Oxygen therapy, aerosol therapy and airway clearance

In short

Oxygen therapy is the controlled delivery of an inspired oxygen fraction above 21% to correct tissue hypoxia; aerosol therapy delivers drugs and water as suspended particles to the airway; airway clearance uses breathing techniques, pressure devices and suction to move retained secretions out. Together they are the three bedside skills that define a respiratory therapist's day in an ICU, emergency department, neonatal unit, pulmonology ward or home-care service. In Indian B.Sc Respiratory Therapy / Cardio-Pulmonary Technology programmes this topic carries the largest practical and viva weight, and it is the staple of technical rounds at Apollo, Fortis, Manipal, Narayana Health, Medanta, AIIMS and the ESIC and state-health services, as well as of oxygen-equipment and homecare employers. Medical oxygen is a drug in India (Drugs and Cosmetics Act, 1940 and Rules, 1945; monographs for *Oxygen* and *Oxygen 93 per cent* in the Indian Pharmacopoeia 2026) and is in the National List of Essential Medicines, so dose, documentation and device choice are clinical decisions, not comfort measures.

Explained

Why oxygen is given: hypoxia, hypoxaemia and targets

• Hypoxaemia = low oxygen in arterial blood (PaO₂ < 60 mmHg or SaO₂/SpO₂ < 90% on room air). Hypoxia = inadequate oxygen at the tissue.
• Four types of hypoxia: hypoxaemic (low PaO₂), anaemic (low Hb or carboxyhaemoglobin), circulatory/stagnant (low cardiac output, shock), histotoxic (cyanide - cells cannot use oxygen). Only the first responds well to raising FiO₂.
• Five mechanisms of hypoxaemia: low inspired PO₂ (altitude), hypoventilation, diffusion limitation, V/Q mismatch (responds to oxygen) and shunt (responds poorly - needs PEEP/recruitment).
• Targets (broad consensus, varies by protocol): SpO₂ 94-98% for most acute patients; 88-92% in COPD and others at risk of hypercapnic respiratory failure; 90-95% in preterm neonates to limit retinopathy of prematurity. Carbon monoxide poisoning and pneumothorax are treated with the highest available FiO₂ regardless of SpO₂, because SpO₂ is falsely high in CO poisoning.
• Long-term oxygen therapy (LTOT): ≥15 h/day for stable PaO₂ ≤ 55 mmHg, or 56-59 mmHg with cor pulmonale, oedema or haematocrit > 55%. Improves survival in COPD; nocturnal-only use does not.
• Oxygen is not a treatment for breathlessness with a normal SpO₂ - it does not relieve dyspnoea in a non-hypoxaemic patient.

