Computerised tomography for the detection of pulmonary emboli in intensive care patients--a retrospective cohort study.
Pulmonary emboli are frequently considered as a cause for respiratory deterioration in intensive care unit (ICU) patients, however empirical observation suggests that computerised tomographic (CT) angiography is infrequently positive after the first 24 hours. This study aimed to determine the rate and risk factors for detection of pulmonary emboli by CT angiography in ICU patients.
All patients undergoing CT angiography >24 hours after ICU admission for respiratory deterioration from April 2000 until January 2004 were included. The positivity rate for pulmonary emboli was determined and risk factors analysed.
Seven (6%) out of 113 CT angiograms were positive for pulmonary emboli. All were found in trauma patients. Comparing positive to negative scans, predefined risk factors including head injury (517 positive scans, 71% vs. 231106 negative scans, 22%, P=0.005), spine injury with neurological impairment (417, 57% vs. 91106, 8%, P=0.002) and lower limb injury (317, 43% vs. 121106, 9%, P=0.039) were significantly more frequent in patients with positive scans. Deep vein thrombosis prophylaxis was employed less frequently prior to a positive scan (in 317, 43% patients with positive scans vs. 911106, 86% patients with negative scans P=0.015). Only the predefined risk factors were independently associated with positive CT angiography on limited logistic regression (OR 24.7 per risk factor, 95% C12.38 to 255.1, P=0.007).
Pulmonary emboli were infrequently diagnosed using CT angiography in ICU patients admitted for more than 24 hours and found only inpatients with recognised risk factors.
Key Words: pulmonary embolus, computerised tomography, intensive care
Amongst intensive care patients, deteriorations in oxygenation or ventilation are very common with the possibility of pulmonary emboli frequently raised as the cause. This presents a diagnostic dilemma, since 1) pulmonary emboli are difficult to diagnose clinically, especially in the unconscious ventilated patient (1), 2) deep vein thrombosis (DVT), a precursor to pulmonary emboli is found in as many as 40% of intensive care unit (ICU) patients (2-4) and 3) undiagnosed pulmonary emboli may be fatal, being found in 12 to 15% of autopsies in hospitalised patients (5-8). In other words, pulmonary embolus as a cause for respiratory deterioration is difficult to exclude, but potentially dangerous if missed.
For these reasons, ICU patients are often sent for computerised tomography (CT) pulmonary angiography to rule out pulmonary emboli. However, empirical observation suggests that CT angiograms performed after the first 24 hours in the ICU are rarely positive for pulmonary emboli.
This study was therefore directed at determining the detection rate for pulmonary emboli in ICU patients sent for CT pulmonary angiography after the first 24 hours in ICU. In addition, an attempt was made to identify patient characteristics associated with a positive CT angiogram. These data were considered important as they would substantiate the empirical observations above and potentially improve utilisation of CT angiography amongst ICU patients.
MATERIAL AND METHODS
This is a retrospective chart review study performed on patients admitted to the 21-bed ICU of Sunnybrook and Women's Health Sciences Center, Toronto, Ontario, Canada over 45 months from April 2000 until January 2004. This hospital is a tertiary referral centre and the largest trauma centre in Ontario. All mechanically ventilated patients in the hospital are admitted to this ICU, with the exception of patients following cardiac and vascular surgery and burns. The study population included two groups of patients who underwent CT angiography of the chest, those whose CT angiogram was performed after the first 24 hours in the ICU during the period from April 2000 to January 2004 (the 'Late Scan' group) and those whose CT angiogram was performed during the period from 24 hours before ICU admission until 24 hours thereafter during 2003 (the Admission Scan' group). The Late Scan group represented the primary focus of investigation, with the Admission Scan group used for control measures as described below.
