Haemoglobin S occurs when thymine is exchanged for adenine at the sixth codon of the β-globin gene. This exchange encodes valine instead of glutamine in the β-globin molecule. Haemoglobin S accounts for over 50% of all haemoglobin and is as high as 80–90% of the total haemoglobin in sickle cell anaemia.Reference De Baun and Vichinsky 1
HerrickReference Herrick 2 first described the characteristic sickle-shaped erythrocytes in 1910. It is prevalent in many parts of India, including Central India, where its prevalence in different communities ranges from 9.4 to 22.2%.Reference Shukla and Solanki 3 Cardiac complications are an important cause of the morbidity and mortality associated with this disease.
The chronic anaemia associated with sickle cell disease results in an increase in cardiac output with only a minimal increase in heart rate. The left ventricular stroke volume increases with significant dilation of the left ventricle,Reference Lester, Sodt, Hutcheon and Arcilla 4 and the degree of left ventricular dilation is closely linked to the degree of anaemia.Reference Grossman, Jones and McLaurin 5 The dilated left ventricle adapts to the increased wall stress by developing eccentric hypertrophy in which the wall is thickened and the myofibers are elongated. Eccentric hypertrophy allows the left ventricle to adapt to chronic volume overload by initially preserving diastolic compliance and maintaining normal filling pressures. Over time, progressive dilation leads to increased wall stress and an increase in left ventricular mass.Reference Gerry, Baird and Nicholas 6
The study of heart abnormalities associated with sickle cell anaemia has become essential because these cardiac alterations occur frequently in adulthood, but may also occur at a much earlier stage in sickle cell anaemia patients. This has been observed despite the current context of sickle cell anaemia care, in which improvements to health conditions have decreased mortality, especially in the first 5 years of life, thus leading to delayed disease-related mortality.Reference Hassell 7 These children have been increasingly able to reach adulthood, thus suffering the consequences of chronic cardiac damage. Overall, the search for better understanding of this issue has increasingly become more relevant.
The aim of this study was to determine echocardiographic changes in left ventricular function in children suffering from sickle cell disease in Mumbai, Western India.
Materials and methods
Study population
This prospective controlled study was conducted from 1 January, 2013 to 31 January, 2014 at our institute. The study comprised 48 cases of sickle cell anaemia and 30 non-anaemic controls (Hb>11 gm/dl) with normal haemoglobin and electrophoresis pattern. The control group patients were comparable in age and sex, free from any cardiovascular disorder and not taking any cardioactive drugs. Detailed clinical examination and investigations including haemogram, chest radiogram, and electrocardiogram were obtained in the study cases. Each patient underwent an echocardiographic study including M-Mode, two-dimensional, colour Doppler indices using a GE Vivid i echocardiographic machine (GE Medical Systems, Israel Ltd., Israel). Standard techniques were used to obtain the measurements in a quiet, wakeful, and non-sedated state, and all the measurements were made by only one cardiologist in Lokmanya Tilak Medical and General Hospital, Sion, Mumbai, while in a clinically stable state. The examination was conducted with the patient lying in supine position. Patients were crisis free for at least 2 weeks before the study.
Inclusion criteria
Diagnosed cases of sickle cell anaemia by haemoglobin electrophoresis under 18 years of age.
Exclusion criteria
Those who:
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∙ had received recent – in the preceding 3 months – blood transfusion;
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∙ had haemoglobinopathies other than sickle cell disease;
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∙ had known congenital or acquired cardiac or pulmonary diseases;
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∙ had electrocardiographic abnormalities that might affect the interpretation of the echocardiographic findings; or
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∙ had any medical conditions other than anaemia affecting their myocardial performance. Patients with inadequate acoustic windows were also excluded.
