, gender and treatment intensity with five-year survival after AMI. BACKGROUND: Utilization rates of specialized cardiac services inversely correlate with age. Gender-specific practice patterns may also vary with age in a manner similar to known age-gender survival differences after AMI. METHODS: Using linked population-based administrative data, we examined the association of age and gender with treatment intensity and long-term survival among 25,697 patients hospitalized with AMI in Ontario between April 1, 1992, and December 31, 1993. A Cox proportional hazards model was used to adjust for socioeconomic status, illness severity, attending physician specialty and admitting hospital characteristics. RESULTS: After adjusting for baseline differences, the relative rates of angiography and follow-up specialist care for women relative to men, respectively, fell 17.5% (95% confidence interval [CI], 13.6 to 21.3, p < 0.001) and 10.2% (95% CI, 7.1 to 13.2, p < 0.001) for every 10-year increase in age. Conversely, long-term AMI survival rates in women relative to men improved with increasing age, such that the relative survival in women rose 14.2% (95% CI, 10.1 to 17.5, p < 0.001) for every 10-year age increase. CONCLUSIONS: Gender differences in the intensity of invasive testing and follow-up care are strongly age-specific. While care becomes progressively less aggressive among older women relative to men, survival advantages track in the opposite direction, with older women clearly favored. These findings suggest that biology is likely to remain the main determinant of long-term survival after AMI for women.

Adult↗
PubMed2002

Utilisation of coronary angiography after acute myocardial infarction in Ontario over time: have referral patterns changed?

OBJECTIVE: To examine how physicians in Ontario, Canada, have altered their referral patterns for coronary angiography after acute myocardial infarction (AMI) over time. DESIGN: Retrospective analysis of multilinked administrative data. SETTING: Province of Ontario, Canada. PATIENTS: 146 365 Ontario AMI patients hospitalised between 1 April 1992 and 31 March 1999. MAIN OUTCOME MEASURES: Utilisation trends of coronary angiography among all patients, as well as within six subgroups: elderly (versus young), women (versus men), high (versus low) risk of 30 day mortality, high (versus low) socioeconomic status, cardiology (versus non-cardiology) attending physician specialty, and hospitals with (versus without) onsite revascularisation capacity. Cox proportional hazard models were adjusted for variations in patient, physician, and hospital characteristics over time. RESULTS: Angiography rates in Ontario increased from 23.2% in 1992 to 35.5% in 1999 (p < 0.0001). Increases in utilisation of coronary angiography were most pronounced among the elderly (12.4-24.3% v 39.3-54.4% for non-elderly patients, p < 0.0001), the affluent (24.6-38.7% v 22.0-32.3% for less affluent patients, p = 0.01), and those tended to by cardiologists (32.0-47.1% v 20.3-30.1% for non-cardiology attending specialties, p < 0.0001) after adjusting for changes in baseline patient, physician, and hospital characteristics over time. CONCLUSIONS: Despite universal health care availability, not all patients benefited equally from increases in service capacity for coronary angiography after AMI in Ontario. Wider implementation of data monitoring and explicit management systems may be required to ensure that appropriate utilisation of cardiac services is allocated to patients who are most in need.

Adolescent↗
PubMed2001

Relationship between annual volume of patients treated by admitting physician and mortality after acute myocardial infarction.