Oxygen sources, supply and Indian regulation

• Cylinders: seamless steel/aluminium, filled to about 137-150 kgf/cm² (bar). Indian hospital sizes: B type ~10 L water capacity (~1,400 L gas) and D type ~46.7 L (~6,400-7,000 L gas). Governed by the Gas Cylinders Rules, 2016 under the Explosives Act, 1884, enforced by PESO (Petroleum and Explosives Safety Organisation, Nagpur). Colour identification of gas cylinders follows the relevant Indian Standard - medical oxygen cylinders in India are black with a white shoulder; always confirm by the label and valve fitting, not colour alone.
• Pressure regulation: cylinder pressure is reduced by a pressure-reducing valve/regulator to about 4 bar (400 kPa), the standard pipeline terminal pressure, then metered by a Thorpe tube flowmeter (read at the centre of the ball/float, held vertical). A pressure-compensated flowmeter reads correctly against downstream resistance.
• Liquid medical oxygen (LMO): stored cryogenically at about −183 °C in a VIE (vacuum-insulated evaporator); 1 L of liquid oxygen ≈ 860 L of gas. Cheapest for large hospitals; needs a reserve manifold bank.
• PSA (pressure swing adsorption) plants: separate oxygen from air on a zeolite molecular sieve on site; product is 93% ± 3% oxygen ("Oxygen 93 per cent", IP), the balance mostly argon and nitrogen. Hundreds were installed in Indian hospitals after the 2021 oxygen crisis under PM-CARES and the NHM; NABH and state health departments now expect an on-site source plus a cylinder bank.
• Oxygen concentrators: small PSA units for homecare, typically 5 L/min or 10 L/min at 90-96% purity; purity and FiO₂ fall as flow is raised beyond the rated output, so they are unsuitable for high-flow needs.
• Medical gas pipeline systems (MGPS): colour-coded copper lines, area valve service units, non-interchangeable NIST/DISS terminal units, alarms at 20% above and below working pressure. Designed and tested to the Indian Standard for medical gas pipeline systems (IS 7396 series) and verified at commissioning by purity, pressure, flow, cross-connection and particulate tests.
• Safety: oxygen is not flammable but violently supports combustion. No oil, grease or petroleum jelly on fittings or on the patient's face; no smoking, spark or open flame; cylinders chained upright, trolley-transported, valve opened slowly away from the body. "Oxygen enrichment" fires in ICUs are a recurring Indian incident report.
• Tax/commercial note: medical oxygen and oxygen delivery devices sit in the lower GST slab (slabs since 22 September 2025 are 5%, 18% and 40%); exact classification can change with notifications, so verify the current rate before quoting it in a tender.

Delivery devices: low-flow, reservoir, high-flow

• Low-flow (variable-performance) devices supply less than the patient's peak inspiratory flow, so room air dilutes the oxygen and FiO₂ varies with tidal volume, respiratory rate and flow:
  • Nasal cannula (prongs): 1-6 L/min, FiO₂ roughly 24% at 1 L/min rising about 4 percentage points per litre to ~44% at 6 L/min. Needs humidification above 4 L/min; above 6 L/min it only dries and hurts the nose.
  • Simple face mask (Hudson): 5-10 L/min, FiO₂ ~35-60%. Never run below 5 L/min - the mask body becomes a dead space and the patient rebreathes CO₂.
• Reservoir devices:
  • Partial rebreathing mask: bag plus mask, no valve between them; 8-12 L/min, FiO₂ ~0.4-0.7.
  • Non-rebreathing mask (NRBM): one-way valve between bag and mask and flaps on the exhalation ports; 10-15 L/min, FiO₂ ~0.6-0.8 in practice (0.9+ only with a perfect seal). The bag must not collapse on inspiration.
• High-flow (fixed-performance) devices meet or exceed inspiratory demand, so FiO₂ is predictable:
  • Air-entrainment (venturi) mask: oxygen jets through a constriction and entrains a fixed proportion of air. Interchangeable colour-coded jets give 24, 28, 31, 35, 40, 50 (sometimes 60)%. The device of choice in COPD where a known, modest FiO₂ matters.
  • Air-entrainment nebuliser / large-volume nebuliser with aerosol mask, face tent, T-piece (Briggs adaptor) or tracheostomy collar - gives controlled FiO₂ *and* bland aerosol humidity.
  • High-flow nasal cannula (HFNC/HFNO): heated, fully humidified gas at 20-60 L/min with FiO₂ independently set from 0.21 to 1.0; washes out nasopharyngeal dead space, gives a small distending pressure (~2-5 cmH₂O) and reduces work of breathing. Monitored with the ROX index; used widely in Indian ICUs since COVID, along with awake prone positioning.
• Beyond these: oxygen hood/head box and incubators for neonates, CPAP/NIV (a separate topic) and mechanical ventilation.