Echocardiographic measurement
The left atrial dimensions, left ventricular end-systolic, and left ventricular end-diastolic dimensions were measured by M-mode according to the recommendation of the American Society of Echocardiography. The left ventricular ejection fraction, fractional shortening, and left ventricular mass were calculated. The left ventricular mass and left ventricular mass index were calculated according to the formula described by Devereux et al.Reference Devereux, Alonso and Lutas 8
To record left ventricular inflow velocities, the pulsed-wave Doppler sample volume was placed at the level of the tips of the mitral valve leaflets in the apical four-chamber view where the highest peak velocity was recorded. Peak flow velocities of the left ventricular inflow in early diastole (E) and late diastole with atrial contraction (A) were measured. E/A velocity ratios were calculated for each cardiac cycle. The deceleration time was measured as the interval between the peak of the E wave and the point at which the descending segment of the E wave crosses the zero velocity line. Left ventricular isovolumic relaxation time was measured as the interval between aortic valve closure and the onset of mitral flow. The pulse wave Doppler Tei index was calculated.Reference Tei, Ling and Hodge 9 Tissue Doppler was performed from the four-chamber apical view. The peak mitral annular velocity (e') was measured using the echo machine calibration tool and electronic callipers. The E/e' ratio was used as a measure of global left ventricular diastolic function.
Each measurement was obtained for three cardiac cycles and their arithmetic average was obtained.
With continuous-wave Doppler, the maximum peak tricuspid jet velocity recorded from any view was used to determine the pulmonary artery systolic pressure with the simplified Bernoulli equation, with right atrial pressure assumed to be 10 mmHg. A tricuspid regurgitant jet velocity of 2.5 m/second or more, which corresponds to a systolic pulmonary artery pressure of 30 mm Hg, has been used to define elevated pulmonary artery pressure in patients with sickle cell disease.Reference Gladwin, Sachdev and Jison 10 , Reference Ataga, Sood and De Gent 11
Statistical analysis
Data were represented as means±standard deviation for continuous variables and as proportions for categorical variables. Comparisons of continuous variables between groups were created by independent Student’s t-test. For discrete variables, distribution between groups was compared with the χ2 test. The multivariate Pearson’s correlation coefficient was applied to determine the relationships of diastolic dysfunction parameter E/e' with pulmonary artery systolic pressure, age, haemoglobin level, and number of crises. All statistical analyses were carried out using the Statistical Packages for Social Sciences (SPSS Inc., Chicago, Illinois, United States of America) software version 11.0 and EPi-Info version 3.4. Statistical tests with two-tailed probability values <0.05 were considered statistically significant. A value of p<0.05 was considered statistically significant. The figure was constructed using the application GhaphPad Prism 5 (GraphPad Software, San Diego, California, United States of America).
Informed consent from the patients and/or their guardians was obtained.
Results
In the study cases, the age range was from 5 to 18 years, with the mean age of 11.19±03.63 years. There were 27 boys and 21 girls in the study group. On cardiovascular examination, mean heart rate was 90±11 beats/minute, which was slightly higher compared with controls, mean blood pressure was 92±12/ 62±7 mmHg. Mean haemoglobin level was 8.4±1.38 gm% in study cases (Table 1). Severe anaemia, haemoglobin levels under 7 gm%, was present in 26 (54%) cases. On chest X-ray, cardiomegaly was detected in 22 (46%) cases. The electrocardiographic changes of left ventricular hypertrophy were present in two (5%) cases.
Table 1 Demographic characteristics of sickle cell disease patients versus normal controls.
BP=blood pressure; Hb=haemoglobin; HR (bpm)=heart rate (beats per minute)
* Significant
** By the Student’s t-test
*** By chi square test
All the patients reported taking vitamin supplements with B complex vitamins. In all, five patients received hydroxyurea, none used deferasirox, and six children used prophylactic antibiotics. Almost all the patients were hospitalised at some point of time and received blood transfusions, exact details of which were not available.
