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(Hypertension. 2000;35:48.)
© 2000 American Heart Association, Inc.
Scientific Contributions |
From Dipartimento Cardiotoracico (R.P., G.D., M.M.), Malattie del Metabolismo (G.P., S.B.), Medicina Interna (A.B., V.D.B.), Università di Pisa, Pisa, Italy.
| Abstract |
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15 µg/min) in nondiabetic subjects,
brachial blood pressure, echocardiographic left
ventricular mass, and other cardiovascular
and metabolic parameters were evaluated in 211
untreated males (38 normal controls, 109 uncomplicated stage 1 to 3
essential hypertensives, and 64 patients with clinically stable
atherosclerotic peripheral vascular disease either with
[n=44] or without [n=20] essential hypertension) with normal
cardiac and renal function. Compared with normoalbuminuric
subjects, microalbuminuric subjects (n=67) were characterized
by higher systolic blood pressure, comparable
diastolic blood pressure, and, therefore, wider pulse
pressure. Greater prevalence of hypertension, peripheral
vascular disease, left ventricular hypertrophy,
and reduced HDL cholesterol values further distinguished
microalbuminuric from normoalbuminuric subjects in
univariate comparisons. The risk of
microalbuminuria increased by ascending pulse pressure
quintiles in age-corrected logistic regression models, in which pulse
pressure was more predictive than systolic pressure and was
independent of mean pressure. When microalbuminuric status was
regressed against a series of dichotomous (vascular and active smoker
status) and continuous (age, pulse and mean pressure, left
ventricular mass index, and HDL and LDL
cholesterol) variables, only pulse pressure, left
ventricular mass index, and smoking status were independent
predictors. The association of increased albuminuria with
wider pulse pressure, a correlate of the pulsatile
hemodynamic load and conduit vessel stiffness as well
as an important cardiovascular risk factor, may explain
why microalbuminuria predicts
cardiovascular events in nondiabetic subjects. The
independence from concomitant vascular disease also suggests that wider
pulse pressure, rather than representing a simple marker
for atherosclerotic disease, influences albuminuria directly.
Key Words: albuminuria pressure hypertension, essential vascular diseases mass, ventricular
| Introduction |
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To specify to a better extent the biological covariates of microalbuminuria in nondiabetic subjects, we have evaluated cross-sectionally a series of cardiovascular and metabolic parameters in a large group of normal control subjects, subjects with essential hypertension (EH), and patients with atherosclerotic vascular disease.
| Methods |
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Clinical screenings were based on history, physical examination, routine tests, and neck and lower limb color-echo Doppler sonography in most subjects. All parameters reported in the present study were collected in a 2-week period.
Diagnosis of hypertension was based on casual blood pressure (BP) values >140/90 mm Hg and/or antihypertensive treatment at the first contact, and secondary forms of hypertension were excluded by routine examinations, including renal color-echo Doppler sonography and, if needed, angiography. If patients had received treatment, antihypertensive drugs were withdrawn 4 weeks before the study; no patient had ever been on lipid-lowering drugs. PVD was diagnosed by intermittent claudication (pain-free walking distance >200 m on a treadmill) confirmed by ankle-brachial pressure measurements <0.9 at one or both sides. According to institutional guidelines, subjects were aware of the investigational nature of the study and agreed to participate. The study was carried out in accordance with the Declaration of Helsinki, and the protocol was approved by the local Ethical Committee.
Experimental Procedures
UAE was measured by nephelometry (Behring; limit of detection
0.1 mg/dL, interassay variation coefficient 3.5%) on samples collected
from 8:00 PM to 8:00 AM during 3 consecutive
days. Sitting BP was measured in the early afternoon by an automated
oscillometric device (SpaceLabs 90207) throughout a 2-hour period. Wall
thickness and chamber diameters were determined at or just below the
mitral valve tips, by the leading edgetoleading edge method on
monodimensional and bidimensional echocardiograms (Hewlett Packard
Sonos 1000, with 2.5- and 3.5-MHz transducers) according to
recommendations of the American Society of
Echocardiography. Clinically significant aortic
insufficiency was excluded in all subjects by Doppler examinations.
Anthropometric measurements (height and weight) were made after each
participant had removed his shoes and upper garments. Blood samples
were obtained between 8:00 and 9:00 AM after an overnight
fasting and 15 minutes of supine rest. Total cholesterol,
HDL cholesterol, and triglycerides were
assessed by enzymatic colorimetric techniques
(Boehringer-Mannheim). Serum and urine creatinine
levels were assayed by standard colorimetric
methods.
