Diffuse Alveolar Damage Correlation with Clinical Diagnosis of Pediatric Acute Respiratory Distress Syndrome

Abstract

Diffuse alveolar damage (DAD) is one of the pathological hallmarks of acute respiratory distress syndrome (ARDS). We aimed to compare pathological findings of DAD with clinical ARDS criteria. We re-evaluated 20 patients whose clinical autopsy revealed DAD. Total 11/20 patients with DAD (55%) met the 1994 American–European Consensus Conference and 7/17 (41%) met the 2012 Berlin clinical criteria. DAD showed only moderate correlation with current clinical ARDS definition. Oxygenation index (OI), seems to be the most valuable tool in predicting pulmonary damage severity, though OI is not listed in either of the previous definitions. We support the recommended use of OI by 2015 consensus conference.

Introduction

Acute respiratory distress syndrome (ARDS) represents clinical conditions with hypoxemia and respiratory failure. An inflammatory response due to different causes results in damage to alveolar barriers, and disturbs gas exchange and lung compliance. ¹ ARDS incidence in children is around 3.9/100,000 population per year. ²

Ashbaugh et al described ARDS in 1967. ³ In 1994, the American–European Consensus Conference (AECC) report set simplified criteria based on arterial hypoxia, bilateral radiographic opacities, and lack of left heart failure. Acute arterial hypoxemia with PaO ₂ /FiO ₂ (P/F) ratio between 200 and 300 mm Hg was defined as acute lung injury (ALI) and below 200 mm Hg as ARDS. ⁴ These criteria have been widely used, but researchers argued that specific additions should be incorporated regarding ventilator settings, the definition of acuteness, and hydrostatic edema. ⁵ As a result, the Berlin definition was formed in 2012. ⁶ Berlin criteria classify ARDS based on oxygenation with a P/F ratio as mild (200–300 mm Hg), moderate (100–200 mm Hg), or severe (< 100 mm Hg), as long as positive end expiratory pressure (PEEP) level is ≥ 5 cm H ₂ O within 1 week of an insult. ⁶ There were objections to the Berlin criteria as well, based on differences between pediatric and adult ARDS. Many authors argued that children have a more heterogeneous spectrum, and the difficulty of measuring arterial oxygenation could result in an underestimation of pediatric ARDS. The requirement of bilateral infiltration was also discussed because of its low sensitivity, interobserver variability, and the lack of contribution on predicting poor outcome as opposed to the degree of hypoxemia. In 2015, Pediatric Acute Lung Injury Consensus Conference (PALICC) introduced a pediatric focused definition. ^{7 8} Major changes included: the removal of bilateral pulmonary involvement criterion, the use of pulse oximetry levels when PaO ₂ is unavailable, and inclusion of oxygenation index (OI) and oxygen saturation index instead of P/F ratio. However, the PALICC definition still needs validation, and there are opinions against the sufficiency of unilateral infiltration and the use of OI to measure intrapulmonary shunts. ⁹

The morphological hallmark of ARDS is diffuse alveolar damage (DAD) with disruption of the alveolocapillary unit, described by Katzenstein et al. ¹⁰ Alveolocapillary unit is damaged. Histopathologically, hyaline membrane formation, alveolar cell type I or endothelial cell necrosis, edema, fibrosis, and alveolar cell type II proliferation are seen. The changes during DAD formation pursue a histopathological course after lung injury. An early exudative phase is followed by an organizing phase, evolving into fibrosis unless recovery occurs. ¹¹

Although most pathological exams of ARDS patients show DAD, these two entities are not always together. DAD was found in 45% of patients fulfilling clinical ARDS criteria. ¹² Conversely, not all DAD cases had clinical ARDS; infections, sepsis, shock, and radiation may also cause this kind of damage. ^{11 13}

The main objective of our study was to compare pathology findings of DAD with clinical criteria for ARDS in pediatric intensive care unit (PICU).

Materials and Methods

Lung tissue samples from 20 autopsies that were postmortem diagnosed with DAD were collected at the Hacettepe University Ihsan Dogramaci Children's Hospital in a period of 12 years (from January 1, 2000 to December 31, 2011). Our hospital is a university-affiliated, multidisciplinary, national referral center. Newborns were not included in the study. The study population consisted of patients from 2 months to 18 years of age.