Patients in the Late Scan group were identified from the radiology department database. All contrast-enhanced CT scans of the chest performed for respiratory deterioration according to the angiography protocol later than 24 hours after ICU admission were eligible for inclusion in the Late Scan group. CT scans performed for non-respiratory reasons as recorded on the examination request (such as diagnosis of major vascular injury or to exclude pneumothorax) were not included. The results of the CT angiograms, as recorded by the attending radiologist (either positive, negative or indeterminate) were recorded.
For patients in the Admission Scan group, inclusion criteria were the same as above; however CT angiograms performed from 24 hours prior to ICU admission until 24 hours after ICU admission were selected. These CT angiograms were identified by examination of the individual patient's records. By virtue of the fact that these patients were being admitted to the ICU from non-critical care areas, they were not initially ventilated, or only recently intubated. These patients were thus more similar to emergency room or ward patients included in other predominantly outpatient studies examining the incidence of pulmonary emboli, in that they may have been able to report symptoms associated with pulmonary emboli and their blood gas evaluations would be less affected by mechanical ventilation. The Admission Scan group patients could thus act as a control group in order to examine the ability of physicians in our institution to diagnose pulmonary emboli in patients comparable to other studies, and to compare the detection rate of CT angiography for the diagnosis of pulmonary emboli specifically in our institution.
For all the patients identified above, demographic, clinical and outcome data were obtained from the ICU admissions database, the hospital clinical information database and patient charts. All data were collected by a single investigator (AL) onto a standard data collection form. Demographic data including age, gender, dates of hospital and ICU admission and discharge were collected. ICU admission aetiology (with particular reference to trauma, medical or post surgical aetiology), medical history (chronic health elements of APACHE II score), performance of surgical procedures prior to CT angiography, outcome and APACHE II score were also recorded. For trauma patients, the presence of head injury, spine injury with neurological deficit and leg injury (femur or pelvic fracture) were specifically sought. On the day of the CT angiography, physiological data were recorded including the minimum systolic and diastolic blood pressure, [P.sub.a][O.sub.2] and platelet count and the maximum pulse rate, Fi[O.sub.2] and prothrombin time (PT). The use of any mechanical ventilation and all inotropic drugs was recorded. In addition, the use of DVT prophylaxis, including unfractionated heparin, low molecular weight heparin, warfarin, mechanical compression devices or inferior vena cava filter was sought during the 48 hours prior to CT angiography.
The databases were also searched to identify the performance of pulmonary angiography and ventilation perfusion scans to identify pulmonary emboli.
For both patient groups, the proportion of CT angiograms found to be positive for pulmonary emboli was calculated and the characteristics of patients with positive and negative CT angiograms compared in order to determine potential risk factors for a positive CT angiogram. As all positive CT angiograms in the Late Scan group occurred in trauma patients, a subgroup analysis was performed comparing trauma patients only with positive or negative CT angiograms in the Late Scan group.
Despite the small number of positive CT angiograms in the Late Scan group, an attempt was made to employ logistic regression to differentiate between decreased use of DVT prophylaxis and defined risk factors for DVT as potential risk factors for positive CT angiography in the Late Scan group. The total number of predetermined DVT risk factors (defined as head injury, spine injury with neurological deficit or leg injury) (9) and use of DVT prophylaxis were included in a logistic regression analysis with CT angiography outcome as the dependent variable. The proportion of positive CT angiograms in the Admission Scan group was compared to the Late Scan group and to the detection rate for CT angiography reported in the literature.
Continuous variables were compared using Student's t test or the Wilcoxon rank sum test according to normality of data distribution. Categorical variables were compared with the Chi square using continuity correction or Fisher's exact test as appropriate. All P values are two-tailed and statistical significance was determined as P <0.05. All statistical tests were performed with SAS version 8.2 (SAS Institute, Inc., Cary, NC, U.S.A.).