Data Processing
UAE (µg/min) was the median of 3 consecutive overnight
collections (median variation coefficient 25%).
Microalbuminuria was defined as any UAE value
15
µg/min6 in the presence of a negative dipstick test for
urine protein. Systolic BP (SBP) and diastolic BP
(DBP) values were the mean of at least 10 recordings taken over
a 2-hour period. Pulse pressure (PP) was the arithmetic difference
between averaged SBP and DBP values. Mean BP (MBP) was DBP +1/3 PP. BMI
(body weight/squared surface area), creatinine clearance,
and LDL cholesterol [total cholesterol-(HDL
cholesterol+triglyceride/5)] were calculated
from standard formulas. Smoking status was defined as active smokers
versus nonsmokers, without distinction between former smokers and those
who have never smoked.
Left ventricular mass (Penn convention) was corrected for
height to derive the left ventricular mass index (LVMI,
g/m). Myocardial hypertrophy was defined as LVMI
143 g/m,
a partition value associated with all-cause mortality and sudden death
in population-based investigations.7 8 Stroke volume (SV)
was the difference between end-diastolic and
end-systolic left ventricular volume.
Statistics
The association of microalbuminuria (coded as
follows: 0, normoalbuminuria; 1,
microalbuminuria) with continuous and categorical
(including quintiles of PP) covariates was analyzed by logistic
regression (maximum likelihood method) using a backward stepwise
procedure (probability to remove, P<0.05) to identify the
independent regressors. Odds ratios (ORs, ie, the exponentiated
regression coefficients) were used to estimate relative risks and 95%
CIs. Intraindividual association of continuous variables was
evaluated by nonparametric Spearman rank correlation
coefficients. Differences among continuous variables were tested by
ANOVA covariated for age, and a multiple range test was used to
evaluate differences among means. Categorical parameters
were tested by Cochran-Armitage exact tests.
Data were reported as mean±1 SD or median and range in the presence of skewed data. Statistical significance was set at P<0.05 unless otherwise indicated.
| Results |
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Clinical Characteristics by Vascular Status
Irrespective of BP, subjects with arteriopathy were older and had
a more atherogenic lipid profile. Clinical characteristics of the study
subjects are shown in Table 3.
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Compared with patients with EH, patients with EH/PVD showed a greater frequency of microalbuminuria, higher UAE and SBP, lower DBP, wider PP, and similar MBP.
Cardiovascular parameters did not differ between normotensive PVD patients and controls.
Intraindividual Correlations
Age-related influences were most evident on creatinine
clearance (inversely, r=-0.44, P<0.001), PP
(r=0.40, P<0.001), and, in decreasing order, SBP
(r=0.23, P<0.001), UAE (r=0.20,
P<0.004), and LVMI (r=0.18, P<0.01).
Lipids and DBP were independent of age.
UAE was correlated with PP (Figure 1), SBP (r=0.38 for both, P<0.001), MBP (r=0.28, P<0.001), and, to a lower extent, DBP (r=0.18, P<0.02). Other statistically significant covariates of albuminuria were LVMI (r=0.21, P<0.01), total cholesterol (r=0.14, P<0.05), LDL cholesterol (r=0.21, P<0.01), and HDL cholesterol (inversely, r=-0.22, P<0.01).
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PP was closely correlated with SBP (r=0.75, P<0.001) and MBP (r=0.43, P<0.001) but not DBP (r=0.10). Systolic and diastolic values were also correlated (r=0.69, P<0.001). The rank correlation of PP and SBP with LVMI was 0.17 (P<0.01) and 0.33 (P<0.001), respectively.
Risk of Microalbuminuria by PP Quintiles
The proportion of subjects with microalbuminuria
(Figure 2, left panel;
P<0.001) and their UAE levels (Figure 2, right
panel; P<0.007) increased by increasing PP quintiles
(cutoff points 45, 50, 59, and 68 mm Hg; n=40, 39, 46, 43,
and 43).
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The age-corrected risk of microalbuminuria did not differ from 1 in the intermediate PP quintiles and increased by 3.4-fold (95% CI 1.24 to 9.3, P<0.03) and 5.3-fold (95% CI 1.9 to 15, P<0.003) in the fourth and fifth quintile, respectively (Figure 3). The trend did not change when PVD patients were excluded from the analysis (cutoff points 44, 50, 54, and 60 mm Hg; n=29, 28, 30, 30, and 30; OR of the upper quintile versus baseline 5.8; 95% CI 1.41 to 24; P<0.02, n=147).
