“The doctor wrote 2 LPM. So I just need a 2 LPM concentrator, right?” This is the first question a newly-prescribed LTOT patient asks, and the answer a surprising number of Indian dealers give is “yes, sir, our 3 LPM model is the one for you, very economical.” The answer is wrong on two counts. A concentrator’s rated maximum flow is not the same as the patient’s prescribed flow. And a unit that has to run at its maximum rated flow to deliver the prescription operates at the noisiest, hottest, lowest-purity point of its curve — precisely where you do not want the machine to live.
This article covers how flow-rate prescriptions map onto concentrator selection across the common home oxygen indications: COPD long-term oxygen therapy, exercise desaturation, ILD, sleep-related hypoxemia, and paediatric use. It explains why a 5 LPM concentrator is usually the right purchase even for a patient prescribed 2 LPM. And it covers the exceptions — the small number of situations where a higher-flow unit is genuinely necessary or a lower-flow portable is the right call.
The prescription maps to flow, not to concentrator size
An oxygen prescription specifies the delivered flow at the patient’s nasal cannula in litres per minute. The prescription usually takes one of three forms:
- A single flow value (“2 LPM continuous”), which applies around the clock.
- A dual flow value (“2 LPM at rest, 4 LPM during exertion”), which requires the patient to adjust the regulator for activity.
- A titrated flow (“1–4 LPM, titrate to maintain SpO₂ ≥ 90%”), which requires the patient or caregiver to watch a pulse oximeter and adjust.
The concentrator’s rated maximum flow must be at least equal to the highest prescribed flow. That is the floor. But the ceiling — how much above the prescription the concentrator’s rated maximum should sit — is a design decision, not a clinical one, and it is the design decision most Indian buyers get wrong.
COPD long-term oxygen therapy — the 1–3 LPM category
The most common indication for home oxygen in the Indian adult population is COPD hypoxemia meeting LTOT criteria (resting SpO₂ ≤ 88% on room air, or PaO₂ ≤ 55 mmHg, per the standard guidelines (GOLD Report)). Prescribed flows for this population cluster tightly:
- Typical resting prescription: 1–2 LPM.
- Typical activity-adjusted prescription: 2–3 LPM during exertion.
- Rare higher values: 3–4 LPM in severe disease.
The clinical target is to maintain SpO₂ at 90% or above at rest; activity titration can push toward higher flow if desaturation with exertion is documented. The vast majority of stable Indian COPD LTOT patients operate at 1.5–2.5 LPM continuous.
For this patient, our recommendation is a standard 5 LPM home stationary concentrator. The reasoning below on the purity curve explains why.
Exercise-induced desaturation — the transient higher-flow category
Some patients desaturate with exertion despite normal resting oxygenation, or desaturate further from an already low resting baseline. The prescription pattern is:
- 2 LPM at rest, with instruction to increase to 4–6 LPM during physical activity (walking, stair-climbing, physiotherapy).
During the higher-flow bursts, which typically last 10–30 minutes, the concentrator is called upon to deliver near its maximum rated output. A 5 LPM unit can technically meet a 5 LPM prescription, but it runs at the top of its flow-purity curve the entire time the patient is exercising. For a patient with frequent exertion-adjusted use, a 10 LPM unit provides substantially better headroom — delivering 5 LPM at the middle of a 10 LPM unit’s curve rather than the top of a 5 LPM unit’s curve.
Our recommendation: if activity-adjusted flow exceeds 3 LPM and is used more than 30 minutes a day, upgrade to a 10 LPM class unit.
Interstitial lung disease — the 3–6 LPM category
ILD (including idiopathic pulmonary fibrosis, hypersensitivity pneumonitis, and post-COVID fibrosis, which has become meaningfully more common in the Indian population since 2021) typically requires higher oxygen flows than COPD because the underlying diffusion impairment is more severe and patients desaturate more rapidly:
- Typical resting prescription: 3–4 LPM.
- Typical activity prescription: 5–8 LPM, sometimes higher.
- Progressive disease: eventual requirement of 8–10 LPM at rest and non-invasive ventilation support.