Humidification and the humidity deficit

• Gas from a cylinder or pipeline is anhydrous. Inspired gas must reach ISB (isothermic saturation boundary) conditions - 37 °C, 100% RH, 44 mg/L absolute humidity - by about the 5th generation bronchus.
• The humidity deficit is the water the airway must supply itself; a persistent deficit thickens secretions, impairs ciliary beat and causes mucus plugging and atelectasis.
• Devices: bubble humidifier (low-flow nasal oxygen; adds little), passover/wick and heated-wire humidifier (ventilators, HFNC - delivers 33-41 mg/L), HME ("artificial nose") - returns ~70% of exhaled heat and moisture, 26-32 mg/L; contraindicated with thick/bloody secretions, large leaks or low tidal volumes, and it adds dead space and resistance.
• A bypassed upper airway (ETT, tracheostomy) must have active humidification or an HME - this is an exam favourite.

Aerosol therapy: physics, devices and drugs

• An aerosol is a suspension of solid or liquid particles in gas. The key descriptor is MMAD (mass median aerodynamic diameter) with GSD (geometric standard deviation; <1.2 = monodisperse).
• Deposition mechanisms: inertial impaction (>5 µm, high velocity, at bends and the oropharynx), gravitational sedimentation (1-5 µm, favoured by breath-hold) and Brownian diffusion (<1 µm, alveolar).
• Target sizes: >10 µm - mouth and throat; 5-10 µm - large airways; 1-5 µm (optimal therapeutic range) - lower airways and parenchyma; <1 µm - mostly exhaled.
• Device types:
  • Jet (small-volume) nebuliser: needs a driving gas of 6-8 L/min; fill 4-5 mL; dead (residual) volume 0.5-1 mL - never fill below 2-3 mL. Breath-enhanced and breath-actuated versions cut waste. Driving a nebuliser with oxygen in a COPD patient can raise FiO₂ - use compressed air with nasal oxygen if hypercapnia is a risk.
  • Ultrasonic nebuliser: piezoelectric crystal; higher output, heats the drug, unsuitable for suspensions like budesonide.
  • Vibrating mesh nebuliser: low residual volume, silent, efficient, no driving gas; preferred in-line on ventilators and HFNC.
  • pMDI (pressurised metered-dose inhaler): HFA propellant; technique = shake, exhale to FRC, slow deep inhalation (~30 L/min) with actuation at the start of inspiration, then 10-second breath-hold; 1 min between puffs; prime after disuse. A spacer/valved holding chamber removes the need for coordination, cuts oropharyngeal impaction and is mandatory in children (with a mask below ~4 years - tidal breathing, 5-6 breaths per actuation).
  • DPI (dry powder inhaler): breath-actuated, needs a fast, forceful inhalation (≥30-60 L/min); useless in a patient with poor inspiratory effort and spoilt by humidity - never exhale into the device.
  • SMI (soft mist inhaler): slow-moving fine mist, long plume duration, high lung deposition.
• Common nebulised drugs: salbutamol 2.5-5 mg, levosalbutamol 0.63-1.25 mg, ipratropium 500 µg, budesonide 0.5-1 mg, adrenaline (croup, 1:1000), hypertonic saline 3-7% and mannitol (mucoactive/osmotic), N-acetylcysteine (mucolytic), dornase alfa (cystic fibrosis), colistin/tobramycin (bronchiectasis), surfactant in neonates (instilled, not nebulised, in standard practice).
• Hazards: paradoxical bronchospasm, tachycardia and tremor, hypokalaemia with repeated SABA, oral candidiasis and dysphonia with inhaled steroids (rinse the mouth), infection from a shared or wet nebuliser cup, and ambient drug exposure to staff - a reason for mouthpieces and filters.