Patient files were retrospectively assessed for clinical criteria of ARDS. Pathological specimens were re-evaluated by a distinct pediatric pathologist to confirm histopathological analysis. The ethics review committee of Hacettepe University approved the study. (GO 13/187).

Data Collection

Retrospective data on clinical and radiological information were collected through chart review. Pediatric logistic organ dysfunction (PELOD), pediatric index of mortality (PIM), pediatric risk of mortality (PRISM II) scores, and (OI = FiO ₂  × mean airway pressure × 100/PaO ₂ ) were calculated.

The AECC criteria were applied: P/F ratio, bilateral infiltrates on chest radiograph, and absence of echocardiographic evidence of left heart failure. Patients were classified as having clinical ARDS if they met all the AECC criteria. The Berlin definition criteria were analyzed for 17 of 20 whose data were fully obtained. The Berlin definition defines ARDS as low oxygenation when PEEP is ≥ 5 cm H ₂ O and bilateral opacities on imaging occurring within 1 week of clinical insult or worsening respiratory symptoms without edema related to cardiac reasons or fluid overload. ARDS severity was based on oxygenation level and was graded as mild (200–300), moderate (100–200), and severe (≤ 100) by the Berlin definition. ⁶

Pathological Examination

Routine pathological examination was based on representative samples from all five pulmonary lobes. Lungs were examined with a minimum of five slides, if needed new slides were prepared for histochemical and microbiological investigation.

Lung tissues were infused with 10% neutral formalin and fixed tissue samples were processed in the Ototechnicon, and were embedded in paraffin to prepare 2- to 4-μm thick sections. These sections were stained with hematoxylin-eosin, Masson's trichrome, and Verhoeff-Van Gieson elastic solutions.

DAD was diagnosed with presence of hyaline membranes, alveolar cell type I or endothelial cell necrosis, edema, and organizing interstitial fibrosis. ^{9 10} Inflammation and congestion were also added as parameters. The severity of these parameters was graded by four levels, 4 being diffuse and intense in all lobes, 3 being local manifestations in all lobes, 2 being diffuse involvement of three lobes, and 1 being focal presence in one or two lobes.

DAD was accepted as severe if hyaline membrane and alveolar type I cell necrosis parameters were 4+ both or one 3+ when the other is 4 + . Grades equal to or less than 3 + for these were classified as mildly damaged.

Statistical Analysis

Statistical analysis was performed using SPSS statistical software package version 21 (SPSS, Chicago, IL). The proportions and qualitative data were compared using the Chi-square test and quantitative data using the Fischer's exact test, Mann–Whitney U test, and Wilcoxon's W test. The p -values less than 0.05 were considered significant.

Results

We analyzed lung tissues of 20 patients over a 12-year period, all of which had DAD. The patient group ranged in age from 2 months to 18 years (mean = 38.5 months and median = 12 months) and consisted of 12 girls and 8 boys. Just over half of the patients (55%) had weight and height below the 3rd percentiles for their age.

Pathological Examination

Hyaline membrane formation was observed in all except one case which showed other aspects of alveolar damage such as alveolar cell type I necrosis. Inflammation was present in all but one, ten patients had inflammation in all lobes, with five diffuse and four focal. Alveolar cell type I necrosis was a universal finding with necrosis being diffuse and intense in 14 patients. Endothelial cell necrosis, congestion, and edema were seen in eight patients. Pulmonary fibrosis was not present.

Mild versus Severe Diffuse Alveolar Damage

Pathological findings were graded according to hyaline membrane and alveolar cell type I necrosis, and two groups composed of 10 patients each were formed (mild and severe DAD). The mild DAD group had longer hospital and PICU stays, and mechanical ventilation times when compared with the severe group ( p  > 0.05). Severe DAD patients tended to have similar P/F ratios but higher oxygenation indices and mean airway pressures ( p  > 0.05). Median OI at the onset of respiratory failure was 19 (4–83) and 30.5 (9–60) in the mild DAD and severe DAD groups, respectively. Also, this discrepancy was similar at the time of death with the median OI of 37 (11–97) and 59.5 (18–257) in mild DAD and severe DAD groups, respectively ( Table 1 ).

Table 1. Patient demographics and characteristics stratified according to severity of diffuse alveolar damage.

