Volume 90, Issue 2 , Pages 193-200, February 2009
Predictors of Cardiopulmonary Hospitalization in Chronic Spinal Cord Injury
Article Outline
Abstract
Waddimba AC, Jain NB, Stolzmann K, Gagnon DR, Burgess JF, Kazis LE, Garshick E. Predictors of cardiopulmonary hospitalization in chronic spinal cord injury.
Objective
To investigate longitudinal risk factors of hospitalization for circulatory and pulmonary diseases among veterans with chronic spinal cord injury (SCI). Circulatory and respiratory system illnesses are leading causes of death in patients with chronic SCI, yet risk factors for related hospitalizations have not been characterized.
Design
Prospective cohort study.
Setting
Veterans Affairs (VA) Boston Healthcare System, Boston, Massachusetts.
Participants/Data Sources
Veterans (N=309) greater than or equal to 1 year post-SCI from the VA Boston Chronic SCI cohort who completed a health questionnaire and underwent spirometry at study entry. Baseline data were linked to 1996 through 2003 hospitalization records from the VA National Patient Care Database.
Interventions
Not applicable.
Main Outcome Measures
Cardiopulmonary hospital admissions, the predictors of which were assessed by multivariate Cox regression.
Results
Of 1478 admissions observed, 143 were a result of cardiopulmonary (77 circulatory and 66 respiratory) illnesses. Independent predictors were greater age (3% increase/y), hypertension, and the lowest body mass index quintile (<22.4kg/m2). A greater percentage-predicted forced expiratory volume in 1 second was associated with reduced risk. SCI level and completeness of injury were not statistically significant after adjusting for these risk factors.
Conclusions
Cardiopulmonary hospitalization risk in persons with chronic SCI is related to greater age and medical factors that, if recognized, may result in strategies for reducing future hospitalizations.
Key Words: Cardiovascular system, Hospitalization, Proportional hazards models, Rehabilitation, Respiratory system, Spinal cord injuries
List of Abbreviations: ASIA, American Spinal Injury Association, BMI, body mass index, CI, confidence interval, COPD, chronic obstructive pulmonary disease, Dx, diagnosis, FEV1, forced expiratory volume in 1 second, FVC, forced vital capacity, HR, hazard ratio, ICD-9-CM, International Classification of Diseases–9th Revision–Clinical Modifications, LOS, length of stay, SCI, spinal cord injury, VA, Veterans Affairs
THE ANNUAL INCIDENCE of SCI in the United States is an estimated 40 cases a million population or 11,000 new cases a year, and there are approximately 253,000 Americans living with chronic SCI.1 After rehabilitation, persons with SCI return to the community. Despite improvements in medical care, persons with chronic SCI require frequent hospitalization.2, 3 Compared with the development of a medical condition, hospitalization is a widely accepted measure of morbidity associated with a specific illness. Annual hospitalization rates of 25% to 40% were previously reported at 10 to 15 years postinjury in Department of VA hospitals in the 1970s and 1980s.4 Annual hospitalization rates for patients with SCI in non-VA systems are less certain because of incomplete data, but were approximately 25% among persons 5 to 20 years after injury in US SCI Model Systems hospitals.5 Despite the high rates of hospitalization, there has been little research conducted assessing risk factors for specific causes of hospitalization in this population. Circulatory and respiratory system illnesses are leading causes of death in persons with SCI,4, 6 but risk factors for hospitalizations related to these illnesses have not been described.
Since 1994, as part of the VA Boston SCI Cohort Study, we have regularly assessed the health of a cohort of persons with chronic SCI.7 Comprehensive data on personal characteristics, including respiratory health and comorbid illness, were collected by questionnaire, spirometry was obtained, and a neurologic examination at study entry was conducted. Previous studies of hospitalization risk have relied only on administrative data and have lacked information on personal risk factors. In this report, we link baseline personal and clinical information on the study cohort with hospitalization records to assess prospective risk factors for circulatory and respiratory system–related hospitalizations at VA Medical Centers. Our findings suggest ways of reducing these admissions to decrease health service use as well as disruptions to community reintegration and functional independence arising from frequent hospital stays.
Methods
Study Population
Between October 1994 and December 2002, as part of a longitudinal health study conducted at VA Boston, we enrolled 328 veterans with SCI who had received treatment at VA Boston. Persons who had other neurologic conditions or who had recovered from the SCI were ineligible. Because the study was designed to assess pulmonary function in chronic SCI, persons with a tracheostomy or requiring mechanical ventilation were also excluded. Because pulmonary function steadily improves in the first year after acute injury, participants were recruited at 1 or more years post-SCI. Recruitment and baseline assessment methodology have been previously presented.7 Of the 328 enrollees, 3 died before October 1, 1996; 8 had recovered or did not undergo examination; 2 had no data on all baseline variables; and 6 did not complete pulmonary function testing. We included 309 veterans (94%) with complete data on key variables. Approval was granted by our Institutional Review Boards, and informed consent was obtained.
