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Therapeutic positioning, ventilation–perfusion and the zones of the lung

Positioning is the cheapest treatment in the ICU and the papers keep asking about it: positioning in ICU (W22, W25), sitting upright (W21), zones of the lung (S25), causes of altered V/Q (S23) and positions that ease breathlessness (S22). One idea runs through all of them: gravity decides where air and blood go.

Syllabus
Ch 4(c) Therapeutic positioning to improve ventilation and perfusion matching; 4(d) positioning to alleviate dyspnoea
Priority
High yield. In 6 of 20 papers (2016–2026), 3 of them as 7 marks.
Asked as
7m W21W22W25 3m S22S23S25
Usual split
7-mark positioning in ICU or sitting upright; 3-mark zones, V/Q causes, positions for dyspnoea
Checked against
West's zones and regional V/Q (standard respiratory physiology); prone positioning trial (2013); positioning evidence in adults and infants. Full list under Sources.
Content checked
19 October 2026. Next review due October 2027.
My revision

1Ventilation and perfusion in the upright lung

  • Ventilation (V) is the air reaching the alveoli (about 4 L/min at rest); perfusion (Q) is the blood flowing past them (about 5 L/min). The overall V/Q ratio is therefore about 0.8.
  • In upright lungs both ventilation and perfusion are greater at the base than the apex, but perfusion rises much more steeply, because gravity pulls blood down.
  • So V/Q is high at the apex (about 3) and low at the base (about 0.6).
  • Basal alveoli get more ventilation because they start smaller (the pleural pressure there is less negative), so they are on the steep, easily filled part of the pressure–volume curve.

West's zones of the upright lung (3 marks in S25)

Upright lung divided into zones 1, 2 and 3 An outline of an upright lung. A thin top band is zone 1, where alveolar pressure exceeds arterial and venous pressure, so there is no blood flow; it appears only with low blood pressure or high airway pressure. The middle band is zone 2, where arterial pressure exceeds alveolar pressure which exceeds venous pressure, so flow depends on the arterial minus alveolar difference. The lower band is zone 3, where arterial and venous pressures both exceed alveolar pressure, so flow is continuous. An arrow beside the lung shows ventilation and perfusion increasing towards the base, perfusion more steeply. V/Q is about 3 at the apex and about 0.6 at the base. Zone 1 Zone 2 Zone 3 PA > Pa > Pv Pa > PA > Pv Pa > Pv > PA no blood flow: alveolar dead space (only with low BP or high airway pressure) flow depends on arterial minus alveolar pressure ("waterfall") continuous flow, depends on arterial minus venous pressure V and Q rise towards the base apex: V/Q about 3 base: V/Q about 0.6
PA = alveolar pressure, Pa = pulmonary arterial pressure, Pv = pulmonary venous pressure. In a healthy upright person there is normally no zone 1; it appears when blood pressure falls or airway pressure rises (for example high PEEP).
If the question means the airway zones instead

Some books describe the conducting zone (trachea to terminal bronchioles, airway generations 0–16: no gas exchange; anatomical dead space about 150 mL in an adult) and the respiratory zone (respiratory bronchioles, alveolar ducts and alveolar sacs, generations 17–23: gas exchange). Write both if the wording is unclear.

2Causes of altered V/Q (3 marks in S23)

TypeMeaningCauses
Low V/Q (shunt when no ventilation)Blood passes alveoli that get too little airSecretions and mucus plugs; bronchospasm; collapse (atelectasis); consolidation (pneumonia); pulmonary oedema; ARDS; lying flat, obesity, pain-limited breathing after surgery
High V/Q (dead space when no perfusion)Air reaches alveoli that get too little bloodPulmonary embolism; low cardiac output or blood loss; emphysema (capillaries destroyed); high PEEP or over-stretched alveoli; pulmonary vascular disease

Both lower PaO₂. Low V/Q improves with oxygen and with positions that send air to the affected area; a true shunt (no ventilation at all) barely responds to oxygen until the alveoli are re-opened.