CONTEXT: Acute myocardial infarction (AMI) is a common condition that is treated by physicians with varying levels of clinical experience, but whether the level of experience affects outcome remains uncertain. OBJECTIVE: To evaluate the relationship between the average annual volume of cases treated by admitting physicians and mortality after AMI. DESIGN, SETTING, AND PATIENTS: Retrospective cohort study using linked administrative databases containing patient admission information for 98 194 patients treated by 5374 physicians between April 1, 1992, and March 31, 1998, in Ontario, Canada. MAIN OUTCOME MEASURES: Mortality risk rates for 30 days and 1 year post-AMI, adjusted by physician volume and patient, physician, and hospital characteristics. RESULTS: The 30-day mortality rate was 13.5% and the 1-year mortality rate was 21.8%. A strong inverse relationship between the average annual volume of AMI cases treated by the admitting physician and mortality after an AMI was observed. The 30-day risk-adjusted mortality rate was 15.3% for physicians who treated 5 or fewer AMI cases per year (lowest quartile) compared with 11.8% for physicians who treated more than 24 AMI cases annually (highest quartile; P<.001). The 1-year risk-adjusted mortality rate was 24.2% for physicians who treated 5 or fewer AMI cases per year (lowest quartile) compared with 19.6% for physicians who treated more than 24 AMI cases annually (highest quartile; P<.001). CONCLUSION: Patients with AMI who are treated by high-volume admitting physicians are more likely to survive at 30 days and 1 year.

Aged↗
PubMed2001

Long-term MI outcomes at hospitals with or without on-site revascularization.

CONTEXT: Many studies have found that patients with acute myocardial infarction (AMI) who are admitted to hospitals with on-site revascularization facilities have higher rates of invasive cardiac procedures and better outcomes than patients in hospitals without such facilities. Whether such differences are due to invasive procedure rates alone or to other patient, physician, and hospital characteristics is unknown. OBJECTIVE: To determine whether invasive procedural rate variations alone account for outcome differences in patients with AMI admitted to hospitals with or without on-site revascularization facilities. DESIGN: Retrospective, observational cohort study using linked population-based administrative data from a universal health insurance system. SETTING: One hundred ninety acute care hospitals in Ontario, 9 of which offered invasive procedures. PATIENTS: A total of 25 697 patients hospitalized with AMI between April 1, 1992, and December 31, 1993, of whom 2832 (11%) were in invasive hospitals. MAIN OUTCOME MEASURES: Mortality, recurrent cardiac hospitalizations, and emergency department visits in the 5 years following the index admission, adjusted for patient age, sex, socioeconomic status, illness severity, and index revascularization procedures; attending physician specialty; and hospital volume, teaching status, and geographical proximity to invasive-procedure centers and compared by hospital type. RESULTS: Patients admitted to invasive-procedure hospitals were much more likely to undergo revascularization (11.4% vs 3.2% at other hospitals; P<.001). However, many other clinical and process-related factors differed between the 2 groups. Although mortality rates were similar between the 2 institution types, the nonfatal composite 5-year event rate (ie, recurrent cardiac hospitalization and emergency department visits) was lower for patients initially admitted to invasive-procedure hospitals (71.3% vs 80.4%; unadjusted odds ratio [OR], 0.65; 95% confidence interval [CI], 0.52-0.82; P<.001). This advantage persisted after adjustment for sociodemographic and clinical factors and procedure utilization (adjusted OR, 0.68; 95% CI, 0.53-0.89; P<.001). However, the nonfatal outcome advantages of invasive-procedure hospitals were explained by their teaching status (adjusted OR, 0.98; 95% CI, 0.73-1.30; P =.87). CONCLUSIONS: In this sample of patients admitted with AMI, the differing outcomes of apparently similar patients treated in 2 different practice settings were explained by multiple competing factors. Researchers conducting observational studies should be cautious about attributing patient outcome differences to any single factor.

Aged↗
PubMed2001

Development and validation of the Ontario acute myocardial infarction mortality prediction rules.