Hospitalization and Service Eligibility Data
Hospitalization in an acute care VA medical center was determined using VA electronic databases from October 1, 1996, through December 31, 2003, to allow at least 1 year of follow-up. It was not possible to include hospitalization data before October 1996 because of nonuniform reporting standards. We extracted hospitalization data from the VA National Patient Care Database located in Austin, Texas. Each medical record includes an admitting diagnosis, up to 9 secondary diagnoses, and an LOS Dx, the condition responsible for the length of hospital stay. Diagnosis codes follow the ICD-9-CM. Information regarding eligibility for VA benefits8 and Medicare was extracted from VA records in 2006. Given chronic disability, it was assumed that the entitlement status of participants remained stable.
Main Outcome
Starting in October 1, 1996, we calculated the number of days to hospital admission for any cause and from discharge to the next admission. Outcomes based on primary admitting Dx were circulatory (ICD-9-CM 390-459) or respiratory system (ICD-9-CM 460-519) admissions (defined here as cardiopulmonary). There was little difference between primary admitting Dx and LOS Dx with 96% concordance overall when considered in ICD-9-CM categories, and 87% concordance for cardiopulmonary hospitalization. Admissions for other causes were considered censored events. The hospital discharge summaries from a random sample of cardiopulmonary admissions (30%) were reviewed and primary admitting Dx validated in 97%.
Predictors
Factors considered at study entry included sociodemographics, comorbid illnesses, respiratory symptoms, personal behaviors, measures of pulmonary function, and BMI (table 1). Age and injury duration were updated at the start of each observation period (ie, at the end of each hospitalization). Based on the American Thoracic Society adult respiratory questionnaire,7 chronic cough was defined as cough occurring on most days for greater than or equal to 3 consecutive months of the year, and chronic phlegm was described similarly. Any wheeze was defined as wheezing with a cold, or occasionally apart from colds, or wheezing on most days or nights. Persistent wheeze was wheezing on most days or nights, or with a cold and occasionally apart from colds. Pneumonia was defined as a history of pneumonia since hospital discharge after SCI. Heart disease was defined as any heart condition requiring treatment in the past 10 years. Participants were asked whether a physician had ever diagnosed asthma, emphysema, or chronic bronchitis, diabetes, or hypertension. COPD included emphysema or chronic bronchitis. Self-reports of these illnesses were previously validated by examining their electronic medical records.6
Table 1. Baseline Characteristics
| Variables | Cardiopulmonary Admission (N=75) | No Cardiopulmonary Admission (N=234) | Total (N=309) |
|---|---|---|---|
| n (%) Died 1996–2003 | 26 (35%) | 36 (15%) | 62 (20%) |
| Sociodemographics | |||
 Age, mean ± SD (y) | 60.1±13.2 | 52.4±14.0 | 54.3±14.2 |
| Men (%) | 100.0 | 98.3 | 98.7 |
| White (%) | 96.0 | 94.0 | 94.5 |
| Married (%) | 61.3 | 47.4 | 50.8 |
| Employed or student (%) | 17.3 | 26.5 | 24.3 |
| Educated beyond high school (%) | 48.0 | 58.6 | 56.0 |
| Enrolled in Medicare (%) | 25.3 | 31.6 | 30.1 |
| 50% to 100% VA service eligibility (%) | 41.3 | 39.7 | 40.1 |
| Health behaviors | |||
| BMI, mean ± SD (kg/m2) | 25.9±5.0 | 27.3±5.4 | 27.0±5.3 |
| Underweight (<18.5) (%) | 5.3 | 2.6 | 3.2 |
| Normal (18.5 to <25) (%) | 38.7 | 32.1 | 33.7 |
| Overweight (25 to <30) (%) | 33.3 | 38.5 | 37.2 |
| Obese (≥30) (%) | 22.7 | 26.9 | 25.9 |
| Smoking status (%) | |||
| Never smoked | 25.3 | 27.8 | 27.2 |
| Ex-smoker | 50.7 | 42.7 | 44.7 |
| Current smoker | 24.0 | 29.5 | 28.1 |
| Alcohol use (%) | 85.3 | 90.2 | 89.0 |
| Alcohol consumed in preceding year, mean ± SD (kg) | 2.23±5.49 | 4.88±9.20 | 4.24±6.95 |
| Lifetime alcohol consumption (kg-y⁎) median (quartile 1,3) | 257 (69, 840) | 223 (63, 578) | 224 (64, 647) |
| Injury characteristics | |||
| Severity/level (%) | |||
| Cervical motor complete and ASIA C | 40.0 | 24.3 | 28.2 |
| Thoracic motor complete and ASIA C | 42.7 | 43.2 | 43.0 |
| All ASIA D | 17.3 | 32.5 | 28.8 |
