Comparison of the National Early Warning Score in non-elective medical and surgical patients.

BACKGROUND: The National Early Warning Score (NEWS) is used to identify deteriorating patients in hospital. NEWS is a better discriminator of outcomes than other early warning scores in acute medical admissions, but it has not been evaluated in a surgical population. The study aims were to evaluate the ability of NEWS to discriminate cardiac arrest, death and unanticipated ICU admission in patients admitted to surgical specialties, and to compare the performance of NEWS in admissions to medical and surgical specialties. METHODS: Hospitalwide data over 31 months, from adult inpatients who stayed at least one night or died on the day of admission, were analysed. The data were categorized as elective or non-elective surgical or medical admissions. The ability of NEWS to discriminate the outcomes above in these different groups was assessed using the area under the receiver operating characteristic curve (AUROC). RESULTS: There were too few outcomes to permit meaningful comparison of elective admissions, so the analysis was constrained to comparison of non-elective admissions. NEWS performed equally well, or better, for surgical as for medical patients. For death within 24 h the AUROC for surgical admissions was 0·914 (95 per cent c.i. 0·907 to 0·922), compared with 0·902 (0·898 to 0·905) for medical admissions. For the combined outcome of any of death, cardiac arrest or unanticipated ICU admission, the AUROC was 0·874 (0·868 to 0·880) for surgical admissions and 0·874 (0·871 to 0·877) for medical admissions. CONCLUSION: NEWS discriminated deterioration in non-elective surgical patients at least as well as in non-elective medical patients.

Predicting outcome following colorectal cancer surgery using a colorectal biochemical and haematological outcome model (Colorectal BHOM).

AIM: To present a new biochemistry and haematology outcome model which uses a minimum dataset to model outcome following colorectal cancer surgery, a concept previously shown to be feasible with arterial operations. METHOD: Predictive binary logistic regression models (a mortality and morbidity model) were developed for 704 patients who underwent colorectal cancer surgery over a 6-year period in one hospital. The variables measured included 30-day mortality and morbidity. Hosmer-Lemeshow goodness of fit statistics and frequency tables compared the predicted vs the reported number of deaths. Discrimination was quantified using the c-index. RESULTS: There were 573 elective and 131 nonelective interventional cases. The overall mean predicted risk of death was 7.79% (50 patients). The actual number of reported deaths was also 50 patients (χ(2) = 1.331, df = 4, P-value = 0.856; no evidence of lack of fit). For the mortality model, the predictive c-index was = 0.810. The morbidity model had less discriminative power but there was no evidence of lack of fit (χ(2) = 4.198, df = 4, P-value = 0.380, c-index = 0.697). CONCLUSIONS: The Colorectal Biochemistry and Haematology Outcome mortality model suggests good discrimination (c-index > 0.8) and uses only a minimal number of variables. However, it needs to be tested on independent datasets in different geographical locations.

The development of a VBHOM-based outcome model for lower limb amputation performed for critical ischaemia.

BACKGROUND: VBHOM (Vascular Biochemistry and Haematology Outcome Models) adopts the approach of using a minimum data set to model outcome and has been previously shown to be feasible after index arterial operations. This study attempts to model mortality following lower limb amputation for critical limb ischaemia using the VBHOM concept. METHODS: A binary logistic regression model of risk of mortality was built using National Vascular Database items that contained the complete data required by the model from 269 admissions for lower limb amputation. The subset of NVD data items used were urea, creatinine, sodium, potassium, haemoglobin, white cell count, age on and mode of admission. This model was applied prospectively to a test set of data (n=269), which were not part of the original training set to develop the predictor equation. RESULTS: Outcome following lower limb amputation could be described accurately using the same model. The overall mean predicted risk of mortality was 32%, predicting 86 deaths. Actual number of deaths was 86 (chi(2)=8.05, 8 d.f., p=0.429; no evidence of lack of fit). The model demonstrated adequate discrimination (c-index=0.704). CONCLUSIONS: VBHOM provides a single unified model that allows good prediction of surgical mortality in this high risk group of individuals. It uses a small, simple and objective clinical data set that may also simplify comparative audit within vascular surgery.

POSSUM models in open abdominal aortic aneurysm surgery.