	Mild DAD n  = 10 median (min–max)	Severe DAD n  = 10 median (min–max)	Overall median (min–max)	p -Value
Clinically defined ARDS ( n )	5	6	11
Age (mo)	24.5 (3–216)	9.5 (2–158)	12 (2–216)	0.28
Gender (male/female)	4/6	4/6	8/12	0.99
Hospital length of stay (d)	18.5 (8–68)	15 (4–81)	16.5 (4–81)	0.52
PICU length of stay (d)	9.5 (1–31)	3 (1–13)	3.5 (1–31)	0.57
Length of mechanical ventilation (d)	11.5 (1–31)	3 (1–13)	4 (1–31)	0.19
PIM	100%	100%	100%	0.99
PRISM	15.5 (12–29)	13 (10–23)	14.5 (10–29)	0.24
PELOD
Onset of respiratory failure	21 (11–23)	21 (2–32)	21 (2–32)	0.39
Time of death	21 (11–32)	22 (20–41)	22 (11–41)	0.19
PaO 2 /FiO 2	51.8 (17–235)	54.6 (38–97)	52.2 (16.8–235)	0.39
OI
Onset of respiratory failure	19 (4–83)	30.5 (9–60)	26 (4–83)	0.53
Time of death	37 (11–97)	59.5 (18–257)	57 (11–41)	0.28
MAP (H 2 O) cm
Onset of respiratory failure	11 (8–14)	14.5 (9–30)	13 (8–30)	0.12
Time of death	16 (12–18)	16.5 (10–33)	16 (10–33)	0.61
Number of dysfunctioning organs at the beginning of respiratory failure ( n )	3	3.5	3 (1–5)	0.29
Need for renal replacement therapy ( n )	2	4	6	0.51
HFO mode use ( n )	1	4	5	0.15
Pulmonary hypertension ( n )	4	0	4	0.08

Abbreviations: ARDS, acute respiratory distress syndrome; DAD, diffuse alveolar damage; HFO, high frequency oscillation; MAP, mean airway pressure; max, maximum; min, minimum; OI, oxygenation index; PELOD, pediatric logistic organ dysfunction; PICU, pediatric intensive care unit; PIM, pediatric index of mortality; PRISM, pediatric risk of mortality.

On admission to the PICU, the mild DAD group had a higher incidence of central nervous system and renal dysfunction, while the severe DAD group had a higher incidence of cardiovascular, hematologic, and hepatic dysfunction ( Table 1 ).

Inflammation

Inflammation was highly present in all groups. There were 10 patients who had severe inflammation in all lobes. Patients with severe inflammation did not have significantly different hospital or PICU lengths of stay, periods of mechanical ventilation, or serological parameters. Clinical ARDS diagnosis was not higher in the severe inflammation group. Seven of these patients had pathogens in lung tissue, including group A streptococcus, candida, cytomegalovirus, and measles (rubeola).

Serological Values

Median hemoglobin levels were in the anemic range and median thrombocyte levels declined during the hospitalization. Total 90% of the patients received erythrocyte transfusions and 80% received thrombocyte transfusions. Total 15% were neutropenic on admission and 25% became neutropenic during hospital stay. Eight patients received albumin infusions, with a mean albumin level of 2.45 g/dL. Renal replacement therapy was administered in six patients: four by peritoneal dialysis and two by hemodialysis. Four out of these six patients did not have edema in their lung tissues.

Clinical Correlation

Of the 20 patients with a pathologic diagnosis of DAD, 11 (55%) were retrospectively identified as having ARDS using the AECC clinical definition. Seven (41%) fulfilled the Berlin criteria, but three patients could not be evaluated due to the lack of information on PEEP values.

Patients who developed ARDS according to the AECC did not have statistically different age, weight, hospital and PICU length of stay, days on mechanical ventilation, administered infusions, microbiological studies, use of high frequency oscillation ventilator mode or pulmonary hypertension ( Table 2 ).

Table 2. Patient demographics and characteristics stratified according to the American–European Consensus Conference definition of clinical acute respiratory distress syndrome.