| Duration of SCI, mean ± SD (y) | 27.6±15.2 | 18.3±13.1 | 20.0±13.9 |
| Traumatic (%) | 89.3 | 89.3 | 89.3 |
| Locomotive mode >50% of time (%) | |||
| Motorized wheelchair | 21.3 | 15.4 | 16.8 |
| Hand-propelled wheelchair | 65.3 | 57.3 | 59.2 |
| Walks with or without aid | 13.3 | 27.4 | 24.0 |
| Respiratory symptoms (%) | |||
| Any wheeze | 52.0 | 50.0 | 50.5 |
| Persistent wheeze | 25.3 | 20.9 | 22.0 |
| Chronic phlegm | 24.0 | 23.5 | 23.6 |
| Chronic cough | 26.7 | 18.0 | 20.1 |
| Dyspnea | 14.7 | 9.0 | 10.4 |
| Pulmonary function, mean ± SD | |||
| FVC % predicted | 68.0±18.5 | 78.1±17.5 | 75.7±18.3 |
| FEV1 % predicted | 67.2±19.7 | 78.0±18.4 | 75.4±19.3 |
| Comorbid illnesses (%) | |||
| COPD | 10.7 | 9.0 | 9.4 |
| Asthma | 5.3 | 9.4 | 8.4 |
| Pneumonia | 26.7 | 17.1 | 19.4 |
| Hypertension | 44.0 | 29.9 | 33.3 |
| Diabetes mellitus | 12.0 | 15.0 | 14.2 |
| Heart disease | 16.0 | 10.7 | 12.0 |
⁎One kilogram-year (kg-y) = 1½ beers or 1¾ wine glasses or 1¼ shots of liquor a week for a year. |
Smoking intensity and duration and beverage-specific alcoholic consumption were reported. Smokers had smoked greater than or equal to 20 cigarette packs in a lifetime or greater than or equal to 1 cigarette a day for greater than or equal to 1 year. Current smokers reported cigarette use within 1 month of testing. Quantity of wine, beer, or liquor consumed was expressed as grams of alcohol.9 Participants were weighed using a wheelchair scale (subtracting wheelchair weight if required) and transferred to a thin mat where supine length (stature) was measured. If measurement was declined or there were joint contractures that precluded accurate assessment (n=66 [21.4%]), self-reported height was used. Weight was measured in 280 (90.6%) study participants and obtained from self-report in 24 (7.8%) and from the medical record of a recent clinic visit in 5 (1.6%). Height (stature) and weight were used to compute the BMI, which was classified into underweight (BMI <18.5), normal weight (BMI = 18.5 to <25), overweight (BMI = 25 to <30), and obese (BMI ≥ 30kg/m2) and was also considered in quintiles. Spirometry was based on American Thoracic Society standards modified for use in SCI as described previously.10 The best FEV1 and FVC were reported, and 93% of the cohort had at least 3 acceptable expiratory efforts with the best FEV1 and FVC within 200mL. Predicted FEV1 and FVC were calculated using Hankinson equations as we previously described.7 We assessed SCI severity/level by the ASIA criteria as previously described. Motor incomplete SCI included ASIA C (most key muscles below the neurologic level grade <3/5) or ASIA D (most muscles grade ≥3/5). SCI severity/level was grouped into severe quadriplegia (cervical motor complete and cervical grade C), severe paraplegia (other thoracic or lower motor complete and grade C), and others (all ASIA grade D).
Statistical Analysis
Proportional hazards methods for repeated outcomes (TPHREG procedure in Statistical Analysis System software version 9.1a) were used. Secular trends were tested by analyzing calendar year effect in a time dependent manner. Proportional hazards assumption was examined using survival plots and testing the interaction between covariates and the time variable, and confirmed that modeling was appropriate.
Results
Descriptive Characteristics
Participants with cardiopulmonary admissions were more likely to be older and have a lower BMI, a history of pneumonia after SCI, hypertension, heart disease, chronic respiratory symptoms, reduced lung function, and a greater mortality (35%) than persons without cardiopulmonary admissions (15%, P<.001; see table 1). Total time at risk was 1619 person-years, or an average of 5.24 person-years a participant, and 17% of all hospitalizations occurred less than or equal to 30 days after a prior admission. There were 1478 all-cause hospitalizations among 251 participants for an admission rate of 0.91 a person-year, and 143 cardiopulmonary hospitalizations among 75 persons for a rate of 0.09 a person-year. There were 77 admissions for circulatory system diseases (5.2%), and 66 admissions for respiratory system diseases (4.5%). The median (quartile 1, quartile 3) hospital stay for cardiopulmonary admissions was 9 (4, 19) days, and 5 (2, 15) days for noncardiopulmonary admissions.