OBJECTIVES: To develop and validate simple statistical models that can be used with hospital discharge administrative databases to predict 30-day and one-year mortality after an acute myocardial infarction (AMI). BACKGROUND: There is increasing interest in developing AMI "report cards" using population-based hospital discharge databases. However, there is a lack of simple statistical models that can be used to adjust for regional and interinstitutional differences in patient case-mix. METHODS: We used linked administrative databases on 52,616 patients having an AMI in Ontario, Canada, between 1994 and 1997 to develop logistic regression statistical models to predict 30-day and one-year mortality after an AMI. These models were subsequently validated in two external cohorts of AMI patients derived from administrative datasets from Manitoba, Canada, and California, U.S. RESULTS: The 11-variable Ontario AMI mortality prediction rules accurately predicted mortality with an area under the receiver operating characteristic (ROC) curve of 0.78 for 30-day mortality and 0.79 for one-year mortality in the Ontario dataset from which they were derived. In an independent validation dataset of 4,836 AMI patients from Manitoba, the ROC areas were 0.77 and 0.78, respectively. In a second validation dataset of 112,234 AMI patients from California, the ROC areas were 0.77 and 0.78 respectively. CONCLUSIONS: The Ontario AMI mortality prediction rules predict quite accurately 30-day and one-year mortality after an AMI in linked hospital discharge databases of AMI patients from Ontario, Manitoba and California. These models may also be useful to outcomes and quality measurement researchers in other jurisdictions.

Aged↗
PubMed2001

A comparison of a Bayesian vs. a frequentist method for profiling hospital performance.

The objective of this study was to compare the classification of hospitals as outcomes outliers using a commonly implemented frequentist statistical approach vs. an implementation of Bayesian hierarchical statistical models, using 30-day hospital-level mortality rates for a cohort of acute myocardial infarction patients as a test case. For the frequentist approach, a logistic regression model was constructed to predict mortality. For each hospital, a risk-adj usted mortality rate was computed. Those hospitals whose 95% confidence interval, around the risk-adjusted mortality rate, excludes the mean mortality rate were classified as outliers. With the Bayesian hierarchical models, three factors could vary: the profile of the typical patient (low, medium or high risk), the extent to which the mortality rate for the typical patient departed from average, and the probability that the mortality rate was indeed different by the specified amount. The agreement between the two methods was compared for different patient profiles, threshold differences from the average and probabilities. Only marginal agreement was shown between the Bayesian and frequentist approaches. In only five of the 27 comparisons was the kappa statistic at least 0.40. The remaining 22 comparisons demonstrated only marginal agreement between the two methods. Within the Bayesian framework, hospital classification clearly depended on patient profile, threshold and probability of exceeding the threshold. These inconsistencies raise questions about the validity of current methods for classifying hospital performance, and suggest a need for urgent research into which methods are most meaningful to clinicians, managers and the general public.

Aged↗
PubMed2001

The influence of surgical specialty training on the outcomes of elective abdominal aortic aneurysm surgery.

OBJECTIVE: The aim of this study was to determine the independent impact of surgeon speciality training (vascular, cardiac, or general surgery) on the 30-day risk-adjusted mortality rate after elective abdominal aortic aneurysm (AAA) surgery. PATIENTS AND METHODS: All patients undergoing elective AAA surgery in Ontario between April 1, 1992, and March 31, 1996, were included. A retrospective cohort study with linked administrative databases was undertaken. RESULTS: The average 30-day mortality rate was 4.1%. Of the 5878 cases studied, 4415 (75.1%) were performed by 63 vascular surgeons, 1193 (20.3%) by 53 general surgeons, and 270 (4.6%) by 14 cardiac surgeons. After the adjustment for potential confounding factors of annual surgeon AAA volume, type of hospital, and patient age, sex, Charlson comorbidity score, and transfer status, the odds of patients dying were 62% higher when the surgery was performed by a general surgeon than when it was performed by a vascular surgeon. Cardiac surgeons' patient outcomes were similar to those of vascular surgeons. CONCLUSIONS: Patients who undergo elective AAA repair that is performed by vascular or cardiac surgeons have significantly lower mortality rates than patients who have their aneurysms repaired by general surgeons. These results provide evidence that surgical specialty training in vascular procedures leads to better patient outcomes.

Aged↗
PubMed2001

Cross-cultural comparisons of health status in Canada using the Health Utilities Index.