For ILD, a 5 LPM concentrator is often undersized from day one or undersized within 12–18 months of disease progression. Our default recommendation for Indian ILD patients is a 10 LPM unit from the outset, because upgrading later is more expensive than buying correctly the first time, and because ILD progression is often more rapid than COPD progression.
Sleep-related hypoxemia — a separate consideration
Patients with nocturnal desaturation due to sleep-disordered breathing, central hypoventilation, or REM-associated desaturation in restrictive disease may have an oxygen prescription that is lower during the day and higher at night, or oxygen-only at night. Prescription patterns:
- Night-only oxygen: typically 2–3 LPM during sleep, off during the day.
- Continuous with night boost: e.g., 1 LPM day, 3 LPM night.
For night-only use, a standard 5 LPM home concentrator running at its lowest flow is the correct choice — the unit will be off most of the day and on overnight. Noise becomes a primary consideration; see our continuous-operation article for the detailed discussion on bedside noise tolerance.
Importantly, oxygen therapy is not a replacement for CPAP or BiPAP in patients with obstructive sleep apnoea. Oxygen alone corrects the saturation but does not open the airway; the patient continues to experience apnoeas and hypercapnia. If the indication is OSA with nocturnal desaturation, the primary therapy is PAP, and oxygen is added only when PAP alone fails to correct saturation. Consult your physician on this specific differential.
Paediatric use — the low-flow category
Paediatric oxygen prescriptions are weight-adjusted and generally lower than adult prescriptions:
- Neonatal / infant: 0.1–0.5 LPM, often delivered via low-flow meter on a standard concentrator, occasionally via specialised paediatric flowmeters reading in millilitres per minute.
- Toddler / young child: 0.5–1.5 LPM.
- Older child: 1–3 LPM.
The standard home concentrator’s lowest flow setting is typically 0.5 LPM; below that, the flow meter is not accurate. For neonatal or very-low-flow paediatric use, a specialised low-flow meter is required — some dealers offer it as an accessory, others do not. Check before purchase.
For most paediatric home oxygen use at 1 LPM and above, a standard 5 LPM concentrator is adequate. The selection consideration shifts towards noise (children’s sleep is more easily disturbed) and continuous run rating (paediatric use is often 24/7).
Why a 5 LPM unit is usually right even for a 2 LPM prescription
The core technical reason — and it is the one Indian dealers rarely explain — is the flow-purity curve of PSA concentrators.
A PSA concentrator’s output purity is not constant across its flow range. Purity peaks near the mid-low region of the rated flow range and falls as flow approaches the rated maximum. A 5 LPM unit rated for 90–95% purity typically delivers:
- 93–95% at 1–2 LPM
- 91–93% at 3 LPM
- 88–92% at 4 LPM
- 85–90% at 5 LPM (near the rated maximum)
A 3 LPM unit running at 2 LPM is at 66% of its rated maximum; a 5 LPM unit running at 2 LPM is at 40% of its rated maximum. The 5 LPM unit sits in a more comfortable part of its purity curve, produces purity 2–4 percentage points higher at the same prescribed flow, and has headroom if the prescription is increased.
Additional reasons a 5 LPM unit beats a 3 LPM unit even for a 2 LPM prescription:
- Compressor loading. The compressor on a 3 LPM unit running at 2 LPM is at ~66% of capacity; the same 2 LPM pulled from a 5 LPM unit loads its (larger) compressor at ~40%. Lower loading extends compressor life.
- Noise. Both units produce 40–50 dB at mid flows, but the 3 LPM unit at 2 LPM is closer to its peak operating noise while the 5 LPM unit is well below its peak.
- Headroom for prescription changes. COPD progresses. A prescription at 2 LPM today is commonly at 3 LPM in 18 months. A 3 LPM unit forced to meet a new 3 LPM prescription is at its rated maximum; a 5 LPM unit is at 60% of its maximum.
- Price differential is modest. In the current Indian market, a quality 3 LPM unit and a quality 5 LPM unit differ by ₹3,000–8,000. The 5 LPM is almost always worth the difference.