Univariate Models
Predictors of cardiopulmonary admissions were assessed with and without adjustment for age and hospital readmission within 30 days (table 2). Adjusted for age and hospital readmission, persons with severe quadriplegia had a risk of cardiopulmonary hospitalization that was of borderline significance (P=.06; HR=1.89; 95% CI=0.99–3.63) compared with persons with ASIA D SCI, and for persons with severe paraplegia, the risk was not significantly elevated (P=.17; HR=1.60; 95% CI=0.82–3.14). VA service connected disability at greater than or equal to 50%, if in the lowest BMI quintile (compared with other quintiles), and each chronic respiratory symptom, pneumonia, and hypertension were risk factors for cardiopulmonary admission, while persons with greater pulmonary function were significantly less likely to be admitted. Adjusting for age and hospital readmission, current smokers were significantly more likely to be admitted for cardiopulmonary causes, and persons enrolled in Medicare were less likely to be admitted. Wheelchair use, lifetime alcohol consumption or alcohol consumption in the year before study entry, history of diabetes, asthma, COPD, employment status, educational or marital status, and calendar year of admission were not significant risk factors.
Table 2. Hazard Ratios for Cardiopulmonary Hospitalization⁎
| Variable | Unadjusted (95% CI) | Adjusted† (95% CI) |
|---|---|---|
| Age | 1.03⁎(1.01–1.06) | 1.03⁎(1.00–1.05) |
| Married | 1.39(0.77–2.52) | 1.02(0.65–1.62) |
| Currently employed or student | 0.66(0.36–1.22) | 1.04(0.51–2.11) |
| Education level: beyond high school | 0.85(0.49–1.47) | 0.85(0.52–1.37) |
| Enrolled in Medicare | 0.67(0.37–1.21) | 0.58⁎(0.35–0.94) |
| VA service eligibility: ≥50% service connected | 1.77⁎(1.06–2.96) | 1.72⁎(1.10–2.67) |
| Calendar year | ||
| 2002–2003 | 1.48(0.77–2.82) | 0.99(0.52–1.90) |
| 2000–2001 | 0.95(0.55–1.65) | 0.75(0.43–1.31) |
| 1998–1999 | 1.47(0.88–2.47) | 1.18(0.73–1.91) |
| 1996–1997 | 1.00 | 1.00 |
| BMI | ||
| Lowest BMI quintile (BMI <22.43kg/m2) | 2.21⁎(1.17–4.18) | .23⁎(1.32–3.77) |
| Quintiles 2–5 | 1.0 | 1.0 |
| Cigarette smoking | ||
| Current smoker | 1.39(0.63–3.04) | 1.92⁎(1.04–3.56) |
| Ex-smoker | 1.48(0.83–2.65) | 1.42(0.86–2.36) |
| Never smoker | 1.0 | 1.0 |
| Alcohol consumption | ||
| Alcohol (kg) consumed in preceding year | 0.98(0.95–1.02) | 0.99(0.97–1.01) |
| Lifetime alcohol consumption (kg-y‡) | 1.00(1.00–1.00) | 1.00(1.00–1.00) |
| SCI level/severity | ||
| Cervical motor complete and ASIA C | 1.88(0.96–3.69) | 1.89(0.99–3.63) |
| Thoracic motor complete and ASIA C | 1.69(0.81–3.54) | 1.60(0.82–3.14) |
| All ASIA D | 1.0 | 1.0 |
| Duration of SCI | 1.04⁎(1.02–1.05) | 1.02⁎(1.01–1.04) |
| Locomotive mode >50% of time | ||
| Motorized wheelchair | 1.37(0.59–3.14) | 1.33(0.59–2.99) |
| Hand-propelled wheelchair | 1.79(0.85–3.79) | 1.63(0.80–3.30) |
| Walks with or without aid | 1.00 | 1.00 |
| Respiratory symptoms | ||
| Any wheeze | 1.40(0.83–2.39) | 1.55⁎(1.01–2.39) |
| Persistent wheeze | 2.20⁎(1.17–4.15) | 1.84⁎(1.04–3.25) |
| Chronic cough | 2.44⁎(1.34–4.43) | 2.17⁎(1.31–3.60) |
| Chronic phlegm | 2.10⁎(1.14–3.87) | 1.76⁎(1.00–3.08) |
| Pulmonary function | ||
| FEV1 % predicted | 0.97⁎(0.95–0.99) | 0.97⁎(0.96–0.99) |
| FVC % predicted | 0.97⁎(0.96–0.99) | 0.97⁎(0.96–0.98) |
| Comorbid illnesses | ||
| COPD | 0.66(0.34–1.28) | 0.62(0.35–1.12) |
| Asthma | 1.15(0.35–3.81) | 1.23(0.48–3.15) |
| Pneumonia | 2.22⁎(1.14–4.30) | 1.94⁎(1.09–3.47) |
| Hypertension | 2.17⁎(1.27–3.72) | 1.86⁎(1.08–3.20) |
| Diabetes | 0.98(0.47–2.05) | 0.81(0.41–1.58) |
| Heart disease | 2.19⁎(1.14–4.20) | 1.52(0.90–2.56) |
⁎Statistical significance (ie, P<.05). |
†Adjusted for age, and readmission in ≤30 days. |