Airway clearance: mechanisms and techniques

• Normal clearance = mucociliary escalator (cilia beating ~1,000/min in a sol layer under a gel layer) + cough. Cough has four phases: irritation, inspiration (to ~85-90% TLC), compression (glottis closed, intrathoracic pressure 100-200 cmH₂O for ~0.2 s) and expulsion. Anything that breaks a phase - tracheostomy (no glottis), weak abdominals, pain, sedation, neuromuscular disease - needs an assisted technique.
• Indications: retained secretions with ineffective cough, atelectasis from mucus plugging, bronchiectasis, cystic fibrosis, post-operative and neuromuscular patients, ventilated patients.
• Techniques:
  1. Postural drainage - position the involved segment uppermost so gravity drains it; the classical set covers all segments of both lungs, up to 11 positions; each 3-15 min. Head-down positions are now used sparingly.
  2. Percussion (clapping) with cupped hands, 3-5 min per segment, over the chest wall - never over spine, kidneys, breast, fractured ribs or drains.
  3. Vibration/shaking during expiration only.
  4. ACBT (active cycle of breathing techniques) - breathing control → 3-4 thoracic expansion exercises with a 3-s hold → FET (huff/forced expiratory technique). A huff with an open glottis moves the equal pressure point peripherally and avoids airway collapse - gentler and often more effective than a cough.
  5. Autogenic drainage - breathing at low, then mid, then high lung volumes to mobilise mucus centrally.
  6. PEP therapy - exhaling against 10-20 cmH₂O to splint airways open and fill distal units through collateral channels; oscillatory PEP (Flutter, Acapella, Shaker, Aerobika) adds vibration that shears mucus and is cheap, portable and self-administered - the practical default for Indian outpatients.
  7. High-frequency chest wall oscillation (HFCWO, "the vest") and IPV (intrapulmonary percussive ventilation) - equipment-intensive, used in CF and neuromuscular centres.
  8. Mechanical insufflation-exsufflation (MI-E, cough assist) - typically about +40 then −40 cmH₂O; indicated when peak cough flow < 270 L/min (ineffective below ~160 L/min), chiefly in neuromuscular disease.
  9. Incentive spirometry, IPPB, lung-volume recruitment ("breath stacking"), early mobilisation and adequate systemic hydration support all of the above.
• Suctioning: oropharyngeal (Yankauer), nasotracheal, or via ETT/tracheostomy. Pre-oxygenate with FiO₂ 1.0, sterile technique, catheter outer diameter ≤ half the tube's internal diameter, negative pressure 100-150 mmHg in adults (80-100 in children, 60-80 in neonates), suction applied only on withdrawal, total pass ≤ 10-15 s, monitor SpO₂, ECG and secretion character. Routine saline instillation is not recommended - it dislodges biofilm into the lower airway. Closed in-line suction preserves PEEP and reduces desaturation and aerosol spread; subglottic-secretion-drainage tubes reduce ventilator-associated pneumonia.
• Complications of suctioning: hypoxaemia, arrhythmia and bradycardia (vagal), mucosal trauma and bleeding, bronchospasm, atelectasis, raised intracranial pressure, infection.
• Cautions/contraindications for chest physiotherapy: untreated pneumothorax, frank haemoptysis, unstable spine or rib fractures, raised ICP (avoid head-down), recent oesophageal or neurosurgery, severe osteoporosis, coagulopathy, haemodynamic instability.

Oxygen hazards, monitoring and weaning

• Oxygen-induced hypoventilation/hypercapnia in chronic CO₂ retainers - loss of hypoxic drive plus reversal of hypoxic pulmonary vasoconstriction worsening V/Q matching, and the Haldane effect. Do not withdraw oxygen abruptly; titrate to 88-92% and recheck gases.
• Absorption atelectasis - nitrogen washout at high FiO₂ leaves units that collapse on absorption.
• Oxygen toxicity - FiO₂ > 0.5-0.6 beyond 24-48 h causes tracheobronchitis then diffuse alveolar damage (Lorrain Smith effect, pulmonary); very high partial pressures (hyperbaric) cause seizures (Paul Bert effect, CNS).
• Retinopathy of prematurity and bronchopulmonary dysplasia in neonates; fire risk.
• Monitoring: continuous SpO₂ (unreliable with poor perfusion, motion, nail polish, dark pigmentation at low saturations, and falsely high in CO poisoning and methaemoglobinaemia), arterial blood gases, respiratory rate and work of breathing, FiO₂ with a calibrated oxygen analyser, and documentation of device, flow, FiO₂ and target range on the chart as a prescription.
• Wean by stepping FiO₂/flow down and confirming SpO₂ stays in target at rest and on walking before discharge.