3Effects of body positions

PositionPhysiological effectUsed for
Upright sitting or standingLargest lung volume (FRC); diaphragm moves best; least airway closure at the basesAlmost everyone: post-op, ICU, weaning trials, breathlessness
SupineAbdominal contents push the diaphragm up; FRC falls; the back of the lungs collapses; aspiration risk risesAvoid long periods; exception: tetraplegia, where vital capacity is often better lying than sitting (use an abdominal binder in sitting)
Side-lying, breathing spontaneously (adult)The lower (dependent) lung gets more ventilation and more perfusionGood lung down to improve oxygenation in one-sided lung disease; affected lung up to drain it
Side-lying, on a controlled ventilatorAir goes more to the upper lung, blood to the lower lung, so matching can worsenWatch SpO₂ after every turn
Side-lying, infantVentilation goes preferentially to the upper lung (soft chest wall)In babies with one-sided disease, put the good lung up
ProneMore even ventilation; re-opens the large back (dorsal) areas of lung; better V/QSevere ARDS (PaO₂/FiO₂ below 150), 16 hours or more a day: lowered deaths in the PROSEVA trial
Head-down tiltGravity drains the lower lobes; but increases work of breathing, reflux and intracranial pressurePostural drainage only when no contraindication
Head-up 30–45°Less reflux and aspiration; better FRC than flatAll ventilated patients (prevents ventilator-associated pneumonia); raised ICP
Forward-lean sitting or standingAbdomen pushes a flattened diaphragm up into a better shape; arms fixed so accessory muscles helpBreathlessness in COPD

Exceptions to "good lung down": a lung abscess or bleeding in the affected lung (pus or blood could spill into the good lung below) and infants.

4Sitting upright as a therapeutic position (7 marks in W21)

SystemPhysiological application
RespiratoryDiaphragm descends and abdominal contents fall away: larger FRC and tidal volume; fewer collapsed basal alveoli; better V/Q and oxygenation; lower work of breathing; a deeper breath before a cough, so a stronger cough; less reflux and aspiration
CardiovascularKeeps the blood pressure reflexes working (prevents the fall in BP on standing that follows bed rest); with legs down it lowers venous return, which eases breathlessness in pulmonary oedema and heart failure (orthopnoea)
Neurological and otherWakefulness, orientation, communication and safer swallowing; head-up lowers intracranial pressure; first step towards standing and walking; relieves pressure on the back

Used in: post-operative patients, ICU and ventilated patients (including breathing trials), COPD and asthma (with forward lean), pulmonary oedema, neurological patients, older people on bed rest. Watch for sliding into a slumped position, which squashes the abdomen and loads the sacrum.

5Positioning in the ICU (7 marks in W22 and W25)

Physiological effects and benefits (W25)

  • Improves V/Q matching and oxygenation; increases lung volume; re-opens collapsed areas.
  • Drains secretions with gravity; lowers the work of breathing.
  • Head-up positions reduce aspiration and ventilator-associated pneumonia.
  • Regular turns prevent pressure injuries, contractures and nerve compression.
  • Upright positions keep the blood pressure reflexes and muscles working; they prepare for mobilisation and raise alertness.
  • Comfort, communication and dignity.

Examples (W22)