OBJECTIVES: This study evaluated the Physiological and Operative Severity Score for the enUmeration of Mortality and Morbidity (POSSUM), Portsmouth (P) POSSUM and Vascular (V) POSSUM. The primary aim was to assess the validity of these scoring systems in a population of patients undergoing elective and emergency open AAA repair. The secondary intention was in the event that these equations did not fit all patients with an aneurysm; a new model would be developed and tested using logistic regression from the local data (Cambridge POSSUM). METHODS: POSSUM data items were collected prospectively in a group of 452 patients undergoing elective and emergency open AAA repair over an eight-year period. The operative mortality rates were compared with those predicted by POSSUM, P-POSSUM, V-POSSUM and Cambridge POSSUM. RESULTS: All models except V-POSSUM (physiology only) showed significant lack of fit when predicting mortality after open AAA surgery. It was found that the locally generated single unified model (Cambridge POSSUM) could successfully describe both elective and ruptured AAA mortality with good discrimination (chi(2)=9.24, 7 d.f., p=0.236, c-index=0.880). CONCLUSIONS: POSSUM, V-POSSUM and P-POSSUM may not be robust tools for comparing mortality between populations undergoing elective and emergency open AAA repair as once thought. The development and successful validation of Cambridge POSSUM provides a unified model to describe both elective and emergency AAAs together and should be validated in other geographical settings.

The use of routine laboratory data to predict in-hospital death in medical admissions.

The ability to predict clinical outcomes in the early phase of a patient's hospital admission could facilitate the optimal use of resources, might allow focused surveillance of high-risk patients and might permit early therapy. We investigated the hypothesis that the risk of in-hospital death of general medical patients can be modelled using a small number of commonly used laboratory and administrative items available within the first few hours of hospital admission. Matched administrative and laboratory data from 9497 adult hospital discharges, with a hospital discharge specialty of general medicine, were divided into two subsets. The dataset was split into a single development set, Q(1) (n=2257), and three validation sets, Q(2), Q(3) and Q(4) (n(1)=2335, n(2)=2361, n(3)=2544). Hospital outcome (survival/non-survival) was obtained for all discharges. An outcome model was constructed from binary logistic regression of the development set data. The goodness-of-fit of the model for the validation sets was tested using receiver-operating characteristics curves (c-index) and Hosmer-Lemeshow statistics. Application of the model to the validation sets produced c-indices of 0.779 (Q(2)), 0.764 (Q(3)) and 0.757 (Q(4)), respectively, indicating good discrimination. Hosmer-Lemeshow analysis gave chi(2)=9.43 (Q(2)), chi(2)=7.39 (Q(3)) and chi(2)=8.00 (Q(4)) (p-values of 0.307, 0.495 and 0.433) for 8 degrees of freedom, indicating good calibration. The finding that the risk of hospital death can be predicted with routinely available data very early on after hospital admission has several potential uses. It raises the possibility that the surveillance and treatment of patients might be categorised by risk assessment means. Such a system might also be used to assess clinical performance, to evaluate the benefits of introducing acute care interventions or to investigate differences between acute care systems.

Characterisation of aorto-iliac arterial stenoses in terms of pressure and flow.

Aorto-iliac stenoses were characterised in terms of pressure drop and flow velocity in a canine model and in patients with occlusive arterial disease. Pressure above and below the stenosis was measured intra-arterially and flow related measurements were made at rest and during reactive hyperaemia in the dog, and following papaverine administration in patients. The addition of flow velocity information to the pressure drop across a stenosis gave an increased separation of stenoses in the experimental animal and also in man.

The measurement of cerebrospinal fluid flow.

A technique for the measurement of cerebrospinal fluid (CSF) velocity-flow is described. It enables the flow of CSF in response to pressure pulses to be measured whilst allowing the simultaneous measurement of pressure through a lumber puncture needle. The physical principles which govern the operation of the flow probe are presented together with practical forms of the probe. The application of the technique is demonstrated by experiments on dog.

Doppler ultrasound assessment of the femoro-popliteal segment: comparison of different methods using ROC curve analysis.