Over the study period, 164 chest CT angiographies performed for respiratory deterioration were identified amongst 4559 ICU patients. These patients had a mean age of 56 [+ or -] 21 years, a mean APACHE II score of 8.4 [+ or -] 17.1 and 2833 patients (62%) were male. Of the 164 chest CT angiographies, 113 (identified from amongst 3498 patients) were performed after the first 24 hours in ICU (the Late Scan group) and 51 angiographies (identified from amongst 1061 patients) were performed during the period from 24 hours before ICU admission until 24 hours thereafter (the Admission Scan group). No patients were sent for direct pulmonary angiography or ventilation perfusion scans to detect pulmonary emboli during the study period.
In the Late Scan group 6% of the patients (7/113) had positive angiographies. A comparison of patient characteristics, physiological parameters, outcome and use of DVT prophylaxis for patients with positive and negative angiographies in the Late Scan group is shown in Table 1. Despite the finding that trauma represented the admission aetiology for only 54/113 (48%) of the Late Scan group patients, all positive CT angiographies were found in trauma patients (P=0.006). In contrast, none of the patients with a medical ICU admission aetiology (40 patients, 38% of Late Scan group) had positive angiography (P=0.049).
Amongst trauma patients, head injury (found in 5/7 (71%) patients with positive scans vs. 23/106 (22%) patients with negative scans, P=0.005), spine injury with neurological impairment (4/7, 57% vs. 9/106, 8%, P=0.002), leg injury (3/7, 43% vs. 12/106, 11%, P=0.039) or a combination of these were significantly more frequent amongst those with positive vs. negative angiography. Thromboembolism prophylaxis was employed for >24 hours prior to positive CT angiography in three of the seven patients (43%) with a positive scan vs. 91 of the 106 patients (86%) with a negative scan (P=0.015).
As all the positive CT angiograms in the Late Scan group occurred in trauma patients, these patients were analysed as a separate subgroup. Patients with positive CT angiography had significantly more risk factors (defined as head injury, spine injury with neurological deficit or leg injury) than those with negative CT angiography (2.0 [+ or -] 0.6 vs. 0.9 [+ or -] 0.6 risk factors, P=0.003). Patients with positive CT angiography also showed a trend to decreased use of DVT prophylaxis (used in 79% of those with negative vs. 43% of those with positive CT angiograms, P=0.065). On limited logistic regression analysis including use of DVT prophylaxis and number of risk factors, only the number of risk factors was significantly associated with a positive CT angiography (OR 24.7 per risk factor, 95% CI 2.38 to 255.1, P=0.007).
The positivity rate for CT angiography performed in the Admission Scan group was 27% (14/51), significantly higher than the 6% positivity rate in the Late Scan group (P <0.001). Patients with positive CT angiography in the Admission Scan group had a significantly lower Pao2 than those with negative CT angiograms (83 [+ or -] 20 mmHg vs. 118 [+ or -] 80 mmHg, P=0.0330) and a lower proportion were ventilated (29% vs. 76%, P=0.0011) at the time of scanning (Table 2). No significant difference was noted in the use of DVT prophylaxis amongst those with positive and negative admission CT angiography.
Amongst patients with respiratory deterioration being referred for CT pulmonary angiography >24 hours after ICU admission, only seven out of 113 scans (6%) were positive for pulmonary emboli. All these CT angiographies were found in trauma patients, the majority of whom had one or more of three pre-defined risk factors for pulmonary embolism: head injury, spine injury with neurological impairment or leg injury. Lack of use of thromboembolism prophylaxis was significantly more common amongst patients with CT angiography positive for pulmonary emboli, however, only the presence of the defined risk factors was a significant predictor of pulmonary emboli on limited logistic regression analysis. Amongst patients included in the Late Scan group, no physiological variables showed clinical usefulness as predictors of CT angiography positive for pulmonary emboli. These data are important as they confirm the empiric observation that in the specific population tested (patients after their first 24 hours in ICU), CT angiography rarely detects pulmonary emboli. Further, the data suggest that CT angiography positive for pulmonary emboli is limited to patients with specific risk factors.
