Introduction

Patients with chronic kidney disease are at significantly increased risk for both morbidity and mortality from cardiovascular disease,^1,2.. Cardiovascular disease accounts for at least 50% of deaths in end stage renal disease ³. The increase in cardiovascular risk associated with CKD may be due to several mechanisms, including age, haemodynamic overload due to hypertension and anaemia, left ventricular hypertrophy, dyslipidaemia, a high prevalence of diabetes mellitus and impaired glucose tolerance, prothrombin changes, neurohormonal overactivity, and divalent ion abnormalities.^{4, 5} More recently other factors known as ‘new or non-classic’ factors have been identified. These include homocysteine, inflammation, and oxidative stress^{6, 7}. This increased risk of cardiovascular mortality may exist in mild renal insufficiency ^8-10 and progresses with the severity of renal disease.^{11, 12}. ^{Dialysis patients are a high risk group for all cause death. The death rate for all US dialysis patients between 1996 and 1998 was 231/1000 patient years.13 Cardiac disease is the major cause of death in dialysis patients, accounting for approximately 45% of all-cause mortality.13 Sudden cardiac death may be implicated in 60% of all cardiac deaths in dialysis patients. In the USRDS database, ‘cardiac arrest; cause unknown ‘ accounted for 47% of all cardiac deaths and another 13% were attributed to arrhythmia.14 Prolonged QT interval, a measure of heterogeneity of vascular repolarization, has been linked with increased risk of sudden death in dialyzed patients and it may be more prolonged, after haemodialysis.15-17}

This cross sectional, prospective study was designed to study the QTc in patients with CKD stages 3 to 5 at first presentation before the commencement of renal substitution. The mean QTc of the patients was compared with that of age and sex matched control subjects. The association between the QTc and other factors was studied.

Materials and Methods

Chronic kidney disease patients attending the Nephrology Clinic of University of Nigeria Teaching Hospital, Enugu, a tertiary health institution were recruited for the study. The study was conducted between February 2003 and January 2004 after approval by the hospital’s Ethics Committee. Informed consent was obtained from each patient and control subject. Chronic kidney disease was defined as by the National Kidney Foundation Quality Outcome Initiative guidelines (K/DOQI)¹⁸. Patients were recruited if they were not on dialysis and had calculated glomerular filtration rate (GFR) < 60 ml/min/1.73m². No patient received medication known to affect the QT interval. The medical history, medications, height, weight, and blood pressure were documented. Venous blood samples were collected for haemoglobin concentration, serum electrolytes and creatinine assays. The biochemical assays were measured by standardized methods on autoanalyzer. A 12 lead electrocardiogram was recorded for each patient using Cardiette auto ruler electrcardiographic machine. The QT interval was measured manually by one observer using calipers on ECG enlarged on the same photocopier by a factor of 3. The QT interval was measured by first deflection of the QRS complex to the point of T-wave offset, defined by the return of the terminal T-wave to the isoelectric TP baseline. Each QT interval was corrected for patient’s heart rate using the Bazette formula: QTc = QT/square root of RR (sec).¹⁹ To measure renal function, estimated glomerular filtration rate (GFR) using the Cockcroft-Gault formula ²⁰ was used. A total of 148 consecutive patients were recruited out of which 91 completed the study. Thirty two control subjects matched for age and sex were also recruited for the study.

Statistical analysis

Results

The characteristics of the patients and control subjects are detailed in table 1.

Ninety-one patients (47 males and 44 females) and 32 control subjects (18 males and 14 females) completed the study. There was no difference in the ages (p = 0.176) and the BMI (p = 0.071) of the patients and control subjects.

All the control subjects had normal QTc interval of < 0.44, with a mean of 0.38 ±0.22 (range 0.30 to 0.42). Fifty four (59.3%) of the patients had QTc interval above the normal value of 0.44, (range 0.45 to 0.60). The mean QTc interval of the patients was 0.46±0.05. There was statistically significant difference between the mean QTc interval of the patients and the control subjects, p < 0.001. There was no difference in the mean QTc of the different stages of CKD (0.46±0.05, 0.45±0.06 and 0.46±0.05 respectively for CKD stages 3, 4 and 5), p = 0.707. Pearson and Spearman Rho bivariate correlation analysis of QTc and all the variables tested (age, gender, BMI, aetiology of CKD, SBP, DBP, Hb, CKD stage, serum calcium, serum phosphate, and calcium-phosphate product) showed that SBP, DBP and aetiology of kidney disease positively correlated with QTc while BMI and Hb showed negative correlation with QTc, Table 2. A linear regression analysis of the variables that showed association with QTc using QTc as the dependent variable showed that only haemoglobin predicted the presence of prolonged QTc, Table 3

The causes of chronic kidney disease in this study were chronic glomerulonephritis 26 (28.6%), diabetic nephropathy, 19 (20.9%), hypertensive nephrosclerosis 16 (17.6%), and unknown aetiology 20 (22.0%), Table 4. Of the 91 patients, 3 (3.3%), 21 (23.1%) and 67 (73.6%) were in stages 3, 4 and 5 respectively.

Figure 1

Table 1: Characteristics of the patients and control subjects

Figure 2

Table 2. Pearson and Spearman Rho bivariate correlation analysis of QTc and variables as appropriate

Figure 3

Table 3. Linear regression analysis of QTc and variables

Figure 4

Table 4. Aetiology of kidney disease in the study population

Discussion

Corrected QT interval in chronic kidney disease.

Factors related to the presence of prolonged QTc

Conclusion

This study has shown a high prevalence of prolonged QTc in predialysis CKD patients compared to normal control subjects in a single centre tertiary hospital in Nigeria. This abnormality was seen as early as CKD stage 3 and persisted through stages 4 and 5 CKD. The QTc abnormality in predialysis CKD in this study correlated with blood pressure, BMI, and haemoglobin concentration. Of all these only haemoglobin concentration predicted the presence of prolonged QT interval.

Limitations of the study

Acknowledments

The authors are grateful to Mrs. Dorothy Okorogu and Mrs. Rose Kalunta (ECG Technicians) for assisting with the electrocardiogram of the study population.