Echocardiographic measurements in patients with sickle cell anaemia were compared with those of non-anaemic controls with a normal haemoglobin electrophoresis pattern (Table 2). Patients with anaemia (study cases) had higher left ventricular internal dimension in diastole, left atrial dimension in systole, and left ventricular mass in comparison with those who were non-anaemic (controls) with a normal haemoglobin electrophoresis pattern (p<0.001) (Table 2). On Doppler study, “E” (100.9±11.95, 89.30±2.73, p<0.001) and “A” (71.85±15.65, 42.33±2.38, p<0.001) wave amplitudes were higher in sickle cell anaemia cases as compared with the controls. The A wave height was increased and the E/A ratio was decreased, whereas the deceleration and isovolumetric relaxation times were prolonged (p<0.001), which is typically seen in slowed or impaired myocardial relaxation. In addition, by tissue Doppler, E/e' ratio was higher (p<0.001) (Table 2). A total of 11 patients (23%) had pulmonary artery systolic pressure >30 mmHg (Fig 1).
Figure 1 Prevalence of pulmonary artery hypertension in sickle cell patients.
Table 2 Comparison among variables between the study and control groups (LV structure, volumes, systolic and diastolic function.)
E/A=early diastole (E) and late diastole with atrial contraction (A); LA=left atrial dimension in systole; LVIDd=left ventricular internal dimension in diastole; LVIDs=left ventricular internal dimension in systole; LV E=mitral inflow velocity in early diastole; LV A=mitral inflow velocity in late diastole; IVRT=isovolumic relaxation time; DT=deceleration time; SV=stroke volume; FS=fractional shortening; EF=ejection fraction; LV=left ventricular; PASP=pulmonary artery systolic pressure; TR=tricuspid regurgitation; e'=peak mitral annular velocity
By Student’s t-test
* Significant
Systolic parameters showed no statistically significant difference between sickle cell anaemia patients and healthy controls (Table 2). Pulse wave Doppler left ventricular myocardial performance index – that is, the Tei index was slightly on the higher side.
Left atrial dimension in systole (mean 21.27±4.27 mm) and left ventricular internal dimension in diastole (mean 36.91±3.02 mm) were more in patients with haemoglobin levels under 7 gm% as compared with those with haemoglobin levels over 7 gm% (19.58±5.22, 33.96±5.39 mm). Although cardiac chambers were dilated, no statistically significant correlation was found between echocardiographic parameters, M-mode and Doppler, and severity of anaemia in the sickle cell patients (Tables 3 and 5).
Table 3 Echocardiographic and echo-doppler parameters of sickle cell disease patients versus controls among haemoglobin levels.
E/A=early diastole (E) and late diastole with atrial contraction (A); Hb=haemoglobin; LA=left atrial dimension in systole; LVIDd=left ventricular internal dimension in diastole; LVIDs=left ventricular internal dimension in systole; LV E=mitral inflow velocity in early diastole; LV A=mitral inflow velocity in late diastole; IVRT=isovolumic relaxation time; DT=deceleration time; LV=left ventricular; PASP=pulmonary artery systolic pressure; TR=tricuspid regurgitation
By the Student’s t-test
* Significant
Left atrial dimension in systole (mean 22.31±3.86 mm) and left ventricular internal dimension in diastole (mean 36.69±4.06 mm) were higher in patients above 10 years of age, as compared with those below 10 years of age (mean 17.37±4.73, 33.21±4.83 mm) (Table 4). Again, although cardiac chambers were dilated, echocardiographic parameters, M-mode and Doppler, showed no statistically significant correlation with age in the sickle cell anaemia patients (Table 5).
Table 4 Echocardiographic and echo-doppler parameters of sickle cell disease patients versus controls among different age groups.
E/A=early diastole (E) and late diastole with atrial contraction (A); LA=left atrial dimension in systole; LVIDd=left ventricular internal dimension in diastole; LVIDs=left ventricular internal dimension in systole; LV E=mitral inflow velocity in early diastole; LV A=mitral inflow velocity in late diastole; IVRT=isovolumic relaxation time; DT: deceleration time; LV=left ventricular; PASP=pulmonary artery systolic pressure; TR=tricuspid regurgitation
By Student’s t-test
* Significant
Table 5 Diastolic dysfunction parameter (E/e'): correlation with number of crises, PASP, age, and haemoglobin.
r=Pearson’s correlation coefficient; p=probability value, a Spearman correlation; PASP=pulmonary artery systolic pressure
* Statistically significant
A significant correlation of left ventricular E/e' ratio with frequency of crisis (Pearson’s correlation=0.3122*, p=0.03075*) was found but not with pulmonary artery systolic pressure (Table 5).