	Patients with clinical ARDS median (min–max)	Patients without clinical ARDS median (min–max)	Overall median (min–max)	p -Value
Age (mo)	13 (2–96)	7 (2–216)	12 (2–216)	0.94
Gender male/female	4/7	4/5	8/12
Length of hospital stay (d)	16 (4–68)	18 (7–81)	16.5 (4–81)	0.65
Length of PICU stay (d)	3 (1–13)	4 (1–31)	3.5 (1–31)	0.76
Length of mechanical ventilation (d)	3 (1–13)	7 (1–31)	4 (1–31)	0.29
PIM	100%	100%	100%	0.99
PRISM	13 (11–25)	17 (10–29)	14.5 (10–29)	0.46
PELOD
Onset of respiratory failure	21 (11–32)	21 (2–32)	21 (2–32)	0.99
Time of death	22 (13–40)	22 (11–41)	22 (11–41)	0.76
OI
Onset of respiratory failure	27 (9–83)	22 (4–40)	26 (4–83)	0.53
Time of death	61.5 (14–257)	51 (11–97)	57 (11–41)	0.33
MAP
Onset of respiratory failure	13.5 (9–30)	11 (8–17)	13 (8–30)	0.46
Time of death	18 (10–33)	16 (13–19)	16 (10–33)	0.53
Number of dysfunctioning organs at the beginning of respiratory failure ( n )	3 (1–5)	2 (1–4)	3 (1–5)
PaO 2 /FiO 2	50 (16.8–97)	53 (30.8–235)	52.2 (16.8–235)	0.99
Renal replacement therapy ( n )	3	3	6	0.57
HFO mode use ( n )	3	2	5	0.6
Pulmonary hypertension ( n )	2	2	4	0.62
Pathological severity mild/severe ( n )	5/6	5/4	10/10

Abbreviations: ARDS, acute respiratory distress syndrome; HFO, high frequency oscillation; MAP, mean airway pressure; max, maximum; min, minimum; OI, oxygenation index; PELOD, pediatric logistic organ dysfunction; PICU, pediatric intensive care unit; PIM, pediatric index of mortality; PRISM, pediatric risk of mortality.

When the AECC criteria were evaluated separately among the 20 patients with DAD, 15 had echocardiographic evidence of normal heart function, and 15 had bilateral pulmonary infiltrations. At the time of ARDS diagnosis, the median P/F ratio was 52.

The median P/F ratio was 50.2 (17–97) for patients with clinical ARDS and 53.3 (31–235) for patients without. The median OI values were higher for the clinical ARDS group. At the beginning of respiratory failure, the OI was 27 versus 22, and at the time of death it was 61.5 versus 51 when comparing the clinically diagnosed ARDS versus nonclinically diagnosed groups, respectively.

Patients with clinical ARDS tended to have higher MAP and lower PRISM scores, although this difference did not reach statistical significance. PELOD scores and number of dysfunctioning organs were equal ( Table 2 ).

Discussion

Acute respiratory distress is a life-threatening condition. Mortality rates increase when associated with sepsis, high PRISM scores, multiorgan dysfunction, and malignancies. ¹⁴ In the study by Flori et al, ¹⁵ three independent variables were identified affecting mortality: (1) low P/F ratio upon admission, (2) organ failures other than pulmonary and central nervous system, and (3) central nervous system dysfunction. All our patients had low P/F ratios and extrapulmonary organ failures.

Acute inflammation results in pulmonary congestion, increased work of breathing, and hypoxemia. Dysregulated inflammatory response is a key element. It leads to altered permeability of alveolar barriers, formation of microthrombi, alveolar edema, and decreased pulmonary compliance. ^{16 17} Histopathological correlate of ARDS is DAD; characterized by hyaline membranes, alveolar cell type I or endothelial cell necrosis, edema, and organizing interstitial fibrosis.

Although pathological diagnosis is the gold standard, biopsy is rarely used in clinical practice.

In this study, we designed a retrospective clinical analysis of patients with a postmortem diagnosis of DAD.

According to an adult series, pneumonia was found in half of the patients who met the AECC criteria for ARDS without findings of DAD. ¹² Prognostic scores and clinical criteria do not distinguish pneumonia from ARDS. In children, ARDS is much more common in response to viral infections, thus complicating the clinical understanding of ARDS. ^{14 18} In our cohort, clinical ARDS diagnoses were not higher in the severe inflammation group.

DAD was classified into two groups as severe and mild DAD to see if pathological severity may be foreseen. Mildly damaged patients had longer hospital and PICU stays and mechanical ventilation periods.