This study was not designed to investigate the incidence of pulmonary emboli in ICU patients, but rather to assess the utility of CT pulmonary angiography. The study suggests that this test has a low yield and this despite the study population being a selected subgroup of ICU patients who might be expected to be at high risk of pulmonary emboli (ICU patients who were identified by ICU physicians as suffering from respiratory deterioration and selected to undergo an invasive test to exclude pulmonary emboli). Four potential explanations for the low yield of CT angiography in the study population are discussed below, namely 1) that pulmonary emboli are rare amongst ICU patients, 2) that clinicians are unable to correctly identify patients at high risk for pulmonary emboli, 3) that CT angiography is not a good tool to diagnose pulmonary emboli in ICU patients in general or specifically at our institution and 4) that patients with pulmonary emboli are being diagnosed by other means or not detected.
1) Owing principally to the invasive nature of diagnostic testing for pulmonary emboli, the true incidence of this condition in ICU patients is undetermined at present--to repeatedly perform pulmonary angiography or even CT angiography on all ICU patients is unethical. Incidence rates for pulmonary emboli have, however, been estimated from cohort studies based on clinically indicated diagnostic tests. These show that pulmonary emboli are present in 0.4% of patients around admission and develop in a further 0.5% during the course of their ICU admission 2. In our study, 14 pulmonary emboli were identified on admission amongst 1061 patients (1.3%) and seven pulmonary emboli were identified in 3498 patients after the first 24 hours of ICU admission (0.2%). Our data thus concur with the suggestion that pulmonary emboli occur relatively rarely after the first 24 hours of ICU care.
2) It is also possible, however, that the low positivity rate for CT angiography was a result of the wrong patients being sent for CT angiography, potentially indicating that ICU physicians are not able to identify ICU patients at risk of pulmonary emboli. Tools commonly used to assist in the clinical diagnosis of pulmonary emboli (such as the Wells (10) or Wicki" scores) were developed on non-critically ill, predominantly emergency room patient populations. These tools are based on data taken from the medical history and basic clinical tests. For example the Wells score includes points given for past surgery, tachycardia and immobilisation (10), while the Wicki criteria include similar variables and blood gas analysis (11). Amongst ICU patients, the clinical criteria accruing points in the two scores may be present in a majority of patients from conditions unrelated to pulmonary emboli, while blood gas variables will be determined by ventilator parameters, rather than spontaneous ventilation. Indeed, while these scores are effective in predicting the risk of pulmonary emboli in emergency room patients (1,12), their effectiveness is decreased amongst other hospital populations, particularly in surgical and ICU patients (13).
The ability of our ICU physicians to identify patients at risk of pulmonary embolism can, however, be assessed from the Admission Scan group. Amongst these patients, the positivity rate for CT angiography was 27%. This is similar to the positivity rate in other larger studies investigating the diagnosis of pulmonary emboli (11,14-16)). For example in the recent PIOPED II study (14), pulmonary emboli were diagnosed in 192/824 (23%) patients included with clinically suspected pulmonary emboli. Thus, even though scoring systems were not specifically utilised in our ICU, amongst patients being admitted to the ICU our physicians were able to effectively identify patients at risk of pulmonary emboli. Whether a lower incidence of pulmonary emboli in the Late Scan group, or a more complex clinical presentation explains the difference in positivity rates between the Admission and Late Scan groups was not investigated in this study.
3) and 4) CT angiography has been repeatedly shown to be effective in diagnosing significant pulmonary emboli with both a high sensitivity and specificity in non critically ill patient populations (14,17-19). Smaller studies have also shown CT angiography to be effective in ICU patients (20). The Admission Scan group showed that at our institution, CT angiography is as effective as expected, displaying a positivity rate in line with that found in other studies. This suggests that technical failure was not the reason for the low positivity rate of the Late Scans in our study. Further, as stated in the results section, alternative diagnostic strategies to detect pulmonary emboli (ventilation perfusion scanning and pulmonary angiography) were not employed during the study period. It is possible that pulmonary emboli are going undetected in our ICU, however the overall mortality rate for patients in our ICU is not higher than the regional average (data not shown).