Coexisting aorto-iliac and femoro-popliteal occlusive lesions remain a problem in vascular surgery. Arteriography does not provide information on their relative contributions to the presenting symptoms. The success of proximal reconstruction alone in such cases depends to some extent on the haemodynamic significance of the femoro-popliteal disease which will remain. Several noninvasive Doppler methods have been recommended for haemodynamic assessment of the femoro-popliteal segment. These methods were studied in 72 limbs of 38 patients. The results are compared using receiver operating characteristic curve analysis. The best single test in this group of patients was normalised transit time which was significantly better than pulsatility index damping factor (p less than 0.01). The addition of damping factor to normalised transit time tended to give some improvement but this was not statistically significant in the clinically relevant part of the ROC curve.

Some failings of pulsatility index and damping factor.

Pulsatility index (PI) is a commonly used method of objective assessment of the Doppler waveform. PI falls with increasing proximal stenosis and is raised by increasing peripheral resistance. Damping factor (DF) for an arterial segment is calculated by dividing the proximal by the distal PI. DF rises with increasing severity of disease of the arterial segment. DF is not, however, sufficiently accurate to be used alone but is usually combined with transit time measurements to provide information of diagnostic use. Both PI and DF have been examined in a canine model of combined segment disease. With increasing stenosis, distal PI falls as expected but so also does proximal PI. Such a stenosis is, in effect, a flow-throttling resistance so that although the characteristics of blood flow are altered by its presence, similar changes are observed both above and below the stenosis. The reduction of PI by a stenosis distal to the insonation site may result in the false interpretation of a low PI as indicating disease proximal to the insonation site. The observed similarity between PI proximal and distal to a stenosis reduces the usefulness of pulsatility index damping factor, particularly in the assessment of the femoro-popliteal segment in combined segment disease.

VBHOM, a data economic model for predicting the outcome after open abdominal aortic aneurysm surgery.

BACKGROUND: Vascular Biochemistry and Haematology Outcome Models (VBHOM) adopted the approach of using a minimum data set to model outcome. This study aimed to test such a model on a cohort of patients undergoing open elective and non-elective abdominal aortic aneurysm (AAA) repair. METHODS: A binary logistic regression model of risk of in-hospital mortality was built from the 2002-2004 submission to the UK National Vascular Database (NVD) (2718 patients). The subset of NVD data items used comprised serum levels of urea, sodium and potassium, haemoglobin, white cell count, sex, age and mode of admission. The model was applied prospectively using Hosmer-Lemeshow methodology to a test data set from the Cambridge Vascular Unit. RESULTS: The validation set contained 327 patients, of whom 208 had elective AAA repair and 119 had emergency repair of a ruptured AAA. Outcome following elective and non-elective AAA repair could be described accurately using the same model. The overall mean predicted risk of death was 14.13 per cent, and 48 deaths were predicted. The actual number of deaths was 53 (chi(2) = 8.40, 10 d.f., P = 0.590; no evidence of lack of fit). The model also demonstrated good discrimination (c-index = 0.852). CONCLUSION: The VBHOM approach has the advantage of using simple, objective clinical data that are easy to collect routinely. The VBHOM data items potentially allow prediction of risk in an individual patient before aneurysm surgery.

Risk-adjusted predictive models of mortality after index arterial operations using a minimal data set.

BACKGROUND: Reducing the data required for a national vascular database (NVD) without compromising the statistical basis of comparative audit is an important goal. This work attempted to model outcomes (mortality and morbidity) from a small and simple subset of the NVD data items, specifically urea, sodium, potassium, haemoglobin, white cell count, age and mode of admission. METHODS: Logistic regression models of risk of adverse outcome were built from the 2001 submission to the NVD using all records that contained the complete data required by the models. These models were applied prospectively against the equivalent data from the 2002 submission to the NVD. RESULTS: As had previously been found using the P-POSSUM (Portsmouth POSSUM) approach, although elective abdominal aortic aneurysm (AAA) repair and infrainguinal bypass (IIB) operations could be described by the same model, separate models were required for carotid endarterectomy (CEA) and emergency AAA repair. For CEA there were insufficient adverse events recorded to allow prospective testing of the models. The overall mean predicted risk of death in 530 patients undergoing elective AAA repair or IIB operations was 5.6 per cent, predicting 30 deaths. There were 28 reported deaths (chi(2) = 2.75, 4 d.f., P = 0.600; no evidence of lack of fit). Similarly, accurate predictions were obtained across a range of predicted risks as well as for patients undergoing repair of ruptured AAA and for morbidity. CONCLUSION: A 'data economic' model for risk stratification of national data is feasible. The ability to use a minimal data set may facilitate the process of comparative audit within the NVD.

