Ultrasound and Pulse Ox vs CXR and ABGs for ARDS Diagnosis
Ultrasound and Pulse Ox vs CXR and ABGs for ARDS Diagnosis
One hundred and twenty three study assessments were conducted on 77 patients, all of whom were mechanically ventilated and on PEEP of at least 5 cmH2 O. The characteristics of the study patients and assessments are given in Table 1. The median age of patients was 56 years (IQR 41–67); 52 (68 %) were male, and 24 patients underwent more than one assessment. Eight (33 %) were reclassified with respect to oxygenation criteria for ARDS and 9 (38 %) were reclassified with respect to radiographic criteria for ARDS over serial assessments. These changes included both reclassification as meeting criteria and reclassification as no longer meeting criteria. As patients were assessed no more frequently than every 3 days and because their clinical status often changed significantly in that time period, we chose assessments as our unit of analysis, treating them as independent. The most common diagnoses at the time of assessments were sepsis (n = 36, 29 %) and trauma (n = 31, 25 %).
At the time of ultrasound the median FiO2 was 0.40 (IQR 0.30–0.40) and the median SpO2 was 98 % (IQR 96–100). At the time of ABG, the median FiO2 was 0.40 (IQR 0.30–0.50), median SpO2 was 99 % (IQR 97–100), and median PaO2 was 100 (IQR 78–122). The SpO2 at the time of ABG was used for all further analyses. The relationship between SpO2/FiO2 and PaO2/FiO2 derived by Rice et al. in a cohort of ARDS patients indicated that SpO2/FiO2 threshold values of 315 and 235 corresponded to PaO2/FiO2 of 300 and 200, respectively, when SpO2 ≤ 97 %. To validate this relationship in our cohort of mechanically ventilated patients, we restricted our analysis to the 44 observations where SpO2 ≤ 97 % at the time of ABG. In this subset, the median PaO2/FiO2 was 198 (IQR 155–249) and the median SpO2/FiO2 was 240 (IQR 191–243). The Spearman rank correlation coefficient between PaO2/FiO2 and SpO2/FiO2 was 0.74 (p < 0.0001). We identified one outlier that was characterized by marked discordance between PaO2 and SpO2 due to rapidly dynamic changes in oxygenation at the time of the study. Excluding this observation, the correlation between PaO2/FiO2 and SpO2/FiO2 increased to 0.83 (p < 0.0001) (Fig. 4). We performed receiver operating curve analysis to further evaluate the SpO2/FiO2 ratio. SpO2/FiO2 ratio had modest ability to discriminate PaO2/FiO2 ≤ 300, based on an AUC value of 0.76 (95 % CI 0.34–1.00) (Additional file 1: Figure S1A http://www.ccforum.com/content/19/1/282/additional). The discriminatory ability for SpO2/FiO2 in classifying PaO2/FiO2 ≤ 200, however, was considerably better, with an AUC of 0.89 (95 % CI 0.80–0.99) (Additional file 1: Figure S1B http://www.ccforum.com/content/19/1/282/additional). The sensitivity of SpO2/FiO2 ≤ 315 for PaO2/FiO2 ≤ 300 was 83 % (95 % CI 68–93), and the specificity was 50 % (95 % CI 1–99) (Table 2). The sensitivity of SpO2/FiO2 ≤ 235 for PaO2/FiO2 ≤ 200 was 70 % (95 % CI 47–87), and the specificity was 90 % (95 % CI 68–99).
(Enlarge Image)
Figure 4.
Correlation between SpO2/FiO2 and PaO2/FiO2 when SpO2 ≤ 97 %. FiO2 fraction of inspired oxygen, PaO2 partial pressure of oxygen, SpO2 pulse oximetric oxygen saturation
Of the 738 lung fields evaluated by ultrasound (six fields for 123 assessments), 357 (48 %) demonstrated B line predominance as interpreted by the study physician. B lines were more common in posterior lung fields but were distributed equally on the left and on the right (Fig. 5). One hundred and one ultrasound assessments were conducted within 8 h of a chest radiograph. We used this subset of assessments to evaluate optimal thresholds of UIS for determination of radiographic criteria of ARDS as various thresholds have been reported. In 35 (35 %) assessments, bilateral opacities consistent with ARDS were apparent on chest radiograph. The sensitivity and specificity of UIS in at least one lung field bilaterally (UIS-2) for radiographic ARDS were 86 % (95 % CI 70–95) and 38 % (95 % CI 26–51) (Table 3). The sensitivity and specificity of UIS in at least one field bilaterally and involving a minimum of three lung fields (UIS-3) were 80 % (95 % CI 63–92) and 62 % (95 % CI 49–74). The sensitivity and specificity of UIS in at least two lung fields bilaterally (UIS-4) were 60 % (95 % CI 42–76) and 77 % (95 % CI 65–87). By receiver operating curve analysis, the ability of UIS pattern to discriminate radiographic ARDS was fair (AUC 0.73, 95 % CI 0.63–0.83) (Additional file 2: Figure S2 http://www.ccforum.com/content/19/1/282/additional).