SituationPositionReason
Right lower lobe pneumonia, breathing spontaneouslyLeft side-lying (good lung down)Better oxygenation; the right lung is uppermost and drains
Left lower lobe collapseRight side-lying, slightly turned, foot of bed raised if allowedAir and gravity drainage to the collapsed lobe
Severe ARDSProne, 16 hours or more a dayRe-opens the back of the lungs; better oxygenation and survival
Head injury with raised ICPHead up 30°, neck straight, no tight ties, no head-down tiltVenous drainage from the brain lowers ICP
Every ventilated patientHead of bed 30–45°Less aspiration and pneumonia
Pulmonary oedemaHigh sitting with legs downLess venous return; easier breathing
Abdominal distension or obesitySitting or reverse Trendelenburg (whole bed tilted head-up)Lets the diaphragm descend
TetraplegiaLying or semi-reclined when breathless; binder when sittingAbdominal contents support the diaphragm
Pressure injury risk30° tilted side-lying; heels floated; turns on a scheduleOff-loads sacrum, hips and heels
Burns, hemiplegia, unconscious patientsAnti-deformity and tone-reducing positionsPrevents contractures
Before and after every turn

Check SpO₂, heart rate and BP; secure the tube, lines and drains; give the patient time to settle (oxygenation can dip for several minutes after a turn).

6Positions to ease breathlessness (3 marks in S22)

  • High side-lying with pillows, top arm supported.
  • Forward-lean sitting with forearms on the knees or on pillows on a table.
  • Relaxed sitting back in a chair with forearms supported.
  • Forward-lean standing with forearms on a wall, rail or trolley; leaning back against a wall with feet slightly apart.

Rationale: in forward lean, the abdominal contents push a flat diaphragm up into a dome, so it contracts more effectively; fixing the arms lets pectoral and other accessory muscles help breathing; relaxed, supported positions lower muscle work and anxiety. Details on the dyspnoea card.

7Exam corner

Past questions (MUHS, 2016–2026)

  • 7-mark questionsPhysiological application of sitting upright as a therapeutic position (W21). Positioning of the patient in ICU with examples (W22). Physiological effects and benefits of body positioning in ICU (W25).
  • 3-mark questionsRationale of therapeutic positioning to alleviate dyspnoea (S22). Three causes of altered ventilation–perfusion ratio (S23). Physiological zones of ventilation (S25).

Answer skeleton: positioning in ICU, 7 marks

  1. [1] Definition: therapeutic positioning uses gravity to improve breathing, circulation and comfort, and to prevent complications.
  2. [2] Physiology: V/Q gradient and zones (draw the zones), good lung down, prone, head-up.
  3. [3] Examples table: pneumonia, collapse, ARDS, raised ICP, VAP prevention, pressure care.
  4. [1] Precautions: monitor before and after turns; secure tubes and lines.

Test yourself

3 mWrite the pressure relationships of West's three zones.

Zone 1: alveolar > arterial > venous (no flow). Zone 2: arterial > alveolar > venous (flow depends on arterial minus alveolar). Zone 3: arterial > venous > alveolar (continuous flow).

MCQAn adult with right lower lobe pneumonia, breathing on his own, has SpO₂ 89%. Which position will most likely raise his SpO₂?A. Right side-lyingB. Left side-lyingC. Flat supineD. Head-down tilt

B. Good lung down: the healthy left lung below gets more ventilation and more blood.

MCQThe V/Q ratio at the apex of the upright lung is about:A. 0.6B. 0.8C. 1.0D. 3

D. Little blood reaches the apex, so ventilation exceeds perfusion.

 Sources

  1. MUHS BPTh syllabus (2012) and MUHS IV BPTh question papers 2016–2026 (muhsonline.com, unofficial archive; Dhole Patil College of Physiotherapy past-paper page).
  2. West JB, Luks AM. West's Respiratory Physiology: The Essentials (zones, regional V/Q).
  3. Guérin C, et al. Prone positioning in severe acute respiratory distress syndrome (PROSEVA). N Engl J Med 2013;368:2159–68.
  4. Heaf DP, et al. Postural effects on gas exchange in infants. N Engl J Med 1983;308:1505–8.
  5. Structure and classical content: Frownfelter & Dean (2022), chapters on positioning and mobilisation; Hough, Physiotherapy in Respiratory and Cardiac Care; Main & Denehy (2016).

Educational notes for exam preparation. Not medical advice.

Version 1. Content checked 19 October 2026. Next review October 2027.