In an adult patient series who underwent clinical autopsy, the sensitivity was 75% and the specificity was 84% for the AECC criteria. ¹⁹ After the establishment of the Berlin criteria, Thille et al reported that the sensitivity was 89% and the specificity 63% for identifying ARDS. ¹²

Pediatric knowledge on this topic is limited. Rodriguez Martinez et al analyzed 34 pediatric patients and found that the AECC definition had higher (80.7%) sensitivity with a lower specificity (71.4%) in children. ¹⁸ According to this report 21/26 patients with DAD met the AECC criteria, ¹⁸ whereas in our study the ratio was 11/20, resulting in a sensitivity of 55%. Of the 17 patients whom ventilator settings were accessed, the Berlin definition was met in 7 with a sensitivity of 41%. In our group, the median P/F ratio was 52 at the time of ARDS diagnosis, consistent with the severe degree of hypoxemia according to the Berlin definition. These results indicate that clinical definitions of ARDS are not sensitive enough for pediatric ARDS diagnosis.

In our study, 11 patients fulfilled the AECC definition among the entire cohort with a postmortem diagnosis of DAD ( n  = 20). These patients did not show any significant differences in demographic, serologic, treatment modality, ventilator setting, and organ dysfunction parameters when compared with the patients who did not meet the AECC criteria. Median PRISM scores were higher in clinical ARDS (AECC met) group (13 vs. 17). OI values at admission and at death were higher in the clinical ARDS patients without statistical significance ( Table 2 ).

OI, designed in 1989, ^{20 21} is suggested to have a predictive significance for extracorporeal membrane oxygenation administration and chronic lung disease development in neonates. ²² Seeley et al found that OI independently predicted mortality in ARDS and may be superior to P/F. ²³ The P/F ratio was found to be inadequate as an early predictor of mortality. ^{24 25} The improvement of oxygenation with prone positioning in pediatric ARDS was followed by OI changes rather than P/F ratio ²⁶ because OI considers MAP and gives information on compliance as well as oxygenation. ²⁶ Dechert et al found that OI score on the first 4 days of mechanical ventilation correlates with 28-day survival/mortality outcomes. ²⁰ In a prospective pediatric study, peak OI measured at any time point was identified as the most reliable independent predictor of mortality. ²⁷

In the current study, the median OI values were 19 and 37 in the mild DAD group, 30.5 and 59.5 in the severe DAD group, comparing the onset of respiratory failure versus the time of death, respectively. Higher OI values in the more intensely damaged group lead us to assume OI as an appropriate candidate to anticipate pathological injury. Statistical insignificance of higher OI is attributed to the small sample size. Besides, P/F ratios did not show any difference between mild and severe groups.

The 2015 PALICC recommendations for pediatric ARDS definition also include OI or OSI instead of P/F ratios. PALICC definition stratifies ARDS by OI: less than 4 (at risk for ARDS), 4 to 8 (mild ARDS), 8 to 16 (moderate ARDS), and greater than 16 (severe ARDS). ^{7 8} All of our patients had OI greater than 16, meaning they all were in severe ARDS range according to the PALICC definition.

Accurate diagnosis and severity assessment of ARDS is critical. The Berlin definition improved diagnostic criteria and proposed an ARDS classification for empirical evaluation. There is still need for improvement because these criteria do not have high mortality prediction. We showed the Berlin definition criteria to be lacking as a predictor of pulmonary tissue damage severity. Instead, OI deserves to be an adjunctive tool to assess ARDS severity. Although not completely independent from P/F ratio, OI would add more insight to assessment as recommended by the PALICC definition.

The PALICC definition excluded the need for bilateral infiltrates. In our patient cohort, five patients did not have bilateral infiltration and therefore did not fit in the AECC clinical ARDS group although they had DAD pathology. Medina et al discussed that because all scientific data on ARDS prognosis and treatment were based on bilateral disease, we need more studies to include unilateral ARDS. ⁹

A limitation of the current study is that only patients who died and underwent pathological pulmonary investigation were included in the study. Thus, our group represents patients with the severest prognosis.

Conclusion

In this study, 20 pediatric patients pathologically diagnosed with DAD were evaluated. Only 55% were diagnosed with ARDS according to current clinical criteria. Patients with and without clinical ARDS tended to have similar pathological presentation. Neither the AECC nor the Berlin definition were highly correlated with the histopathological severity.

When pathological evaluation is graded as mild and severe DAD, there was no statistically significant clinical parameter to predict the pathological outcome. Only OI (although statistically not significant) seemed to be higher in patients with severe pulmonary injury, this correlation should be tested in a larger group of pediatric patients. We agree with the PALICC recommendation of using OI instead of P/F ratio in pediatric ARDS evaluation.

Funding Statement

Funding None.