Discussion
Left ventricular systolic and diastolic function
Patients in the present study had Doppler evidence of a diastolic dysfunction. The systolic and diastolic dimensions and left ventricular muscle mass were increased compared with controls; however, systolic function remained normal. The A wave height was increased and the E/A ratio was decreased, whereas the deceleration and isovolumetric relaxation times were prolonged; this pattern is typically seen with slowed or impaired myocardial relaxation. Taken together, these changes indicate early diastolic dysfunction. Decreasing e', and, therefore, increasing E/e' ratio, values reflect a decrease in relaxation ability – that is, increasing stiffness – of the left ventricle. This was supported by the observed significant increase in the E/e' ratio in the study group.
The cardiovascular system is known to be stressed by chronic anaemia, recurrent small pulmonary artery occlusion, and myocardial haemosiderosis. Autopsy studies have revealed that right and left ventricular dilation is common in both children and adults.Reference Gerry, Bulkley and Hutchins 12 Large screening echocardiographic studies have indicated that left ventricular systolic function was preserved in the majority of sickle cell disease patients studied at rest,Reference Sachdev, Machado and Shizukuda 13 – Reference Martins, Mesquita, Cunha, Pinheiro and Romeo Filho 15 and that the presence of segmental wall motion abnormalities is uncommon. Left ventricular dysfunction has mainly been seen in older patients and in association with chronic conditions such as hypertension and renal disease.Reference Willens, Lawrence, Frishman and Strom 16
The almost normal systolic function observed in both the groups was consistent with the literature, independent of the echo parameters used.Reference Lester, Sodt, Hutcheon and Arcilla 4 , Reference Covitz, Espeland, Gallagher, Hellenbrand, Leff and Talner 14 , Reference Seliem, Al-Saad, Bou-Holaigah, Khan and Palileo 17 Ribera et alReference Ribera, Ribera, Koifman and Koifman 18 also demonstrated that sickle cell anaemia resulted in a volume-overloaded heart and a significant increase in left ventricular cardiac mass, both of which were proportional to the degree of anaemia. Despite these abnormal loading conditions, systolic function tends to be preserved. Lester et alReference Lester, Sodt, Hutcheon and Arcilla 4 concluded that the major echocardiographic abnormality in sickle cell anaemia children was enlargement of left heart chambers, which they correlated with the severity of anaemia.
Our study showed that, although there were significant chamber dilatations in sickle cell anaemia patients in comparison with controls, there were no significant correlations with other factors within sickle cell anaemia patients, a fact also observed in the study by Kilinc et al.Reference Kilinc, Acarturk and Kumi 19
Wali et alReference Wali, Venugopalan, Rivera and al-Lamki 20 showed that dilated cardiac chambers in sickle cell anaemia were neither associated with any abnormality in systolic or diastolic left ventricular function nor with significant pulmonary artery hypertension, which they explained by the less severity of sickle cell disease in Omani children.
Of note, in the present study, diastolic dysfunction showed a significant difference between the study group and the healthy control group, although there was no correlation with severity of anaemia within the study group. This observation indirectly supports the study by Seliem et al,Reference Seliem, Al-Saad, Bou-Holaigah, Khan and Palileo 17 who found that the main difference between thalassaemia and sickle cell disease patients was in the higher E wave and E/A ratio in thalassaemia patients and shorter diastolic filling period in sickle cell disease patients. Although it is understood that the haemodynamics in both diseases are similar in some aspects – that is, chronic anaemia leading to dilated ventricles with increased muscle mass – they differ in the basic pathophysiological process – that is, iron overload versus sickling – which may explain the difference in the diastolic abnormalities.
These abnormal patterns suggest an intrinsic myocardial abnormality in patients with sickle cell anaemia and may prove to be early markers of cardiac disease.