OBJECTIVE: The purpose of this study was to examine the differences in health status, as measured by the Health Utilities Index (HUI), among seven cultural groups in Canada defined by place of birth and language. DESIGN: The study analysed cross-sectional data from the National Population Health Survey conducted by Statistics Canada in 1994-95. RESULTS: Age-standardized prevalence of dysfunction, defined as HUI < 0.83, varied from 12.7% in English-speaking immigrants to 17.8% in French-speaking Canadians. Considerable differences between the groups were found in the reporting of pain, emotional function, and cognitive function. The variation in HUI scores across the cultural groups could not be explained by differences in socioeconomic status and self-reported chronic conditions. CONCLUSIONS: Although the healthy immigrant effect is probably responsible for some of the variation in health status among cultural groups in Canada, considerable differences exist within the immigrant and Canadian-born populations. Cultural factors may have a substantial effect on the reporting of pain and mental health problems. Further studies are needed to determine the cross-cultural validity of the HUI.

Canada↗
PubMed2001

Dynamic relationship between sympathetic nerve activity and renal blood flow: a frequency domain approach.

Blood pressure displays an oscillation at 0.1 Hz in humans that is well established to be due to oscillations in sympathetic nerve activity (SNA). However, the mechanisms that control the strength or frequency of this oscillation are poorly understood. The aim of the present study was to define the dynamic relationship between SNA and the vasculature. The sympathetic nerves to the kidney were electrically stimulated in six pentobarbital-sodium anesthetized rabbits, and the renal blood flow response was recorded. A pseudo-random binary sequence (PRBS) was applied to the renal nerves, which contains equal spectral power at frequencies in the range of interest (<1 Hz). Transfer function analysis revealed a complex system composed of low-pass filter characteristics but also with regions of constant gain. A model was developed that accounted for this relationship composed of a 2 zero/4 pole transfer function. Although the position of the poles and zeros varied among animals, the model structure was consistent. We also found the time delay between the stimulus and the RBF responses to be consistent among animals (mean 672 +/- 22 ms). We propose that the identification of the precise relationship between SNA and renal blood flow (RBF) is a fundamental and necessary step toward understanding the interaction between SNA and other physiological mediators of RBF.

Animals↗
PubMed2001

Investigating the determinants of decreasing postmyocardial infarction mortality: an analysis of in-hospital data from 1992 and 1997.

BACKGROUND: Although acute myocardial infarction (AMI) is the leading cause of mortality in the industrialized world, postmyocardial infarction mortality rates have been declining in recent decades. Two possible contributing factors toward this encouraging trend include changing patient characteristics and improved patient management. OBJECTIVES: To compare temporal changes in the characteristics and management of patients with AMI at a tertiary care hospital (Sunnybrook and Women's College Health Sciences Centre) in Toronto, Ontario. METHODS: Two hundred hospital charts of patients with AMI as the most responsible diagnosis were reviewed (100 from 1992 and 100 from 1997). One hundred thirty prespecified variables were extracted from each chart, with emphasis placed on baseline clinical characteristics, AMI management and survival. RESULTS: Between 1992 and 1997, AMI in-hospital mortality declined from 20% to 15%. Most baseline clinical characteristics (age, sex, comorbidity, cardiac history, and presenting symptoms and signs) were similar across the 1992 and 1997 patient populations. The only significant risk factor change involved an increase in the prevalence of hypercholesterolemia. In contrast, between 1992 and 1997 there was an increased in-hospital use of anticoagulants, antiplatelets, thrombolytics, beta-blockers, angiotensin-converting enzyme inhibitors and statins. Similarly, there was an increased use of coronary angioplasty and coronary bypass surgery. There was no significant change in the use of AMI therapies that are potentially harmful, including antiarrythmic agents and calcium channel blockers. CONCLUSIONS: AMI patient characteristics were similar between 1992 and 1997 but there were striking changes in AMI treatment patterns. The increased use of evidence-based pharmacotherapy may be the most significant contributing factor to declining postmyocardial infarction mortality.