‡One kilogram-year (kg-y) = 1½ beers or 1¾ wine glasses or 1¼ shots of liquor a week for a year. |
Multivariable Model
In a multivariate model, significant predictors of cardiopulmonary hospitalization risk were greater age (3% increase a year), a history of hypertension, and BMI in the lowest quintile (<22.4kg/m2), whereas a greater percentage-predicted FEV1 was associated with reduced risk (3% reduction/percentage-predicted FEV1; table 3). When SCI level was added to this model, there was no significant increase in hazard among persons with cervical motor complete and ASIA C SCI (P=.14; HR=1.62; 95% CI=0.85–3.10), nor among those with thoracic or lower motor complete and ASIA C SCI (P=.36; HR=1.32; 95% CI=0.73–2.39) compared with ASIA D SCI. The reduced risk attributable to greater pulmonary function was similar when FVC was included. Separate models of circulatory and respiratory system hospitalizations were also considered. The predictors for circulatory and respiratory system hospitalizations considered separately were similar. The only exception was the greater effect of hypertension on respiratory admissions (HR=2.69; 95% CI=1.47–4.90) in contrast to the effect of hypertension on circulatory admission risk (HR=1.18; 95% CI=0.73–1.90). The effects of age, hypertension, percentage-predicted FEV1, or if in the lowest BMI quintile on cardiopulmonary admission risk were similar and remained significant whether or not VA service–connected disability greater than or equal to 50% (P=.06; HR=1.52; 95% CI=0.98–2.38) or Medicare enrollment (P=.04; HR=0.60; 95% CI=0.37–0.97) were added to the final regression model.
Table 3. Multivariable Models⁎ of Cardiopulmonary Hospitalizations†
| Model | Cardiopulmonary HR (95% CI) (n=143) | Circulatory System HR (95% CI) (n=77) | Respiratory System HR (95% CI) (n=66) |
|---|---|---|---|
| Multivariate model | |||
| Age | 1.03†(1.01–1.05) | 1.05†(1.03–1.07) | 1.01(1.00–1.03) |
| Hypertension | 1.74†(1.12–2.75) | 1.18(0.73–1.90) | 2.69†(1.47–4.90) |
| FEV1 % predicted | 0.97†(0.96–0.98) | 0.99(0.98–1.00) | 0.96†(0.94–0.97) |
| Lowest BMI quintile | 1.79†(1.19–2.68) | 1.46(0.83–2.54) | 1.99†(1.13–3.51) |
| Each variable added | |||
| Any wheeze | 1.02(0.70–1.49) | 1.01(0.63–1.64) | 1.13(0.62–2.06) |
| Persistent wheeze | 1.08(0.71–1.65) | 0.86(0.50–1.49) | 1.32(0.70–2.49) |
| Chronic phlegm | 1.48(0.93–2.38) | 1.07(0.62–1.84) | 1.88(0.94–3.73) |
| Chronic cough | 1.54†(1.02–2.33) | 1.23(0.76–1.99) | 1.91(0.98–3.72) |
| Pneumonia | 1.53(0.99–2.38) | 1.25(0.69–2.28) | 1.52(0.89–2.61) |
| Each variable added in place of FEV1 | |||
| Any wheeze | 1.28(0.85–1.93) | 1.12(0.69–1.83) | 1.59(0.83–3.04) |
| Persistent wheeze | 1.44(0.91–2.30) | 0.98(0.57–1.67) | 2.07†(1.10–3.91) |
| Chronic phlegm | 1.62(0.99–2.66) | 1.10(0.64–1.90) | 2.24†(1.15–4.37) |
| Chronic cough | 1.70†(1.09–2.65) | 1.28(0.78–2.10) | 2.23†(1.14–4.36) |
| Pneumonia | 1.89†(1.18–3.01) | 1.31(0.73–2.33) | 2.47†(1.37–4.47) |
⁎Adjusted for any-cause readmission within ≤30 days. |
†Statistical significance (ie, P<.05). |
Respiratory Symptoms and Pneumonia
When added separately (see table 3) to the multivariate model, any wheeze, persistent wheeze, and cigarette smoking were not significant predictors of circulatory, respiratory, or cardiopulmonary hospitalization. Chronic cough and chronic phlegm were stronger predictors of respiratory than circulatory hospitalizations but were not statistically significant in either model, and overall they were of borderline significance in models including all cardiopulmonary hospitalizations. Effects of respiratory symptoms and of pneumonia were greater when measures of pulmonary function (FEV1 percentage-predicted) were excluded from the multivariate models, particularly for respiratory admissions (see table 3).
