European Journal of Cardiovascular Medicine

Acute Respiratory Distress Syndrome (ARDS) is a life-threatening respiratory condition characterized by hypoxemia and stiff lungs [1]. It is characterized by diffuse alveolar damage leading to the accumulation of secretions in the alveolar sacs, causing a small effective lung volume, decreased lung compliance, and severe hypoxemia refractory to standard treatment [2]. The extent of lung involvement varies among patients, with the posterior dependent part being predominantly affected in supine patients and the basal region in ambulant patients.

Refractory hypoxemia can occur in most patients with acute respiratory distress syndrome. Refractory hypoxemia means inadequate arterial oxygenation despite adequate levels of inspired oxygen [3]. Management of refractory hypoxemia includes opening collapsed alveoli with recruitment maneuvers [4]. One such recruitment maneuver is prone positioning of the patient [4]. During prone positioning, recruitment of the dorsal lung increases the total lung volume and achieves a more even tidal volume distribution [5].

Since the 1970s, prone positioning has been used as a method for improving oxygenation in patients with acute respiratory distress syndrome [6]. Different studies have yielded varying results regarding the usefulness of this procedure. Not all patients with refractory hypoxemia respond to prone positioning. Also, placing the patient in the prone position is not without complications. Avoiding such complications requires trained staff and close monitoring.

Predicting the responsiveness of improved oxygenation during prone ventilation would be beneficial for patients at high risk for complications during prone position ventilation. It is also essential to assess the improvement in prone positioning in terms of lung recruitment and improved gas exchange, which can be evaluated by improvement in PaO2/FiO2 ratio, chest imaging, or even by improvement in lung aeration score estimated by patient bedside ultrasonography. Responders have increased oxygenation while in the prone position as well as after returning to the supine position. This study aimed to assess the change in PaO2/FiO2 score (Partial pressure of Oxygen in arterial blood/Fraction of Oxygen in the Inspired gas) before prone positioning, 2 hours after proning, and 2 hours post a 12-hour prone session [7].

The study design was a cross-sectional study conducted in the Multidisciplinary ICU Department of Anesthesiology at Government Medical College, Thiruvananthapuram. The study population comprised ARDS patients admitted to the ICU over a one-year period following institutional ethics committee clearance.

The inclusion criteria included:

• Patients diagnosed with ARDS according to the Berlin definition with a PaO2/FiO2 ratio < 200mmHg.
• Patients on invasive and non-invasive mechanical ventilation.
• Patients aged >18 years.
• Hemodynamically stable patients.

The exclusion criteria were:

• Patients who were not willing to consent.
• Patients with spinal instability.
• Patients with signs of increased intracranial pressure.
• Patients with unstabilized fractures.
• Patients with open abdominal wounds.
• Uncooperative patients.

1. Description of Study Participants Based on Age Group (Table 1)

The distribution of enrolled patients based on age groups is as follows: 13.3% of patients were ≤ 30 years old, 33.3% were in the 31-40 years group, 41.7% were in the 41-50 years group, and 11.7% were in the 51-60 years group. The mean age of the study group was 40.3 ± 8.1 years, with a gender distribution of 40.0% females and 60.0% males.

1. Description of Study Participants Based on PaO2/FiO2 Score (Table 2A and Table 2B)

Table 2a presents the mean PaO2/FiO2 scores before, during, and after prone positioning. The scores were 124.7 ± 32.7 before prone positioning (P1), 136.7 ± 39.8 two hours after prone positioning (P2), and 139.8 ± 42.6 two hours post 12-hour prone session (P3). Table 2b shows the mean change in PaO2/FiO2 scores post prone positioning, with a change of 12 ± 9.9 between P2 and P1, and 15.1 ± 12.7 between P3 and P1.

1. Association between PaO2/FiO2 Score and Prone Positioning (Table 3a and Table 3b)

Table 3a presents the association between PaO2/FiO2 scores and prone positioning, indicating a significant increase in scores from before prone positioning (mean: 124.7) to two hours after prone positioning (mean: 136.7), with a paired t-test showing p < 0.001. Table 3B further reinforces this association, showing a significant increase in scores from before prone positioning (mean: 124.7) to two hours post 12-hour prone positioning (mean: 139.6), also with a paired t-test showing p < 0.001.

1. Change in PaO2/FiO2 Score after Prone Positioning (Figure 1)

Figure 1 illustrates the change in PaO2/FiO2 scores after prone positioning. It shows that 46.7% of participants experienced an improvement of ≥20 in their PaO2/FiO2 score, while 63.3% had a change < 20. Among those with a change < 20, 36.7% showed mild improvement (1 to < 20 increase), and 16.7% experienced a worsening of their PaO2/FiO2 score after prone positioning.