Aged↗
PubMed2000

Postmenopausal estrogen replacement therapy and increased rates of cholecystectomy and appendectomy.

BACKGROUND: Several studies have indicated that estrogen may prime inflammatory and nociceptive pathways, leading to symptoms that mimic cholecystitis. We set out to confirm the relation between recent estrogen use and cholecystectomy in postmenopausal women and to test the novel hypothesis that a similar relation exists for appendectomy. METHODS: We developed a retrospective cohort using prescribing and surgical procedure information from health administrative databases for approximately 800,000 female residents of Ontario who were over 65 years of age between July 1, 1993, and Mar. 31, 1998. We compared the incidence of cholecystectomy and appendectomy among women recently prescribed estrogen replacement therapy, levothyroxine and dihydropyridine calcium-channel antagonists (DCCA) using age-adjusted Cox proportional hazards models. Patients were followed for a mean of 540 (standard deviation [SD] 449) days. RESULTS: Compared with women taking DCCA, those who had recently begun taking estrogen were significantly more likely to undergo cholecystectomy (age-adjusted risk ratio [aRR] 1.9, 95% confidence interval [CI] 1.6-2.2) and appendectomy (aRR 1.8, 95% CI 1.1-3.0). No significant difference in either outcome measure was found between the levothyroxine users and the DCCA users. INTERPRETATION: This study identifies an increased risk of cholecystectomy and appendectomy among postmenopausal women who have recently begun estrogen replacement therapy.

Aged↗
PubMed2000

The use of the Tobit model for analyzing measures of health status.

Self-reported health status is often measured using psychometric or utility indices that provide a score intended to summarize an individual's health. Measurements of health status can be subject to a ceiling effect. Frequently, researchers want to examine relationships between determinants of health and measures of health status. Regression methods that ignore the presence of a ceiling effect, or of censoring in the health status measurements can produce biased coefficient estimates. The Tobit regression model is a frequently used tool for modeling censored variables in econometrics research. The authors carried out a Monte-Carlo simulation study to contrast the performance of the Tobit model for censored data with that of ordinary least squares (OLS) regression. It was demonstrated that in the presence of a ceiling effect, if the conditional distribution of the measure of health status had uniform variance, then the coefficient estimates from the Tobit model have superior performance compared with estimates from OLS regression. However, if the conditional distribution had non-uniform variance, then the Tobit model performed at least as poorly as the OLS model.

Adult↗
PubMed2000

Rate of stroke recurrence in patients with primary intracerebral hemorrhage.

BACKGROUND AND PURPOSE: Primary intracerebral hemorrhage (PICH) is a devastating illness with high early mortality. Hypertension is a major risk factor both for ischemic cerebrovascular disease and for intracranial hemorrhage. Survivors of PICH are at risk for both recurrent hemorrhage and ischemic cerebrovascular disease. We sought to determine the rate of recurrence of ICH or cerebral ischemia in a cohort of PICH patients at the Toronto Hospital, Toronto, Canada. METHODS: A retrospective search of computerized hospital records from 1986 to 1996 for patients with a discharge diagnosis of intracerebral hemorrhage (International Classification of Diseases, Ninth Revision-Clinical Modification [ICD-9-CM] code 431) was conducted to identify the index cases. Charts were abstracted for demographic and clinical characteristics. CT scans, MR scans, or radiologist reports were reviewed. To determine recurrence, the database was linked to the Ontario Provincial Government Vital Statistics Registry and to the Canadian Institute for Health Information database of hospital discharge abstracts. Logistic regression analysis was used to identify predictive factors for mortality after PICH. A Cox proportional hazards model was fitted to identify predictive factors for recurrent ICH or stroke. RESULTS: A total of 746 charts were identified by computer search. After abstraction, 423 index patients with PICH were identified. Of these, 27.4% died in the first 30 days of their admission. Predictors of death were age, intraventricular rupture of hemorrhage, and trilobar hemorrhage. The recurrence rate for PICH was 2.4% (95% CI 1.4% to 3. 9%) per year, whereas the recurrence rate for ischemic cerebrovascular disease was 3.0% (95% CI 1.8% to 4.7%) per year. The only significant predictor of readmission for ICH was lobar location of the index hemorrhage, with a hazard ratio of 3.8 (95% CI 1.2 to 12.0). CONCLUSIONS: PICH has a high 30-day mortality rate. Survival from the initial insult portends a moderate risk of recurrence of 2. 4% per year for PICH and 3.0% per year for ischemic cerebrovascular disease. Patients with PICH are at risk for both ischemic stroke or TIA and recurrent hemorrhage; thus, PICH may be a marker for ischemic stroke. Patients with lobar hemorrhage have a 3.8-fold increased risk of recurrent ICH.