(Enlarge Image)
Figure 5.
Distribution of B line-predominant lung fields. Black bars indicate reads by the study physician; white bars indicate reads by the independent physician. For right (R) and left (L), zones correspond to locations shown in Fig. 2
To test interobserver reliability of the ultrasound interpretations, all ultrasound videos were independently reviewed by an ultrasound-trained attending physician who had not participated in training or data collection and who was blinded to the clinical scenario. The kappa coefficient between the two interpreters for designating B lines present in a lung field was 0.57 (CI 0.40–0.73), consistent with moderate agreement. In comparison to the study physician, the independent physician classified a higher proportion of lung fields as B line-predominant but the relative distribution of B lines in the left lung compared to right lung, and throughout the upper, middle, and lower lung zones was comparable (Fig. 5). We evaluated the interobserver reliability for different UIS thresholds. For UIS-2, the kappa coefficient was 0.44 (CI 0.05–0.83); for UIS-3, the kappa coefficient was 0.45 (CI 0.09–0.81); and for UIS-4, the kappa coefficient was 0.47 (CI 0.07–0.87). Using the independent physician's classifications, sensitivity and specificity of UIS-2 for radiographic criteria of ARDS were 89 % (95 % CI 73–97) and 15 % (95 % CI 8–26), respectively; 89 % (95 % CI 73–97) and 29 % (95 % CI 18–41) for UIS-3; and 83 % (95 % CI 66–93) and 46 % (95 % CI 33–58) for UIS-4 (Table 3).
We next evaluated the combination of pulse oximetry measurements and pulmonary ultrasound assessments as markers for the coexistence of oxygenation and radiographic criteria for ARDS in our cohort. We combined SpO2/FiO2 ≤ 315 and ultrasound demonstrating UIS-3 in the subset of 33 observations with SpO2 ≤ 97 %, a chest radiograph within 8 h of ultrasound, and ABG within 24 h of ultrasound (Table 4). The sensitivity of the combination of pulse oximetry and ultrasound determinations of oxygenation and radiographic ARDS criteria was 83 % (95 % CI 52–98) and specificity was 62 % (95 % CI 38–82). The positive predictive value was 56 % (95 % CI 31–79) and the negative predictive value was 87 % (95 % CI 60–98). We repeated this analysis using a threshold SpO2/FiO2 ≤ 235, restricting to cases of at least moderate ARDS (PaO2/FiO2 ≤ 200) and found that the sensitivity was 64 % (95 % CI 31–89) and specificity was 86 % (95 % CI 65–97). The positive predictive value was 70 % (95 % CI 35–93) and the negative predictive value was 83 % (95 % CI 61–95).
We considered the ultrasound interpretations of the independent physician and repeated these analyses. The sensitivity of the combination of pulse oximetry (SpO2/FiO2 ≤ 315) and ultrasound (UIS-3) for oxygenation and radiographic ARDS criteria was 91 % (95 % CI 62–100) and specificity was 48 % (95 % CI 26–70) (Table 4). The sensitivity of the combination of pulse oximetry (SpO2/FiO2 ≤ 235) and ultrasound (UIS-3) for oxygenation and radiographic criteria of severe ARDS was 73 % (95 % CI 39–94) and specificity was 77 % (95 % CI 55–92).
To determine whether our treatment of each assessment as independent altered our findings, we performed sensitivity analyses restricted to the first assessment for each of the 77 patients. We did not find any significant differences in the relationships between SpO2/FiO2 and PaO2/FiO2 . Diagnostic accuracy of UIS as a marker for radiographic opacities consistent with ARDS was also similar: the sensitivity and specificity of UIS-3 were 80 % (95 % CI 56–94) and 72 % (95 % CI 57–84). The sensitivity of the combination of pulse oximetry and ultrasound determinations of oxygenation and radiographic ARDS criteria (SpO2/FiO2 ≤ 315 and UIS-3) was 88 % (95 % CI 47–100) and specificity was 69 % (95 % CI 39–91).