These findings collectively suggest a strong association between prone positioning and improved oxygenation levels, as evidenced by the significant increases in PaO2/FiO2 scores post-prone positioning and a notable proportion of participants showing clinically meaningful improvements in their oxygenation status.

The study included 60 patients with moderate to severe ARDS, and the results indicate a noticeable improvement in mean PaO2/FiO2 scores after prone positioning. Specifically, the mean PaO2/FiO2 score increased from 124.7 ± 32.7 before prone positioning (P1) to 136.7 ± 39.8 two hours after prone positioning (P2), and further to 139.8 ± 42.6 two hours post a 12-hour proning session (P3). These findings suggest a consistent and significant improvement in oxygenation levels following prone positioning (Haddam et al., [8]).

Moreover, the study observed mean differences in PaO2/FiO2 scores between P1 and P2 (12 ± 9.9) and between P1 and P3 (15.1 ± 12.7), indicating sustained improvement even after returning to the supine position post-proning. These differences were statistically significant, further supporting the effectiveness of prone positioning in enhancing oxygenation (Haddam et al., [8]).

The study categorized participants into responders and non-responders based on their PaO2/FiO2 score improvement. Responders, constituting 46.7% of the group, showed an improvement of ≥20 in their PaO2/FiO2 score, while non-responders (63.4%) either had a change < 20 or experienced a worsening of their PaO2/FiO2 score after prone positioning.

Analysis of anterior and posterior lung aeration scores revealed interesting findings. Responders had lower mean anterior and posterior lung aeration scores compared to non-responders, indicating that individuals with less lung aeration benefitted more from prone positioning in terms of oxygenation improvement. These differences were statistically significant, suggesting a correlation between lung aeration patterns and response to prone positioning (Haddam et al., [8]; Prat et al., [9]).

These findings align with previous studies by Haddam et al. and Prat et al.,[8,9]  which also noted the importance of lung ultrasound patterns in predicting oxygenation response to prone positioning. Specifically, Prat et al. found that a normal lung ultrasound pattern in supine position predicted significant improvement in PaO2/FiO2 ratio post-proning, consistent with the observations in this study (Prat et al., [9]).

The findings of this study suggest that lung ultrasound can serve as a valuable tool in predicting the oxygenation response to prone positioning in patients with ARDS. Specifically, patients with a higher posterior lung aeration score appeared to benefit more from prone positioning in terms of oxygenation improvement. These insights contribute to our understanding of individualized management strategies for ARDS patients, highlighting the potential role of lung ultrasound in guiding therapeutic interventions such as prone positioning to optimize patient outcomes.

Conflict of interest : Nil

1. Fanelli V, Vlachou A, Ghannadian S, Simonetti U, Slutsky AS, Zhang H. Acute respiratory distress syndrome: new definition, current and future therapeutic options. J Thorac Dis [Internet]. 2013 Jun [cited 2023 Mar 5];5(3). Available from: https://jtd.amegroups.com/article/view/1057
2. Ashbaugh D, Bigelow DB, Petty T, Levine B. ACUTE RESPIRATORY DISTRESS IN ADULTS. The Lancet. 1967 Aug 12;290(7511):319–23.
3. Ambati S, Yandrapalli S. Refractory Hypoxemia and Venovenous ECMO. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2023 [cited 2023 Jun 16]. Available from: http://www.ncbi.nlm.nih.gov/books/NBK560516/
4. Mora-Arteaga JA, Bernal-Ramírez OJ, Rodríguez SJ. The effects of prone position ventilation in patients with acute respiratory distress syndrome. A systematic review and metaanalysis. Med Intensiva. 2015;39(6):359–72.
5. Gattinoni L, Busana M, Giosa L, Macrì MM, Quintel M. Prone Positioning in Acute Respiratory Distress Syndrome. Semin Respir Crit Care Med. 2019 Feb;40(1):94–100.
6. Guérin C, Albert RK, Beitler J, Gattinoni L, Jaber S, Marini JJ, et al. Prone position in ARDS patients: why, when, how and for whom. Intensive Care Med. 2020 Dec;46(12):2385–96.
7. Costamagna A, Pivetta E, Goffi A, Steinberg I, Arina P, Mazzeo AT, et al. Clinical performance of lung ultrasound in predicting ARDS morphology. Ann Intensive Care. 2021 Mar 29;11(1):51.
8. Haddam M, Zieleskiewicz L, Perbet S, Baldovini A, Guervilly C, Arbelot C, et al. Lung ultrasonography for assessment of oxygenation response to prone position ventilation in ARDS. Intensive Care Med. 2016 Oct;42(10):1546–56.
9. Prat G, Guinard S, Bizien N, Nowak E, Tonnelier JM, Alavi Z, et al. Can lung ultrasonography predict prone positioning response in acute respiratory distress syndrome patients? J Crit Care. 2016 Apr;32:36–41.