Measuring clinical performance using routinely collected clinical data.

Following the well-publicized problems with paediatric cardiac surgery at the Bristol Royal Infirmary, there is wide public interest in measures of hospital performance. The Kennedy report on the BRI events suggested that such measures should be meaningful to the public, case-mix-adjusted, and based on data collected as part of routine clinical care. We have found that it is possible to predict in-hospital mortality (a measure readily understood by the public) using simple routine data-age, mode of admission, sex, and routine blood test results. The clinical data items can be obtained at a single venesection, are commonly collected in the routine care of patients, are already stored on hospital core IT systems, and so place no extra burden on the clinical staff providing care. Such risk models could provide a metric for use in evidence-based clinical performance management. National application is logistically feasible.

Portsmouth POSSUM models for abdominal aortic aneurysm surgery.

BACKGROUND: The Portsmouth Physiological and Operative Severity Score for enUmeration of Mortality and morbidity (P-POSSUM) equation permits comparative audit that allows for differences in case mix. The methodology has previously been shown accurately to model general surgical and vascular surgical patients. Patients with a ruptured abdominal aortic aneurysm (AAA) are, however, very different from elective patients and it may be hypothesized that they require their own specific risk model. METHODS: POSSUM data on 444 (213 emergency, 231 elective) admissions for AAA surgery between August 1993 and July 2000 were analysed using the P-POSSUM equation for general surgery and the P-POSSUM equations developed for vascular surgery. RESULTS: All equations successfully modelled the elective aneurysms but failed to fit the emergency aneurysms, and the elective and emergency aneurysms combined. This suggested that admission method (not a POSSUM data item) is an important factor for patients with AAA. However, with these data it was not possible to generate a model, including admission method, that successfully modelled the combined elective and emergency data. The 213 emergency aneurysm repairs were divided into two groups by operation date. The first 106 (training set) were used to form logistic regression models following the P-POSSUM methodology. These models were found successfully to fit the remaining 107 records (test set) on prospective application (chi2 = 4.50, 4 d.f., P = 0.345). CONCLUSION: Ruptured AAAs appear to be different from elective AAAs and other vascular cases and require their own risk model.

Haemodynamic assessment of the femoropopliteal segment: comparison of pressure and Doppler methods using ROC curve analysis.

Combined aorto-iliac and femoropopliteal vascular disease remains a problem in vascular surgery. Arteriography does not provide information on the relative contributions of the two lesions to the presenting symptoms. Aorto-iliac haemodynamics may reveal occult aorto-iliac disease but does not show whether combined proximal and distal reconstruction will be required to provide symptomatic relief. Haemodynamic assessment of both segments may help in this respect. A haemodynamic assessment of the femoropopliteal segment of 72 limbs in 38 patients is reported. The segmental pressure drop between a common femoral arterial cannula and a below-knee occlusion cuff is compared with a non-invasive Doppler method combining both transit time and damping factor. Comparison is made using receiver operating characteristic (ROC) curve analysis. The measurement of segmental pressure drop is more accurate than the Doppler method in detection of femoropopliteal stenoses of greater than 50 per cent of the luminal diameter (P less than 0.05).

On-line classification of arterial stenosis severity using principal component analysis applied to Doppler ultrasound signals.

Principal component analysis is a powerful method of feature extraction which can be applied to continuous-wave Doppler waveforms. A microprocessor system for the on-line calculation of the coefficients of principal components has been devised and tested in an experimental model. Doppler waveforms were obtained from positions distal to stenoses of known severity implanted in the iliac arteries of three dogs and classified into one of four groups. By reference to data from a previous series of experiments the microprocessor correctly classified 75% of stenoses. The remaining 25% were all classified as being one group more severe than they actually were.