Aged↗
PubMed2000

Sex differences and similarities in the management and outcome of stroke patients.

BACKGROUND AND PURPOSE: Previous studies have documented sex differences in the management and outcome of patients with cardiovascular disease. However, little data exist on whether similar sex differences exist in stroke patients. We conducted a study to determine whether sex differences exist in patients with acute stroke admitted to Ontario hospitals. METHODS: Using linked administrative databases, we performed a population-based cohort study. The databases contained information on all 44 832 patients discharged from acute-care hospitals in Ontario between April 1993 and March 1996 with a most responsible diagnosis of acute stroke. The main outcomes measured consisted of sex differences in comorbidities, the use of rehabilitative services, the use of antiplatelet therapy and anticoagulants (in elderly stroke survivors aged > or =65 years only), discharge destination, and mortality. RESULTS: Male stroke patients were more likely than female stroke patients to have a history of ischemic heart disease (18.1% versus 15.3%, respectively; P<0.001) and diabetes mellitus (20.1% versus 18. 7%, respectively; P<0.001), whereas female patients were more likely than male patients to have hypertension (33.8% versus 30.0%, respectively; P<0.001) and atrial fibrillation (12.9% versus 10.2%, respectively; P<0.001). There were no sex differences in the usage of in-hospital rehabilitative services. The overall 90-day postdischarge use of aspirin and ticlopidine was similar in stroke survivors aged 65 to 84 years. However, among stroke survivors aged > or =85 years, men were more likely than women to receive aspirin (36. 0% versus 30.7%, respectively; P<0.001) and ticlopidine (9.2% versus 6.8%, respectively; P=0.007). Use of warfarin was similar for the two sexes. Men were more likely than women to be discharged home (50. 6% versus 40.9%, respectively; P<0.001) and less likely to be discharged to chronic care facilities (16.8% versus 25.2%, respectively; P<0.001). The risk of death 1 year after stroke was somewhat lower in women than men (adjusted odds ratio 0.939, 95% CI 0.899 to 0.980; P=0.004). The mortality differences were greatest among elderly stroke patients. CONCLUSIONS: Elderly men are more likely than elderly women to receive aspirin and ticlopidine and equally like to receive warfarin after a stroke. Despite these differences, elderly women have a better 1-year survival after a stroke.

Aged↗
PubMed2000

Use of antidepressants among elderly subjects: trends and contributing factors.