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Pulsatile and Steady BP Components and Risk of
Microalbuminuria
The relative importance of PP, SBP, DBP, and MBP in predicting
microalbuminuria was addressed in logistic regression
models, including BP parameters alone and in combination.
Because age was an univariate predictor of
microalbuminuria risk (OR 1.39/10 years, 95% CI 1.05 to
1.56, P<0.01), all analyses included age as a
covariate.
PP, as such, was associated (P<0.001) with a 62% increase in risk for every 10 mm Hg rise (95% CI 1.35 to 1.9) compared with a 41% increase for every 10 mm Hg rise in SBP (95% CI 1.22 to 1.59, P<0.001). When PP and SBP were combined, PP remained a significant (P<0.03) predictor of microalbuminuria (OR/10 mm Hg increase 1.43, 95% CI 1.03 to 1.83), and SBP did not (OR/10 mm Hg increase 1.17, 95% CI 0.99 to 1.45, P<0.2). The risk associated with PP (OR/10 mm Hg increase 1.55, 95% CI 1.26 to 1.85, P<0.001) was independent of MBP (OR/10 mm Hg increase 1.17, 95% CI 0.99 to 1.5, P<0.2); when evaluating SBP and DBP in the same model, higher SBP (OR/10 mm Hg increase 1.61, 95% CI 1.34 to 1.89, P<0.001) and lower DBP (OR/10 mm Hg decrease 0.096, 95% CI 0.092 to 0.099, P<0.03) predicted microalbuminuria.
Multivariate Components of the Risk of
Microalbuminuria
Among the dichotomous (vascular and active smoker status) and
continuous (age, PP, MBP, LVMI, and HDL and LDL
cholesterol) variables that distinguished
(P<0.1) microalbuminuric from
normoalbuminuric subjects in one-to-one comparisons, only PP
(OR/10 mm Hg rise 1.6, 95% CI 1.3 to 1.9, P<0.001),
LVMI (OR/10 g/m 1.14, 95% CI 1.03 to 1.25, P<0.02), and
smoking (OR 1.97, 95% CI 1.001 to 3.98, P=0.05) predicted
microalbuminuria independently (Figure 4). The overall percentage of deviance
explained by the model was 18%.
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| Discussion |
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60 mm Hg. A nonlinear trend implies that a threshold
exists above which the renal impact of that hemodynamic
factor becomes more evident and, reciprocally, that its influence may
be obscured by other biological mechanisms at lower PP levels. PP also
provided information additional to SBP, in spite of the strict
collinearity between the 2 parameters. Risk of
microalbuminuria was predicted by both lower DBP and higher
SBP entered in the same logistic regression model, indicating that PP
compounded the exaggerated rises in the latter and the less than
proportional increments or actual decrements in the former.
Determinants of PP include SV, rate of systolic ejection, and
stiffness of the arterial tree.9 10 Because SV
measurements did not vary across the spectrum of UAE values, increased
arterial stiffness was a more likely cause of the wider PP
in our microalbuminuric subjects, probably because of
arteriosclerotic degeneration of the major
capacitance arteries, although other mechanisms cannot be ruled out on
the basis of our data.11 An obvious question regards confounders, primarily atherosclerosis, a disease in which segmental macrovascular lesions and diffuse microvascular abnormalities tend to coexist,12 affecting about one third of the components of our sample. Atherosclerosis may also increase UAE by damaging diffusely vascular endothelium,13 but our finding of a normal UAE in the group of normotensive patients with a fully developed arterial lower limb disease, in itself a harbinger of widespread macroangiopathy,14 does not support that prediction. It was only in the presence of an exaggerated PP, a likely result of the stiffening of large arteries through collagen neoformation and excess calcium deposition at the affected sites,11 15 that the prevalence of microalbuminuria increased strikingly. Our data, therefore, suggest an only indirect relation between atherosclerotic macroangiopathy and renal microvascular changes, with elevated BP, particularly its pulsatile elements, acting as the chain in the link; however, our data do confirm that subclinical kidney damage is frequent in EH that is complicated by PVD.3 16 The increasing prevalence of microalbuminuria by ascending PP quintiles in uncomplicated subjects analyzed separately from atherosclerotic patients and the overall results of the multivariate logistic regression analyses further discount a direct link between advanced atherosclerosis and albuminuria, in agreement with data from our17 and other18 laboratories. This negative conclusion contrasts with the inferences drawn from the results of previous cross-sectional observations of patients with atherosclerotic PVD,3 perhaps because uninterrupted antihypertensive treatment may have obscured pressure-related effects in that later study. Age-related confounding could also be hypothesized because, as expected,19 PP and, to a lesser extent, even UAE showed age-related changes, but demographic differences between