Heart Disease
When added to the multivariate model, a history of heart disease was a significant predictor of hospitalization for circulatory system causes, but not of respiratory diseases (table 4). Hypertension remained a stronger predictor of respiratory admissions than of circulatory system diseases.
Table 4. Assessment of Effects of Heart Disease on Cardiopulmonary Hospitalization⁎†
| Model | Cardiopulmonary HR (95% CI) (n=143) | Circulatory HR (95% CI) (n=77) | Respiratory HR (95% CI) (n=66) |
|---|---|---|---|
| Age | 1.03†(1.01–1.05) | 1.05†(1.03–1.07) | 1.01(0.99–1.03) |
| Hypertension | 1.66†(1.04–2.65) | 1.03(0.64–1.66) | 2.70†(1.47–4.94) |
| FEV1 % predicted | 0.97†(0.96–0.98) | 0.99†(0.98–1.00) | 0.96†(0.94–1.00) |
| Lowest BMI quintile | 1.77†(1.19–2.62) | 1.42(0.85–2.38) | 1.99†(1.13–3.52) |
| Heart disease | 1.40(0.84–2.32) | 1.92†(1.13–3.24) | 0.97(0.38–2.51) |
| Heart disease (excluding hypertension) | 1.66†(1.00–2.74) | 1.9†3(1.14–3.27) | 1.35(0.53–3.47) |
⁎Adjusted for any-cause readmission within ≤30 days. |
†Statistical significance (ie, P<.05). |
Discussion
Our study is important because persons with SCI have very high hospitalization rates, and our study is the first to assess risk factors for cardiopulmonary hospital admissions prospectively using merged administrative and baseline assessment data. Our results demonstrate that cardiopulmonary admissions in SCI are strongly related to modifiable risk factors as in the able-bodied population and include factors that are not unique to SCI. Pulmonary dysfunction is a common outcome of respiratory muscle paralysis, and both respiratory and cardiovascular diseases have previously been identified6 as leading causes of death in chronic SCI. Circulatory and respiratory system illnesses frequently present with similar symptoms and signs, and mortality risk factors often double as determinants of hospitalization,11 providing the basis for jointly examining risk factors for cardiopulmonary hospitalizations. We expected to find that persons with a greater degree of neurologic impairment would have a significantly greater risk of hospital admissions for cardiopulmonary disease because these persons are likely to have greater degrees of respiratory impairment7 and potentially have more cardiovascular disease risk factors such as a greater BMI. However, persons with cervical motor complete and ASIA C SCI (severe quadriplegia) had a risk of cardiopulmonary hospitalization that was of borderline significance after adjusting for age and readmission, and the risk was reduced further after accounting for other risk factors in a multivariate model. In this multivariate model, the risk for cardiopulmonary hospitalization was greater with increasing age, a history of hypertension, and BMI in the lowest quintile, and with lower pulmonary function, adjusting for rehospitalization within less than or equal to 30 days. Although there were some differences when circulatory and respiratory system admissions were considered separately, overall risk models were similar, and we were able to increase the power of the study and the precision of the risk estimates by considering both causes together.