OBJECTIVE: The authors assessed changes over time in antidepressant utilization among elderly subjects regarding the prevalence of antidepressant users, shifts in prescription patterns, and related financial implications. METHOD: The authors conducted a population-based study of more than 1.4 million Ontario residents aged 65 years or older. Cross-sectional data regarding annual antidepressant utilization were obtained from administrative databases for 1993 to 1997. Time series analysis was used to assess trends over time and to make future projections. RESULTS: The proportion of antidepressant users increased from 9.3% of the elderly population in 1993 to 11.5% in 1997. Prescriptions for selective serotonin reuptake inhibitors (SSRIs) accounted for 9.6% of antidepressant prescriptions dispensed in the first 30 days of 1993 and 45.1% of those dispensed by the last 30 days of 1997 and were projected to increase to approximately 56% by the end of 2000. Prescriptions for tricyclic antidepressants fell from 79.0% in the first 30 days of 1993 to 43.1% by the last 30 days of 1997 and were projected to decline to approximately 28% by the end of 2000. Annual antidepressant costs (in Canadian dollars) increased by 150%, from $10.8 million in 1993 to $27.0 million in 1997. Population shifts and an increase in the prevalence of antidepressant users accounted for at least 20% of this increase, whereas the prescribing transition from tricyclic antidepressants to SSRIs accounted for at least 61% of the increase. CONCLUSIONS: The introduction of SSRIs has had a substantial financial impact at the drug utilization level. Future research should address the appropriate balancing of the cost of newer agents versus their ostensible advantages.

Aged↗
PubMed1999

Return to work after heart transplantation: 12-year follow-up.

BACKGROUND: Current statistics show that 50% of heart transplantation recipients survive 10 years or more. Emphasis on cost containment has renewed interest in the employment status of these long-term survivors. METHODS: We have identified 62 patients operated on at one United Kingdom center and evaluated their employment and clinical status by interview at 1, 5, and 12 years from surgery. RESULTS: Employment pattern was good, with 69%, 69% (of 55 survivors), and 57% (of 35 survivors) of the group working at 1, 5, and 12 years, respectively. Blue-collar work status per se was not a deterrent to employment, but older patients with a poor presurgical work history were less successful in finding work, and trade and legislative restrictions forced some, who were willing and capable of returning to their previous employment, into lower paid, less satisfactory jobs. CONCLUSIONS: Subgroups of patients who are likely to be unemployed can be identified prior to surgery. Steps can be taken prior to surgery to anticipate future problems, and establish realistic employment goals.

Adult↗
PubMed1999

Resonance in the renal vasculature evoked by activation of the sympathetic nerves.

We examined the ability of different frequencies in sympathetic nerve activity (SNA) to induce oscillations in renal blood flow (RBF). In anesthetized rabbits the renal nerves were stimulated using modulated sine patterns (base frequency 5 Hz, 5-ms duration pulses) that varied in amplitude between 0 and 10 V at a frequency between 0.04 and 1.0 Hz. The strengths of the induced oscillations in RBF were calculated using spectral analysis. Although faster rhythms in simulated SNA >0.6 Hz contributed to the level of vascular tone, 95% of the power in the frequency response curve was below this frequency, indicating a low-pass filtering/integrating characteristic of the vasculature. Frequencies <0.6 Hz were associated with increasing ability to induce oscillations in RBF. The ability of an SNA rhythm at 0.6 Hz to induce a rhythm in RBF was 21 times less than that at 0.25 Hz. At 0.16 Hz there was a distinct peak in the frequency response curve, indicating the vasculature was more sensitive in this frequency band to sympathetic stimulation. Blockade of endogenous nitric oxide by NG-nitro-L-arginine methyl ester (L-NAME; 20 mg/kg) did not alter resting RBF levels nor was the low-pass filtering/integrating characteristic of the vasculature to nerve stimulation changed (i.e., the curve was not shifted left or right); however, there was a selective increase in the sensitivity to stimulation at 0.16 Hz, i.e., larger oscillations in RBF were evoked. These results indicate an ability of SNA to induce resonant oscillations in the renal vasculature and that there may be active and passive modulators of these responses. Naturally occurring oscillations in SNA <0.6 Hz are likely to contribute to the dynamic control of RBF, ensuring it responds rapidly and with high gain to the stimuli of daily life, while filtering out the faster oscillations ensures stable glomerular filtration.

Animals↗