# Situation: A 46-year-old woman with severe community-acquired pneumonia and septic shock is in the intensive care unit (ICU). On day 2 she is intubated and diagnosed with acute respiratory distress syndrome (ARDS): her chest X-ray shows bilateral opacities, and an echocardiogram shows normal heart function. She is 160 cm tall and weighs 82 kg. The team places her in the prone position for 16 hours a day. Her husband asks how lying face down helps her lungs. Which explanation should the nurse give?

> source: MyMerci (mymerci.kr)  
> url: https://mymerci.kr/pages/nclex_q.php?qn_id=629882  
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> subject: Nursing Practice III — Care of Clients with Problems in Surgery, Oxygenation, Fluid and Electrolytes, Infectious, Inflammatory and Immunologic Response, Cellular Aberrations

## 문제

Situation: A 46-year-old woman with severe community-acquired pneumonia and septic shock is in the intensive care unit (ICU). On day 2 she is intubated and diagnosed with acute respiratory distress syndrome (ARDS): her chest X-ray shows bilateral opacities, and an echocardiogram shows normal heart function. She is 160 cm tall and weighs 82 kg.

The team places her in the prone position for 16 hours a day. Her husband asks how lying face down helps her lungs. Which explanation should the nurse give?

## 보기

1. It rests the lungs, so the ventilator can deliver larger breaths safely
2. It takes pressure off the heart so that it pumps more blood to the lungs
3. It lets secretions drain from the mouth and stops pneumonia from spreading
4. It reopens collapsed lung at the back and matches air to blood flow **✔ 정답**

**정답: 4**

## 해설

Prone positioning lets the dorsal (back) lung regions, which are compressed and collapsed when she lies on her back, reopen. This improves ventilation–perfusion matching and secretion drainage and, in moderate to severe ARDS, reduces mortality when used for at least 12 to 16 hours a day.

## 심화 해설

Why prone positioning helps in ARDS

The most accurate explanation is that prone positioning reopens collapsed dorsal lung regions and improves ventilation–perfusion (V/Q) matching. In ARDS, the lungs are heavy with edema, and when the patient lies supine, the weight of the overlying lung, heart, and abdominal contents compresses the dependent (posterior/dorsal) alveoli, causing them to collapse. Blood flow, however, continues to preferentially go to those same dorsal regions because of gravity. The result is a large area of lung that is perfused but not ventilated — a shunt — which produces severe hypoxemia.

When the patient is turned prone, the dorsal alveoli are no longer compressed by the weight of the heart and edematous lung above them. They reopen and participate in gas exchange again. At the same time, blood flow remains relatively greater in the dorsal regions, so the reopened alveoli now receive both air and blood. This V/Q matching is the primary mechanism by which prone positioning improves oxygenation [1][2][4].

A second, related benefit is that prone positioning makes ventilation more homogeneous. In the supine position, the ventral (anterior) alveoli receive most of the tidal volume and become overdistended, while the dorsal alveoli remain collapsed. This creates dangerous regional differences in lung stress and strain. Turning the patient prone redistributes the gas–tissue ratio more evenly along the dependent–nondependent axis, reducing overdistension of the ventral lung and recruiting the dorsal lung [2][3]. This attenuation of ventilator-induced lung injury (VILI) is thought to be a major reason why prone positioning reduces mortality in moderate-to-severe ARDS, not just because it improves oxygenation [2][3].

The other options do not capture the core mechanism. Prone positioning does not “rest the lungs” in the sense of allowing larger tidal volumes; in fact, it is used together with low tidal volume lung-protective ventilation, and the goal is to avoid large breaths. It does not primarily work by taking pressure off the heart to increase cardiac output, although the prone position can alter the position of the heart relative to the lungs. Secretion drainage is a real but secondary benefit; the dominant effect is alveolar recruitment and V/Q improvement [1][2].

Key point! Prone positioning is indicated early in intubated patients with moderate-to-severe ARDS, typically defined by a PaO2/FIO2 ratio < 150 mmHg, and should be maintained for at least 12 to 16 hours per day to achieve the mortality benefit [1].

| Mechanism | Supine position | Prone position |
| --- | --- | --- |
| Dorsal alveoli | Compressed by heart and edematous lung; collapsed | Reopen as overlying weight is removed |
| Ventilation distribution | Preferentially to ventral lung; dorsal lung underventilated | More even distribution along dependent–nondependent axis |
| Perfusion distribution | Greater in dorsal lung (gravity) | Remains relatively greater in dorsal lung |
| V/Q matching | Poor; large shunt fraction | Improved; reopened dorsal alveoli receive both air and blood |
| Lung stress/strain | Ventral overdistension, dorsal collapse | More homogeneous; less VILI |

Watch out! The improvement in oxygenation after turning prone is not immediate in every patient, and a lack of immediate PaO2 rise does not mean the therapy is failing. The survival benefit is also related to reduced VILI, which is a longer-term protective effect rather than an acute gas-exchange change [2][3].References (research sources)

- [1]Prone positioning in ARDS.Research articleEhrmann S, Li J, Liu L, Guérin C. (2026) · DOI: 10.1007/s00134-026-08543-x

- [2]Prone position in ARDS patients: why, when, how and for whom.Research articleGuérin C, Albert RK, Beitler J, Gattinoni L, Jaber S, Marini JJ (2020) · DOI: 10.1007/s00134-020-06306-w

- [3]Electrical Impedance Tomography for Monitoring Prone Positioning in ARDS.Research articleStanford-Hill R, Moulton AW, Morais CCA, Katira BH. (2026) · DOI: 10.1177/19433654261474271

- [4]Prone positioning improves ventilation-perfusion matching assessed by electrical impedance tomography in patients with ARDS: a prospective physiological study.Research articleWang YX, Zhong M, Dong MH, Song JQ, Zheng YJ, Wu W (2022) · DOI: 10.1186/s13054-022-04021-0

## 임상 시나리오

Prone Positioning in ARDSWhy face-down improves oxygenation
In ARDS, edematous lungs collapse in dorsal (dependent) regions when supine, creating shunt — perfused but non-ventilated alveoli. Turning prone reopens these alveoli and restores ventilation-perfusion (V/Q) matching, the primary mechanism for improved oxygenation.

Prone positioning also makes ventilation more homogeneous, reducing overdistension of ventral alveoli and lowering ventilator-induced lung injury. In moderate-severe ARDS, use for at least 12-16 hours/day reduces mortality.

CautionProne positioning requires careful attention to pressure injuries, endotracheal tube and line security, and hemodynamic stability during turns. Secretion drainage is a secondary benefit, not the primary rationale.

## 핵심 개념

- **Ventilation-perfusion (V/Q) matching** — The balance between air reaching alveoli and blood flow to those alveoli; mismatch causes hypoxemia.
- **Dorsal (dependent) lung regions** — Posterior lung areas that are compressed and collapsed when supine, but reopen in prone position.
- **Shunt** — Perfused but non-ventilated lung units that cause refractory hypoxemia in ARDS.
- **Prone positioning** — Placing patient face down for 12-16 hours daily to improve oxygenation and reduce mortality in moderate-severe ARDS.
- **ARDS** — Acute respiratory distress syndrome with bilateral opacities and non-cardiogenic pulmonary edema.

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