Results
Patients and Study Assessments
One hundred and twenty three study assessments were conducted on 77 patients, all of whom were mechanically ventilated and on PEEP of at least 5 cmH2 O. The characteristics of the study patients and assessments are given in Table 1. The median age of patients was 56 years (IQR 41–67); 52 (68 %) were male, and 24 patients underwent more than one assessment. Eight (33 %) were reclassified with respect to oxygenation criteria for ARDS and 9 (38 %) were reclassified with respect to radiographic criteria for ARDS over serial assessments. These changes included both reclassification as meeting criteria and reclassification as no longer meeting criteria. As patients were assessed no more frequently than every 3 days and because their clinical status often changed significantly in that time period, we chose assessments as our unit of analysis, treating them as independent. The most common diagnoses at the time of assessments were sepsis (n = 36, 29 %) and trauma (n = 31, 25 %).
Relationship Between SpO2/FiO2 and PaO2/FiO2
At the time of ultrasound the median FiO2 was 0.40 (IQR 0.30–0.40) and the median SpO2 was 98 % (IQR 96–100). At the time of ABG, the median FiO2 was 0.40 (IQR 0.30–0.50), median SpO2 was 99 % (IQR 97–100), and median PaO2 was 100 (IQR 78–122). The SpO2 at the time of ABG was used for all further analyses. The relationship between SpO2/FiO2 and PaO2/FiO2 derived by Rice et al. in a cohort of ARDS patients indicated that SpO2/FiO2 threshold values of 315 and 235 corresponded to PaO2/FiO2 of 300 and 200, respectively, when SpO2 ≤ 97 %. To validate this relationship in our cohort of mechanically ventilated patients, we restricted our analysis to the 44 observations where SpO2 ≤ 97 % at the time of ABG. In this subset, the median PaO2/FiO2 was 198 (IQR 155–249) and the median SpO2/FiO2 was 240 (IQR 191–243). The Spearman rank correlation coefficient between PaO2/FiO2 and SpO2/FiO2 was 0.74 (p < 0.0001). We identified one outlier that was characterized by marked discordance between PaO2 and SpO2 due to rapidly dynamic changes in oxygenation at the time of the study. Excluding this observation, the correlation between PaO2/FiO2 and SpO2/FiO2 increased to 0.83 (p < 0.0001) (Fig. 4). We performed receiver operating curve analysis to further evaluate the SpO2/FiO2 ratio. SpO2/FiO2 ratio had modest ability to discriminate PaO2/FiO2 ≤ 300, based on an AUC value of 0.76 (95 % CI 0.34–1.00) (Additional file 1: Figure S1A http://www.ccforum.com/content/19/1/282/additional). The discriminatory ability for SpO2/FiO2 in classifying PaO2/FiO2 ≤ 200, however, was considerably better, with an AUC of 0.89 (95 % CI 0.80–0.99) (Additional file 1: Figure S1B http://www.ccforum.com/content/19/1/282/additional). The sensitivity of SpO2/FiO2 ≤ 315 for PaO2/FiO2 ≤ 300 was 83 % (95 % CI 68–93), and the specificity was 50 % (95 % CI 1–99) (Table 2). The sensitivity of SpO2/FiO2 ≤ 235 for PaO2/FiO2 ≤ 200 was 70 % (95 % CI 47–87), and the specificity was 90 % (95 % CI 68–99).
(Enlarge Image)
Figure 4.
Correlation between SpO2/FiO2 and PaO2/FiO2 when SpO2 ≤ 97 %. FiO2 fraction of inspired oxygen, PaO2 partial pressure of oxygen, SpO2 pulse oximetric oxygen saturation
UIS as a Marker for Radiographic Opacities Consistent With ARDS
Of the 738 lung fields evaluated by ultrasound (six fields for 123 assessments), 357 (48 %) demonstrated B line predominance as interpreted by the study physician. B lines were more common in posterior lung fields but were distributed equally on the left and on the right (Fig. 5). One hundred and one ultrasound assessments were conducted within 8 h of a chest radiograph. We used this subset of assessments to evaluate optimal thresholds of UIS for determination of radiographic criteria of ARDS as various thresholds have been reported. In 35 (35 %) assessments, bilateral opacities consistent with ARDS were apparent on chest radiograph. The sensitivity and specificity of UIS in at least one lung field bilaterally (UIS-2) for radiographic ARDS were 86 % (95 % CI 70–95) and 38 % (95 % CI 26–51) (Table 3). The sensitivity and specificity of UIS in at least one field bilaterally and involving a minimum of three lung fields (UIS-3) were 80 % (95 % CI 63–92) and 62 % (95 % CI 49–74). The sensitivity and specificity of UIS in at least two lung fields bilaterally (UIS-4) were 60 % (95 % CI 42–76) and 77 % (95 % CI 65–87). By receiver operating curve analysis, the ability of UIS pattern to discriminate radiographic ARDS was fair (AUC 0.73, 95 % CI 0.63–0.83) (Additional file 2: Figure S2 http://www.ccforum.com/content/19/1/282/additional).