This study is limited by its retrospective nature, small size and selected population. As only seven positive CT angiograms were identified in the Late Scan group, the statistical basis for logistic regression is very weak. The study was also limited to one ICU that admits a high proportion of trauma patients (representing 40% of the total study cohort) and the conclusions might not be valid for larger populations of medical patients. Finally, the CT angiographies were not interpreted specifically for the study.
The study does however, provide a picture of everyday clinical practice and suggests that while pulmonary emboli are not infrequently diagnosed by CT angiography around admission to the ICU, the utility of CT angiography later during ICU admissions in unselected patients with respiratory deterioration may be limited.
Accepted for publication on September 28, 2007.
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A. LICHT *, W J. SIBBALD ([dagger]), E D. LEVIN ([double dagger])
Departments of Critical Care and Medicine, Sunnybrook Health Sciences Centre, University of Toronto, Toronto, Ontario, Canada
* M.D., Fellow, Department of Critical Care.
[dagger] M.D., Professor.
[double dagger] M.B., B.Chir., Fellow, Department of Critical Care, Hadassah University Hospiral, Jerusalem, Israel.
Address for reprints: Dr P D. Levin, Department of Anesthesia and Critical Care, Hadassah University Hospital, PO Box 12000, Jerusalem 91120, Israel.
TABLE 1 Late Scan group (CT scans performed after 24 hours in the ICU: n=113) Negative CT angiography patients (%) Number of CT angiograms 106 (94%) Demographics Age (years) 57 [+ or -] 22 Gender (male) 61 (58%) APACHE II score 19 [+ or -] 6 Trauma aetiology 47 (44%) Postoperative 40 (38%) Medical aetiology 40 (38%) Head injury 23 (22%) Spine injury with neurological 9 (8%) deficit Leg injury 12 (11%) ICU mortality 16 (15%) ICU length of stay (days) * 9.9 (4.6-21.9) Time from admission to CT 7.8 (2.7-15.1) angiography (days) * DVT prophylaxis DVT prophylaxis >24 h prior to CT 91 (86%) Physiological parameters Heart rate (bpm) 98 [+ or -] 24 Min systolic blood pressure 135 [+ or -] 23 (mmHg) Min diastolic blood pressure 65 [+ or -] 13 (mmHg) Max Fi[O.sub.2] 0.51 [+ or -] 0.19 Min [P.sub.a][O.sub.2] (mmHg) 94 [+ or -] 32 Min oxygen saturation ( (%)) 97 [+ or -] 2 PT (INR) 1.2 [+ or -] 0.3 Platelet count (x[10.sup.9]/l) 329 [+ or -] 275 Mechanical ventilation 68 (64%) Inotropic drug use 19 (18%) Positive CT angiography patients (%) P Number of CT angiograms 7 (6%) Demographics Age (years) 45 [+ or -] 20 0.171 Gender (male) 6 (86%) 0.237 APACHE II score 18.9 [+ or -] 7.4 0.997 Trauma aetiology 7 (100%) 0.006 Postoperative 5 (71%) 0.083 Medical aetiology 0 (0%) 0.049 Head injury 5 (71%) 0.005 Spine injury with neurological 4 (57%) 0.002 deficit Leg injury 3 (43%) 0.039 ICU mortality 1 (14%) 1.000 ICU length of stay (days) * 12.6 (2.5-19.9) 0.826 Time from admission to CT 11.8 (2.5-14.4) 0.893 angiography (days) * DVT prophylaxis DVT prophylaxis >24 h prior to