normoalbuminuric and microalbuminuric subjects were not huge, and, besides, statistics accounted for this factor. Cardiovascular risk factors such as dyslipidemia, diabetes, and smoking may influence both UAE20 and conduit vessel distensibility,10 but total and LDL cholesterol values were evenly distributed, and HDL cholesterol, albeit reduced in the present and in a previous study20 of microalbuminuric subjects, did not emerge as a critical factor in the statistical analyses. Furthermore, diabetics were excluded from the trial, and the albuminuric action of smoking, already shown in past studies21 22 and confirmed in the present study, was additive to that of PP. Thus, the strength of statistical association, its persistence after adjusting for potential confounders, and inferential reasoning make plausible a direct connection between microalbuminuria and elevated PP. Similar results were obtained in the studies that examined the behavior of PP23 24 and, more frequently, SBP (a measure of vascular compliance25 and a variable strictly related to PP) as determinants of albuminuria in unselected populations26,27 and hypertensive patients.22,28 Regarding mechanisms, an excessive pulsatile load altered gluteal arteriolar structure,29 and the same may happen at the renal level, where wall hypertrophy characterized the nephrosclerotic biopsies harvested from mildly proteinuric EH patients.30 In light of the recent demonstration of the relation between microalbuminuria and future development of renal insufficiency in EH,2 it may be relevant, in this context, to consider the independent association between increasing serum creatinine and faster pulse-wave velocity, a measure of arterial stiffness, in EH patients.31 SBP also frequently overcame DBP as a predictor of overt kidney damage and end-stage renal disease,32 a clinical condition in which arterial stiffness is almost universal.33 Cross-sectional associations are to be taken with caution, though, and we are aware that our present cross-sectional findings might be compatible with exactly opposite interpretations, eg, an effect of urine albumin on PP. Prospective follow-ups of populations (less selected than ours and mainly composed of subjects referred to a tertiary diagnostic center) will be needed to evaluate in further detail any role of the pulsatile BP components on the kidney. The discrepancy between overnight measurement of urine and early afternoon BP recordings or gender-related phenomena in our all-male group should also be considered. Technical limits may also exist in that peripheral and central PPs differ to a variable extent,9 which is not a critical point, though, in view of the fact that differences in hemodynamic regimens are attenuated in middle-aged sedentary subjects.9
Because brachial PP determination predicts morbid events in prospective
cohorts of essential hypertensive subjects,34 35 our data
may help to understand better why microalbuminuria behaves
as a marker of cardiovascular risk in nondiabetic
hypertensive subjects. Moreover, the association with other primary
risk factors, such as increased LVMI and left ventricular
hypertrophy, decreased HDL cholesterol levels,
and tobacco smoking, legitimately qualifies
microalbuminuria as an integrated marker of
cardiovascular risk. The list of damaging factors
associated with the presence of microalbuminuria may,
however, be incomplete; as a matter of fact, additional mechanisms are
likely (eg, see References 21 and 3621 36 ), in view of the fact that our
logistic model left
80% of the total microalbuminuria
deviance unexplained. Furthermore, our cardiac mass data confirm the
correlation with UAE,37 an association usually interpreted
as a reflection of the prevailing pressor load on the kidney and the
heart. Quite unexpectedly, however, the statistical link between the 2
parameters was independent of pressure, a result also
recently reported by Gatzka et al.38 The data might
perhaps be explained by circulating factors that are produced in
greater amount by a hypertrophic heart39 and by an
increasing renal permeability to albumin.40 This
interesting possibility requires further evaluation in future
studies.
In conclusion, the association of increased albuminuria with wider PP, a correlate of the pulsatile hemodynamic load and conduit vessel stiffness, which is in itself an important cardiovascular risk factor, and left ventricular hypertrophy may help to explain why microalbuminuria predicts cardiovascular events in nondiabetic subjects. The independence from concomitant vascular disease also suggests that wider PP, rather than representing a simple marker for atherosclerotic disease, influences albuminuria directly.
| Footnotes |
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Received July 1, 1999; first decision July 16, 1999; accepted August 18, 1999.
| References |
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