The case against is thin. A 3 LPM unit saves a small amount of money, occupies marginally less space, and uses perhaps 25–50 W less electricity. None of these advantages outweighs the flow-curve, headroom, and noise considerations for a long-term oxygen patient.
Our strong recommendation: for adult COPD, adult sleep-related hypoxemia, and paediatric use at 1 LPM or above, buy a 5 LPM home stationary concentrator unless there is a specific reason (space, transportability) requiring a smaller unit. For ILD, adults with exercise desaturation exceeding 4 LPM, or any condition with documented clinical trajectory towards higher flows, buy a 10 LPM unit.
When a portable concentrator is the right call
Portable oxygen concentrators (POCs) are a separate category. They deliver lower maximum continuous flow (typically 1–3 LPM) but add pulse-dose capability for activity. POCs are indicated for:
- Patients who need to leave the house regularly and require supplemental oxygen during transit. For most LTOT patients in India this is not the case; a home stationary unit plus a small cylinder for transit suffices.
- Travel, including air travel. POCs with FAA approval are the only oxygen option on commercial flights. If the patient flies more than 2–3 times a year, a POC makes sense as a secondary unit.
- Active patients in early disease. A POC supports mobility that a stationary unit does not; for a patient still working or caregiving, the mobility is worth the cost.
POCs are not cost-effective as primary home oxygen for a sedentary or elderly LTOT patient. Battery management, the smaller compressor, and pulse-dose limitations all make them worse as a home unit than a stationary. For most Indian LTOT use, the correct configuration is a home stationary concentrator plus a backup cylinder for outages.
Failure modes of flow-rate mis-selection
Several patterns we see in Indian dealer sales and their consequences:
- Underselling: 3 LPM unit for a 2 LPM patient. Unit runs hot, noise climbs, purity marginal, compressor fails at 14–18 months rather than 36+. Patient thinks the 3 LPM unit “was a bad machine.” The machine was fine; it was mis-specified.
- Overselling: 10 LPM unit for a 2 LPM COPD patient. Unit delivers the prescription comfortably but costs 70–100% more than needed, uses 50–80% more electricity, and is physically larger. Patient family resents the purchase when a neighbour bought a 5 LPM that works equally well.
- Mis-titration: patient left at 2 LPM when prescription has moved to 3 LPM. Clinical saturation marginal, patient assumes they need a “better machine” when they actually need to turn the regulator knob clockwise. Always reconfirm the prescribed flow and visibly verify the flowmeter reading.
- Humidifier bottle mis-seating causing reduced delivered flow. The unit produces the set flow into the bottle but delivers less at the cannula. A daily flow check at the cannula tips (described in the maintenance article) catches this.
The decision summary
- 1–2 LPM prescription, stable: 5 LPM home stationary unit. Not 3 LPM; not 10 LPM.
- 2–3 LPM prescription with activity titration: 5 LPM unit is adequate; 10 LPM unit provides headroom for progression.
- 3–6 LPM prescription (ILD, severe COPD): 10 LPM unit.
- Above 6 LPM, especially with non-invasive ventilation: 10 LPM unit, often paired with a bilevel device.
- Paediatric below 0.5 LPM: 5 LPM unit with specialised low-flow meter accessory.
- Mobile / travelling patient: POC as secondary unit, not as primary home device.
Flow-rate selection is the most consequential pre-purchase decision for an oxygen patient, and it is one of the few decisions where the dealer’s incentive does not align with the patient’s interest — most dealers push either the cheapest unit that meets the prescription or the most expensive unit the buyer can afford. Neither is automatically right. The right answer is usually a 5 LPM unit, and the right reason is the flow-purity curve, compressor headroom, and trajectory of most adult oxygen-dependent conditions.
Consult your physician for the specific flow prescription appropriate for your diagnosis and clinical status. This article addresses equipment selection against a known prescription and does not substitute for clinical assessment.
Background references: GOLD report on COPD long-term oxygen therapy [CITATION]; ATS/ERS ILD home oxygen guidance [CITATION]; ISO 80601-2-69 flow-purity specification requirements [CITATION].