Key points

• FiO₂ - fraction of inspired oxygen; room air 0.21. Beyond 0.6 for long periods risks toxicity and atelectasis.
• Low-flow device - delivers less than inspiratory demand, so FiO₂ is variable (cannula, simple mask).
• High-flow/fixed-performance device - meets or exceeds demand, so FiO₂ is predictable (venturi mask, HFNC, air-entrainment nebuliser).
• Nasal cannula rule of thumb - ~24% at 1 L/min, +4 percentage points per extra litre up to 6 L/min.
• Simple mask minimum 5 L/min - below this the mask becomes dead space and CO₂ rebreathing occurs.
• NRBM - one-way valves, 10-15 L/min, FiO₂ ~0.6-0.8; bag must stay inflated through inspiration.
• Air:O₂ entrainment ratio - falls as FiO₂ rises, so total flow falls; high-FiO₂ venturi jets are only marginally "high flow".
• HFNC - 20-60 L/min heated humidified gas, dead-space washout plus small PEEP, FiO₂ set independently.
• COPD target 88-92% - uncontrolled oxygen can cause hypercapnic narcosis.
• Humidity at ISB - 37 °C, 100% RH, 44 mg/L; a bypassed upper airway always needs active humidification or an HME.
• MMAD 1-5 µm - the respirable range; deposition by impaction, sedimentation and diffusion.
• pMDI technique - slow deep breath with a 10-s hold; DPI technique - fast forceful breath. Confusing the two is the commonest patient error.
• Spacer - improves lung dose, removes coordination need, cuts oral candidiasis; mask spacer for under-4s.
• Nebuliser dead volume 0.5-1 mL - fill to at least 2-3 mL and drive at 6-8 L/min.
• Huff (FET) - open-glottis forced expiration; moves the equal pressure point peripherally and avoids airway collapse.
• Oscillatory PEP - cheap, portable, self-administered; largely replaces routine percussion and postural drainage in stable disease.
• Suction rules - catheter OD ≤ ½ tube ID, 100-150 mmHg, ≤15 s, on withdrawal only, pre-oxygenate, no routine saline.
• Peak cough flow < 270 L/min - cough is at risk; consider lung-volume recruitment or mechanical insufflation-exsufflation.

Formulas and examples

Example 1 - venturi mask flow check. A COPD patient is ordered 40% oxygen by air-entrainment mask with the jet set at 8 L/min oxygen.

• Air:O₂ = (100 − 40)/(40 − 21) = 60/19 = 3.16 : 1
• Air entrained = 3.16 × 8 = 25.3 L/min; total flow = 8 + 25.3 = 33.3 L/min
• An adult's peak inspiratory flow is roughly 30 L/min, so 33.3 L/min just meets demand - FiO₂ is reliable. At the 60% jet the ratio is (100−60)/(60−21) = 40/39 = 1.03:1, so even at 15 L/min total flow is only 15 × 2.03 ≈ 30.4 L/min - barely adequate, which is why high-FiO₂ venturi masks lose their "fixed performance".

Example 2 - cylinder duration. A D-type cylinder of 46.7 L water capacity reads 120 kgf/cm². Flow ordered 6 L/min; keep a 10% reserve.

• Contents = 46.7 × 120 = 5,604 L; usable = 5,604 − 560 = 5,044 L
• Duration = 5,044 / 6 = 841 min ≈ 14 h 0 min
• At 15 L/min on an NRBM: 5,044 / 15 = 336 min ≈ 5 h 36 min - plan the transport, and carry a spare.

Example 3 - oxygen content and the limits of SpO₂. Two patients, both SpO₂/SaO₂ 96%, PaO₂ 85 mmHg, cardiac output 5 L/min; Hb 14 g/dL and 9 g/dL.