Rather than restrict ourselves merely to investigating risk factors for the development of cardiopulmonary disease, we studied predictors of hospitalization, which is a more broadly acceptable measure of attributable morbidity. Frequent or avoidable admissions are costly to the health care system and disruptive to the rehabilitating patient's functional independence and community reintegration. Understanding the causes of hospitalization provides insight into the potential for its reduction. Our selection of predictor variables was based on a conceptual model of health care use suggested by Andersen and Newman12, 13 and on known relationships between these variables and cardiopulmonary diseases. Cardiopulmonary hospitalization may occur for a variety of risk factors both related and unrelated to the specific development of cardiopulmonary diseases. The Andersen model12, 13 considers determinants of health service use to include various predisposing and enabling factors unrelated to a specific medical condition as well as medical reasons for admission. In older adults, medical conditions have been found to be an important determinant of use,14 and risk factors for the development of cardiopulmonary disease may also serve as risk factors for worsening of the disease leading to hospital admission. In this cohort we were able to consider specific medical conditions and personal risk factors related to cardiopulmonary disease as well as pulmonary function and respiratory symptoms. Predisposing factors include various sociodemographic characteristics, and enabling factors relate to the logistical aspects of obtaining care including access to health care, income, and health insurance status. In this study all participants were eligible for VA care, but there was variation in priority and access based on service-connected status and Medicare enrollment.
Findings regarding SCI level and completeness of injury have been mixed in previous studies, and persons with tetraplegia are reported to have a greater risk when assessed within 10 years since injury.4, 15, 16, 17, 18, 19, 20 In contrast, our study included many persons assessed 10 or more years since injury when factors related to secondary medical conditions were important but SCI level and completeness of injury were not. All-cause hospitalization rate in our study (0.91/person-year) was also higher than in the Model Systems (0.38/person-year) 20 years post-SCI5 and greater than that previously reported in VA hospitals in the 1970s and 1980s (0.55/person-year).4 The causes of hospitalization, by organ system, in our study resembled those of Middleton et al,20 who found that 4.5% of readmissions were a result of respiratory illness and 4.8% a result of cardiovascular diseases among younger (mean age=37.8±17.4y), predominantly (78%) men with SCI. A previous study from the SCI Model System examined only 1-year risk for admission for any cause.15 There was a lesser risk associated with greater education, functional independence, and ability to ambulate independently.
Our results suggest that cardiopulmonary admission risk was greater if the participant had a VA service–connected disability of greater than or equal to 50% and was less likely if a participant was enrolled in Medicare. Veterans with service-connected disabilities rated by VA greater than or equal to 50% belong to the highest priority group8 and are most eligible for hospitalization benefits.21 These results indicate that although all persons with SCI in this cohort are veterans who were recruited at VA Boston and eligible for care, including hospitalization, the extent to which they are hospitalized for cardiopulmonary diseases in VA medical centers varied based on whether they had a VA service–connected disability greater than or equal to 50% and had Medicare benefits. However, adjustment for these factors did not change the contribution of the risk factors identified in our multivariate model, and further suggests that our results are generalizable to persons with SCI hospitalized elsewhere regardless of the degree of VA service disability or Medicare enrollment. Previous studies in persons with SCI suggest that social factors such as education15 are significant predictors, but our results highlight personal and medical variables as more important risk factors than most sociodemographics.
We assessed motorized and hand-propelled wheelchair use compared with others who walk as a risk factor for cardiopulmonary admission because persons who use these locomotive modes are likely to be less active compared with persons who walk. The activity level hypothesis in the literature suggests that hospitalization rates among persons with SCI are lower among persons with greater degrees of functional independence.15, 22 Immobility is a risk factor for pressure ulcers, and unemployed persons with SCI are reported to be at greatest risk for hospitalization partly because of a greater risk of developing pressure ulcers.15 We found no evidence for the activity level theory in this study, because employment status and mode of locomotion were not significant risk factors.
A greater level of pulmonary function was a significant protective factor against cardiopulmonary hospitalization in this study. This is consistent with observations in the able-bodied that reduced pulmonary function is a risk factor for respiratory, cardiovascular, and all-cause mortality.23 In our SCI cohort, previous analysis of deaths up to the year 2000 showed that greater levels of FEV1 was also a protective factor for mortality.6 Although respiratory symptoms were associated with pulmonary and overall cardiopulmonary hospitalization risk, with FEV1 included in the model, the effects of respiratory symptoms and previous pneumonia were reduced. Although respiratory symptoms have been related to cardiovascular and pulmonary mortality in the general population independent of lung function,24 and for any cause hospitalization independent of lung function in the general population,25 our data suggest that the assessment of pulmonary function in SCI is more important.
Reduced pulmonary function is a well described independent risk factor for cardiovascular mortality,26 and a lower FEV1 may increase a person's vulnerability to a cardiac or pulmonary illness. However, reduced pulmonary function is associated with greater values of circulating markers of systemic inflammation (C-reactive protein) in the able-bodied,27 and therefore may also be a marker for greater cardiovascular disease risk on this basis.