Myocardial performance index
In our study, pulse wave Doppler Tei index was slightly on the higher side. As systolic function was found to be preserved in these patients, the elevated left ventricular myocardial performance index most likely is explained by decreased diastolic function.Reference Ghaderian, Keikhaei, Heidari, Salehi and Azizi Malamiri 21
Pulmonary hypertension
Out of 48 sickle cell disease patients, 11 (23%) had pulmonary artery systolic pressure as indicated by tricuspid regurgitant jet velocity of over30 mmHg. A recent review of published studies comprising over 600 children indicated that the prevalence of a jet velocity of 2.5 m/second or more was about 30%.Reference Kato, Onyekwere and Gladwin 22 Another studyReference Minniti, Sable and Campbell 23 found only 11% prevalence of a jet velocity of over 2.6 m/second. Haemolysis and obliterative thrombosis are likely to be responsible for the development of pulmonary hypertension, as no correlation was found with diastolic dysfunction (Table 5).
Age and sickle cell disease
As diastolic dysfunction is much more prevalent in adults, the incidence of diastolic abnormalities should increase with the increase in the patient’s age. ZilbermanReference Zilberman, Du, Das and Sarnaik 24 et al found no correlations between the age and indices of cardiac enlargement, ventricular hypertrophy, or indices of ventricular stiffness. We had statistically significant chamber dilatations, but were not able to demonstrate a significant difference between Doppler indices of patients younger and older than 10 years in our study. This may be due to a number of factors, including the fact that the severity of anaemia, the number of previous transfusions received, and the number of crises differed in these two age groups. In addition, our study made comparisons between two paediatric age groups and not between paediatric and adult patients. In addition, we believe that it is not the degree of anaemia, but the basic pathophysiological process – that is, sickling – which affects cardiac function. Survival in a relatively asymptomatic state indicates less severity of disease in patients older than 10 years of age.
Conclusion
Our patients had Doppler evidence of a diastolic dysfunction, as compared with controls. Systolic and diastolic dimensions and left ventricular muscle mass were increased compared with the controls. As diastolic dysfunction showed no statistically significant correlation with either haemoglobin or age among sickle cell patients, we believe that sickle cell disease leading to microvascular dysfunction and microvasculopathy leading to ischaemia are likely to be responsible for cardiac dysfunction.
The present study and the study by Seliem,Reference Seliem, Al-Saad, Bou-Holaigah, Khan and Palileo 17 as discussed, suggest that an intrinsic myocardial abnormality due to sickle cell disease – sickle cardiomyopathy – plays a major role in diastolic dysfunction.
We conclude that the increased left ventricular stiffness found in our study, when compared with controls, might be due to fibrosis related to ischaemia caused by sickle cell disease in addition to wall hypertrophy.
Cox et alReference Cox, Soka and Kirkham 25 found that the tricuspid regurgitant jet velocity predicted future hospitalisations. Similarly, early detection of diastolic dysfunction, using simple non-invasive echocardiographic parameters, might also predict future hospitalisations and improve the morbidity and mortality statistics in sickle cell disease patients. Further studies with long-term follow-up are required to test this hypothesis.
Study limitations
The sample size of our study was limited and was evaluated for a short period of time (14 months), and these shortcomings should be considered in the interpretation of the results. Other potentially confounding factors such as body mass index, gender, and haemolysis indicators were not assessed. Furthermore, some diastolic indices such as pulmonary venous flow velocities were not part of the Doppler assessment of these young patients for technical reasons.
Further research is required to assess cardiac function in the long term in patients with sickle cell disease.
Acknowledgements
The authors would like to convey their particular thanks to Dr Vijay Babar, associate professor, preventive and social medicine department, Jawaharlal Nehru Medical College, Wardha, for helping them in statistical analysis.
Financial Support
This research received no specific grant from any funding agency, commercial, or not-for-profit sectors.
Conflicts of Interest
None.
Ethical Standards
An appropriate ethical standard was maintained throughout the study and approved by our institutional committee.