• Hb 14: CaO₂ = 1.34 × 14 × 0.96 + 0.003 × 85 = 18.01 + 0.26 = 18.27 mL/dL; DO₂ = 18.27 × 5 × 10 = 914 mL/min
• Hb 9: CaO₂ = 1.34 × 9 × 0.96 + 0.26 = 11.58 + 0.26 = 11.84 mL/dL; DO₂ = 592 mL/min
• Conclusion: the anaemic patient carries 35% less oxygen with an identical pulse oximeter reading - raising FiO₂ adds only the small dissolved term. Transfusion or cardiac output, not oxygen, is the treatment.

Example 4 - P/F ratio and escalation. A patient on an NRBM, estimated FiO₂ 0.60, has PaO₂ 72 mmHg.

• P/F = 72 / 0.60 = 120 → moderate ARDS range (100-200).
• On HFNC at 50 L/min, FiO₂ 0.60, SpO₂ 92%, RR 28: ROX = (92/0.60)/28 = 153.3/28 = 5.5 - above the ~4.88 threshold reported at 12 h, i.e. HFNC is working; a value falling below it argues for intubation.

Example 5 - humidity deficit. A tracheostomised patient breathes 10 L/min of gas delivered at 30 °C, fully saturated.

• Absolute humidity delivered = 30.4 mg/L; deficit = 44 − 30.4 = 13.6 mg/L
• Relative humidity at body temperature = 30.4/44 = 69%
• Water the airway must supply in 1 h = 13.6 mg/L × 600 L = 8,160 mg ≈ 8.2 mL/h - enough to dry and crust secretions, so the heater is raised to deliver 37 °C saturated gas.

Example 6 - suction catheter. ETT internal diameter 8.0 mm.

• (8.0 − 1) × 2 = 14 Fr (OD 14/3 ≈ 4.7 mm, i.e. 58% of the tube ID; a 12 Fr is safer in a patient who oxygenates poorly). Pressure 100-150 mmHg, single pass ≤ 15 s.

Where it is used

• Hospital ICUs, emergency and casualty departments: respiratory therapists and cardio-pulmonary technologists set devices, titrate FiO₂, run HFNC, perform suction and chest physiotherapy, and do oxygen-consumption audits - routine roles at Apollo, Fortis, Manipal, Narayana, Medanta, AIIMS, PGIMER, CMC Vellore and state medical colleges.
• Neonatal and paediatric units: blended oxygen with air-oxygen blenders, hoods, bubble CPAP, SpO₂ targeting to limit retinopathy of prematurity - a major area under India's NHM newborn-care programme.
• Medical gas engineering and biomedical services: MGPS design, PSA plant operation and purity logging, VIE and manifold management, flowmeter and analyser calibration, PESO compliance for cylinder storage, NABH documentation.
• Homecare and LTOT business: concentrator and CPAP rental and servicing, patient training on inhaler and oscillatory-PEP technique - a large Indian market since 2021.
• Pharmaceutical and device industry: inhaler and nebuliser manufacturers (Cipla, Lupin, Glenmark, Sun) employ product and medical-affairs staff who train clinicians in device technique; aerosol characterisation (cascade impactor, MMAD/GSD) is a QC role.
• Pulmonary rehabilitation and bronchiectasis/CF clinics: airway-clearance prescription, exercise oxygen titration, cough-assist programmes in neuromuscular disease.