(Enlarge Image)
Figure 5.
Distribution of B line-predominant lung fields. Black bars indicate reads by the study physician; white bars indicate reads by the independent physician. For right (R) and left (L), zones correspond to locations shown in Fig. 2
Interobserver Reliability of UIS Interpretation
To test interobserver reliability of the ultrasound interpretations, all ultrasound videos were independently reviewed by an ultrasound-trained attending physician who had not participated in training or data collection and who was blinded to the clinical scenario. The kappa coefficient between the two interpreters for designating B lines present in a lung field was 0.57 (CI 0.40–0.73), consistent with moderate agreement. In comparison to the study physician, the independent physician classified a higher proportion of lung fields as B line-predominant but the relative distribution of B lines in the left lung compared to right lung, and throughout the upper, middle, and lower lung zones was comparable (Fig. 5). We evaluated the interobserver reliability for different UIS thresholds. For UIS-2, the kappa coefficient was 0.44 (CI 0.05–0.83); for UIS-3, the kappa coefficient was 0.45 (CI 0.09–0.81); and for UIS-4, the kappa coefficient was 0.47 (CI 0.07–0.87). Using the independent physician's classifications, sensitivity and specificity of UIS-2 for radiographic criteria of ARDS were 89 % (95 % CI 73–97) and 15 % (95 % CI 8–26), respectively; 89 % (95 % CI 73–97) and 29 % (95 % CI 18–41) for UIS-3; and 83 % (95 % CI 66–93) and 46 % (95 % CI 33–58) for UIS-4 (Table 3).
SpO2/FiO2 and UIS as Marker for Oxygenation and Radiographic Criteria for ARDS
We next evaluated the combination of pulse oximetry measurements and pulmonary ultrasound assessments as markers for the coexistence of oxygenation and radiographic criteria for ARDS in our cohort. We combined SpO2/FiO2 ≤ 315 and ultrasound demonstrating UIS-3 in the subset of 33 observations with SpO2 ≤ 97 %, a chest radiograph within 8 h of ultrasound, and ABG within 24 h of ultrasound (Table 4). The sensitivity of the combination of pulse oximetry and ultrasound determinations of oxygenation and radiographic ARDS criteria was 83 % (95 % CI 52–98) and specificity was 62 % (95 % CI 38–82). The positive predictive value was 56 % (95 % CI 31–79) and the negative predictive value was 87 % (95 % CI 60–98). We repeated this analysis using a threshold SpO2/FiO2 ≤ 235, restricting to cases of at least moderate ARDS (PaO2/FiO2 ≤ 200) and found that the sensitivity was 64 % (95 % CI 31–89) and specificity was 86 % (95 % CI 65–97). The positive predictive value was 70 % (95 % CI 35–93) and the negative predictive value was 83 % (95 % CI 61–95).
We considered the ultrasound interpretations of the independent physician and repeated these analyses. The sensitivity of the combination of pulse oximetry (SpO2/FiO2 ≤ 315) and ultrasound (UIS-3) for oxygenation and radiographic ARDS criteria was 91 % (95 % CI 62–100) and specificity was 48 % (95 % CI 26–70) (Table 4). The sensitivity of the combination of pulse oximetry (SpO2/FiO2 ≤ 235) and ultrasound (UIS-3) for oxygenation and radiographic criteria of severe ARDS was 73 % (95 % CI 39–94) and specificity was 77 % (95 % CI 55–92).
Sensitivity to Repeat Assessments
To determine whether our treatment of each assessment as independent altered our findings, we performed sensitivity analyses restricted to the first assessment for each of the 77 patients. We did not find any significant differences in the relationships between SpO2/FiO2 and PaO2/FiO2 . Diagnostic accuracy of UIS as a marker for radiographic opacities consistent with ARDS was also similar: the sensitivity and specificity of UIS-3 were 80 % (95 % CI 56–94) and 72 % (95 % CI 57–84). The sensitivity of the combination of pulse oximetry and ultrasound determinations of oxygenation and radiographic ARDS criteria (SpO2/FiO2 ≤ 315 and UIS-3) was 88 % (95 % CI 47–100) and specificity was 69 % (95 % CI 39–91).
Source...