Not surprisingly, we found that a history of heart disease was a significant predictor of circulatory disease admissions. We unexpectedly found a history of hypertension to be a strong predictor of hospitalization with respiratory illness. An independent association between hypertension and Chlamydia pneumoniae infection has been suggested in some studies.28 Among the elderly population, hypertension has also been associated with reduced pulmonary function.29 We know of no prior report of hypertension as an independent risk factor for respiratory illness in chronic SCI.30
Although SCI is characterized by an increase in BMI and increase in adiposity, persons in the lowest BMI quintile at study entry had a significantly increased risk of cardiopulmonary hospitalization than persons with a greater BMI. We found that this increased risk was limited to those persons who had BMI near the lower normative range or who were underweight, and including persons who were only underweight provided similar results. Persons who were obese or overweight did not have an increased risk of cardiopulmonary hospitalization. Such an obesity risk factor paradox is also reported for morbidity and mortality outcomes in illnesses that may be associated with cachexia31 including heart failure,32 end-stage renal disease,33 malignancies,34 and acquired immunodeficiency syndrome.35 The cause of a greater cardiopulmonary hospitalization risk in persons with the lowest BMI in chronic SCI is uncertain, particularly because BMI was assessed at study entry often years before hospital admission.
Strengths and Limitations
A strength of this report is its prospective design and the identification of factors at study entry that were not considered in previous studies assessing hospitalization risk in chronic SCI. Although history of heart disease and hypertension were based on self-report, we previously validated the accuracy of self-reports of heart disease in 93% and those of hypertension in 80% of our cohort, which were concordant with clinical diagnoses in electronic charts.6 Another limitation is that data on hospitalizations outside VA medical centers were not available, and that nonveterans were not included. As noted previously, adjustment for degree of VA service–connected disability and Medicare enrollment did not change the results, suggesting that their relevance is not restricted to veterans with SCI. Employment status and locomotive mode might not accurately reflect the full spectrum of activity level, and it is possible that variables assessed at baseline changed over time. Random misclassification in baseline variables would decrease our ability to detect an effect on hospitalization risk, and a decrease in FEV1 over time compared with the baseline or further reduction in BMI could lead to underestimation of their true effect on hospitalization. We also acknowledge that BMI underestimates true adiposity in SCI, and it was not possible to measure stature and weight in all participants. Although our results indicate that the factors included in the final multivariate model are stronger predictors of cardiopulmonary admission than level and completeness of injury, it is possible that in a larger sample, a small residual effect of SCI level and completeness could be detected. When added to the final model, the HR for the effects of level and completeness of SCI decreased and its significance decreased, yet its point estimate remained positive.
Implications
Our findings have long-term implications for the medical care and follow-up of persons with chronic SCI, particularly as survival after SCI increases and medical conditions unrelated to the original injury occur. Previous studies have focused primarily on assessing hospitalization risk based on SCI level and completeness of injury, but our study suggests that cardiopulmonary admissions in chronic SCI are more strongly related to potentially modifiable risk factors. In addition, the strongest risk factors identified in our study are not unique to chronic SCI, but are conditions commonly addressed in the care of the able-bodied. In addition to SCI-related conditions that are more commonly addressed, we suggest that generalist and specialist physicians caring for these patients direct increased efforts at prevention of secondary medical conditions. The training of SCI specialized physicians might need to re-emphasize skills in managing chronic medical illnesses that have, until now, been viewed as necessary mostly for family physicians and general internists. Early identification and aggressive treatment of hypertension, heart disease, excessive weight loss, and reduced pulmonary function among persons with SCI might reduce circulatory and respiratory system–related hospitalizations.
Conclusions
In a multivariate model, independent predictors of cardiopulmonary hospitalization risk were greater age (3% increase a year), a history of hypertension, being in the lowest BMI quintile (<22.4kg/m2), and lower pulmonary function (3% increase/unit decrease in percentage-predicted FEV1). Cardiopulmonary hospitalization risk in chronic SCI was more significantly associated with modifiable factors than with SCI level and severity at an average of 20 years postinjury.
Supplier
Acknowledgments
We are grateful to Kirby Matthess, BS, for her help with data collection, to Hongshu Guan, MS, PhD, of Channing Laboratory for help with statistical programming, and for the assistance of the VA Boston Healthcare System Spinal Cord Injury Service Staff and patients. The views expressed in this article are those of the authors and are not meant to represent the views of the Department of Veterans Affairs or the United States Government.
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Supported by the National Institutes of Health/National Institute of Child Health and Human Development (grant no. R01 HD42141; Garshick), and the Office of Research and Development, Health Services Research and Development, and Massachusetts Veterans Epidemiology Research Information Center Cooperative Studies Program, Department of Veterans Affairs.
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.
Reprints are not available from the author.
PII: S0003-9993(08)01605-5
doi:10.1016/j.apmr.2008.07.026
© 2009 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.
Volume 90, Issue 2 , Pages 193-200, February 2009