Interview questions

• Q: Why is a venturi mask called a fixed-performance device? A: Its jet entrains a fixed proportion of air and delivers total flow above the patient's peak inspiratory flow, so the FiO₂ does not change with breathing pattern.
• Q: What happens to total flow as you select a higher FiO₂ on a venturi mask? A: The air:oxygen entrainment ratio falls, so total flow falls; above about 40% the device may no longer exceed inspiratory demand and the delivered FiO₂ becomes unreliable.
• Q: Why should a simple face mask never be run at 2 L/min? A: The mask volume becomes dead space that is not flushed, so the patient rebreathes exhaled CO₂.
• Q: Why is oxygen targeted to 88-92% in COPD? A: Higher oxygen can blunt hypoxic drive, reverse hypoxic pulmonary vasoconstriction and worsen V/Q matching, causing CO₂ retention and narcosis; it should be titrated, not withdrawn abruptly.
• Q: A patient is SpO₂ 99% but drowsy after a fire rescue - is oxygenation fine? A: No. Carboxyhaemoglobin is read as oxyhaemoglobin by a standard pulse oximeter; give high-flow oxygen and measure COHb by co-oximetry.
• Q: What particle size do you want from a nebuliser and why? A: MMAD 1-5 µm - larger particles impact in the oropharynx, smaller ones are mostly exhaled.
• Q: How does inhalation technique differ between a pMDI and a DPI? A: pMDI needs a slow deep inhalation with a 10-second hold; a DPI needs a fast forceful inhalation to de-aggregate the powder.
• Q: Why add a spacer to a pMDI? A: It slows the plume and evaporates propellant, cutting oropharyngeal deposition and the need for hand-breath coordination, so lung dose and safety improve.
• Q: Why is a huff often better than a cough? A: The glottis stays open, so the equal pressure point moves peripherally and dynamic airway collapse is avoided, clearing secretions with less fatigue and less airway trauma.
• Q: State your suctioning limits for an adult with an 8 mm ETT. A: Catheter 12-14 Fr (outer diameter ≤ half the tube's internal diameter), vacuum 100-150 mmHg, suction only on withdrawal, each pass under 15 seconds, after pre-oxygenation, with SpO₂ and rhythm monitored.
• Q: Should you instil normal saline before suctioning? A: No - the evidence shows it does not thin secretions meaningfully and it pushes bacterial biofilm distally, increasing desaturation and infection risk.
• Q: What purity does a PSA plant or a home concentrator give, and why does that matter? A: About 93% ± 3% (concentrators 90-96%), the balance mostly argon and nitrogen, so delivered FiO₂ is lower than from a cylinder and falls further if the rated flow is exceeded.
• Q: How long will a D-type cylinder at 100 kgf/cm² last at 10 L/min? A: 46.7 × 100 ≈ 4,670 L; less a 10% reserve ≈ 4,200 L; about 420 minutes, i.e. roughly 7 hours.

Common mistakes

• Quoting an exact FiO₂ for a nasal cannula or simple mask - these are variable-performance devices and the figures are estimates only.
• Using the formula (100 − FiO₂)/(FiO₂ − 21) with FiO₂ as a fraction instead of a percentage, or forgetting to add the oxygen's own 1 part when computing total flow.
• Confusing cylinder *pressure* with *contents*: pressure falls linearly with gas remaining in a compressed-gas cylinder but a liquid-oxygen gauge does not behave that way.
• Running a nebuliser at 2-3 L/min, or filling it to 1.5 mL, and then concluding the drug does not work - most of it stays as dead volume.
• Teaching DPI technique for a pMDI (and vice versa), and skipping the mouth rinse after inhaled corticosteroids.
• Withholding oxygen from a hypoxaemic COPD patient for fear of CO₂ retention - the correct action is controlled oxygen to 88-92% with a gas recheck, not no oxygen.
• Suctioning routinely by the clock rather than on indication, with too large a catheter, too long a pass or no pre-oxygenation; and doing percussion or head-down drainage in haemoptysis, raised ICP or an untreated pneumothorax.
• Forgetting humidification for an ETT or tracheostomy, or leaving an HME in place with thick bloody secretions.

Related topics

Arterial blood gas analysis · Pulmonary function tests · Mechanical ventilation basics · Non-invasive ventilation and CPAP · Airway management and intubation · Pulmonary rehabilitation and cardiopulmonary resuscitation.