CT 3 (43%) 0.015 Physiological parameters Heart rate (bpm) 104 [+ or -] 21 0.489 Min systolic blood pressure 129 [+ or -] 13 0.501 (mmHg) Min diastolic blood pressure 68 [+ or -] 12 0.579 (mmHg) Max Fi[O.sub.2] 0.54 [+ or -] 0.22 0.659 Min [P.sub.a][O.sub.2] (mmHg) 124 [+ or -] 63 0.262 Min oxygen saturation ( (%)) 96 [+ or -] 2 0.482 PT (INR) 1.1 [+ or -] 0.1 0.181 Platelet count (x[10.sup.9]/l) 347 [+ or -] 230 0.871 Mechanical ventilation 6 (86%) 0.670 Inotropic drug use 2 (29%) 0.631 Values: value and percentage, or mean [+ or -] standard deviation, or *=median (interquartile range). Min=minimum, Max=maximum values in 24 hours prior to CT angiography. TABLE 2 Admission Scan group (performed from 24 hours prior to 24 hours post ICU admission: n=51) Negative CT angiography patients (%) Number of CT angiograms 37 (73%) Demographics Age (years) 58 [+ or -] 20 Gender (male) 24 (65%) APACHE II score 19 [+ or -] 7 Trauma aetiology 8 (22%) Post operative 11 (30%) Medical aetiology 22 (59%) Head injury 4 (11%) Spine injury with neurological 3 (8%) deficit Leg injury 2 (5%) ICU mortality 9 (24%) ICU length of stay (days) * 4.1 (1.5-11.5) DVT prophylaxis DVT prophylaxis given 21 (57%) Physiological parameters Max heart rate (bpm) 92 [+ or -] 20 Min systolic blood pressure 132 [+ or -] 23 (mmHg) Min diastolic blood pressure 69 [+ or -] 14 (mmHg) Max Fi[O.sub.2] 0.56 [+ or -] 0.23 Min [P.sub.a][O.sub.2] (mmHg) 118 [+ or -] 80 Min oxygen saturation (%) 97 [+ or -] 4 PT (INR) 1.2 [+ or -] 0.3 Platelet count (x[10.sup.9]/l) 264 [+ or -] 171 Mechanical ventilation 28 (76%) Inotrope use 6 (16%) Positive CT angiography patients (%) P Number of CT angiograms 14 (27%) Demographics Age (years) 67 [+ or -] 17 0.127 Gender (male) 8 (57%) 0.748 APACHE II score 20 [+ or -] 7 0.479 Trauma aetiology 2 (14%) 0.797 Post operative 2 (14%) 0.303 Medical aetiology 10 (71%) 0.430 Head injury 1 (7%) 1.000 Spine injury with neurological 0 (0%) 0.546 deficit Leg injury 1 (7%) 1.000 ICU mortality 2 (14%) 0.705 ICU length of stay (days) * 3.5 (1.9-6.8) 0.709 DVT prophylaxis DVT prophylaxis given 9 (64%) 0.754 Physiological parameters Max heart rate (bpm) 105 [+ or -] 28 0.106 Min systolic blood pressure 128 [+ or -] 37 0.696 (mmHg) Min diastolic blood pressure 68 [+ or -] 19 0.981 (mmHg) Max Fi[O.sub.2] 0.66 [+ or -] 0.30 0.314 Min [P.sub.a][O.sub.2] (mmHg) 83 [+ or -] 20 0.033 Min oxygen saturation (%) 94 [+ or -] 6 0.160 PT (INR) 1.3 [+ or -] 0.3 0.650 Platelet count (x[10.sup.9]/l) 203 [+ or -] 129 0.284 Mechanical ventilation 4 (29%) 0.001 Inotrope use 1 (7%) 0.654 Values: value and percentage, or mean [+ or -] standard deviation, or *=median (interquartile range). Min=minimum, Max=maximum values in 24 hours prior to CT angiography.
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|Title Annotation:||Original Papers|
|Author:||Licht, A.; Sibbaldt, W.J.; Levins, P.D.|
|Publication:||Anaesthesia and Intensive Care|
|Article Type:||Clinical report|
|Date:||Jan 1, 2008|
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