Jenkins, Shona Margaret MacRae
Multi-slice computed tomography coronary angiography for the detection of coronary artery disease in a district hospital setting.
MD thesis, University of Glasgow.
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Coronary artery disease (CAD) is the leading cause of mortality in Scotland (population 5.2 million), accounting for around 9000 deaths each year. Accurate diagnosis of the presence and extent of CAD is essential to guide management. Invasive coronary angiography (I-CA) is the gold standard diagnostic investigation but is associated with a small risk of significant vascular complications. Over the last decade multi-slice computed tomography coronary angiography (MSCT-CA) has emerged as a non-invasive imaging modality capable of visualising the coronary arteries. Incremental advances in scanner technology have greatly improved the accuracy of MSCT-CA in comparison to I-CA.
Implementation of MSCT-CA in routine clinical practice in Scotland is desirable in terms of patient safety and convenience in addition to reducing pressure on cardiac catheterisation laboratory time. The latter is particularly relevant considering the recent introduction of primary percutaneous intervention for myocardial infarction. However, at the time of conducting this study the evidence for MSCT-CA accuracy was limited and only minimal guidance on appropriate use of MSCT-CA was available. Furthermore, the majority of existing evidence for MSCT-CA accuracy was derived from specialist academic centres with substantial experience in the technique and the accuracy of MSCT-CA in smaller centres with variable expertise and a more heterogeneous population was unknown.
The aim of this prospective, comparative study was to determine the accuracy of MSCT-CA in comparison to I-CA for the detection of significant CAD in patients presenting to a district general hospital in Scotland and to consider the health economic implications of introducing MSCT-CA into routine clinical practice.
One hundred patients with suspected CAD on the basis of symptoms and non-invasive stress testing underwent both 40-Slice MSCT-CA and I-CA. Studies were reported by independent, blinded radiologists and cardiologists and compared using the 15-Segment model of the American Heart Association. A stenosis of ≥ 50% was considered significant. The accuracy of MSCT-CA was determined in patient-based, artery-based and segment-based analyses and the impact of various patient characteristics on image quality and diagnostic accuracy was evaluated. Inter-observer agreement was determined for both MSCT-CA and I-CA and the possibility of a “learning curve effect” investigated. The cost-effectiveness of an “MSCT-CA first” strategy was considered in a health economic analysis.
The primary analysis considered MSCT-CA accuracy on a per patient basis. Patient prevalence of significant CAD was 38%. Patients with MSCT-CAs deemed not fully evaluable were included and considered to have significant underlying CAD. This strategy was considered clinically relevant as in practice a patient with an unevaluable MSCT-CA would proceed to I-CA for definitive diagnosis. This work demonstrated that 40-Slice MSCT-CA has a high sensitivity (92%) and a high negative predictive value (NPV) (91%) for the detection of significant CAD on a per patient basis. Specificity and positive predictive value (PPV) were less impressive and significantly compromised by the inclusion of patients with scans considered not fully evaluable by MSCT-CA. On segment-based and artery-based analyses respectively, distal segments were more often unevaluable than proximal segments owing to their smaller size, and the right coronary and circumflex arteries were the arteries most often unevaluable, likely due to their higher mobilities.
Heart rate during MSCT-CA was not optimally controlled with oral beta blockers and rate limiting calcium channel blockers. The mean number of MSCT-CA evaluable segments per patient was significantly higher in the lower heart rate group. More than half the study population had coronary artery calcification and there was a non-significant trend towards more unevaluable segments in this patient group. Coronary artery calcification had the effect of reducing NPV while increasing PPV. One third of the study patients were obese and a non-significant trend towards an increasing number of unevaluable segments with increasing body mass index was observed.
Pre-test probability of significant underlying CAD was determined by the Duke Clinical Score. Fifty-nine per cent of patients had a low-intermediate pre-test probability and 41% a high pre-test probability of CAD. The prevalence of significant CAD in the high pre-test probability group and the low-intermediate pre-test probability group was 73% and 14% respectively. Correspondingly, the sensitivity and PPV of MSCT-CA were higher in the high pre-test probability group while specificity and NPV were lower.
Inter-observer agreement of the MSCT-CA reporters was substantial and comparable to that of the I-CA reporters in the patient-based analysis. A small observed improvement in MSCT-CA reporter diagnostic specificity during the study was not statistically significant.
This study demonstrated MSCT-CA to be cost effective in the detection of significant CAD in a patient population with low-intermediate pre-test probability and hence fairly low prevalence of disease. Savings would be increased with improved MSCT-CA specificity. A strategy of screening patients being considered for I-CA on the basis of their risk level and referring ‘low-intermediate risk’ cases for MSCT-CA could affect around 60% of patients currently referred for diagnostic I-CA in North Glasgow and subsequently avoid I-CA in at least half of these patients.
To permit the development of an effective MSCT-CA service future work must focus on ensuring appropriate training for those performing and reporting MSCT-CA and on the development of local guidelines to govern patient selection for MSCT-CA. Audit of MSCT-CA referrals could determine the extent of adherence to guidelines. Further research could be observational in nature with follow-up of patients who have MSCT-CA and are then referred for I-CA and also follow-up of patients with “negative” MSCT-CA who do not have subsequent I-CA in terms of subsequent cardiac events.
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