Too Much of a Good Thing Can be Bad

The day the “little blue knob” scared me

I still remember the 2009 American Thoracic Society poster session in San Diego, USA. My feet hurt, the coffee was terrible, and a very polite gentleman from Philadelphia asked me, “So, Dr. Agarwal, if I turn the flow meter to 2 L/min on a nasal cannula, how much oxygen is my COPD patient actually breathing?” I opened my mouth, closed it, then mumbled something about “roughly 28%.” He smiled and walked off. That was the moment I realised we prescribe oxygen every day, but we rarely stop to ask, “How much of it is the patient really getting?”

Fast-forward 15 years, and I still see the same little blue knob on the wall turned to random numbers—1, 3, 5, “a bit more because the SATS are 86%.” It’s like seasoning soup without tasting it: a pinch of salt, a splash of oxygen, hope for the best.

Why COPD lungs treat oxygen like a frenemy

Picture two friends: one brings pizza, the other brings a boom-box blasting death-metal at 3 a.m. Oxygen is that second friend. We need it, but too much and the brain forgets to tell the lungs to breathe. In COPD, chronically high CO₂ has blunted the “hey, breathe!” chemoreceptors; the only alarm left is hypoxia. Dump in too much O₂ and—boom—hypoxic drive vanishes, CO₂ climbs, the patient gets sleepy, and we’ve hyper-oxygenated our way into a respiratory acidosis. Not fun for anyone.

Bottom line: in COPD, oxygen is medicine, not comfort air. Every litre matters.

The poster that started it all

Our 2009 study was embarrassingly simple. We asked 30 nurses, 30 doctors and 30 respiratory therapists to look at a standard flow meter (the kind you see bolted to every hospital wall) and tell us what FiO₂ they thought the patient was receiving at 1, 2, 4 and 6 L/min via nasal cannula. Answers ranged from “21% plus a smidge” to “about 50%.” The real range? Roughly 24–44%—but the scatterplot looked like someone sneezed on the graph. If the pros don’t know, how can we expect patients to feel confident?

Nasal cannula vs Venturi mask: the eternal cage match

Venturi mask: the control freak

• Delivers exact FiO₂ (24%, 28%, 31% … )
• Colour-coded, snap-on jets, almost impossible to screw up
• Feels like strapping a small lunch-box to your face; patients hate it after hour two

Nasal cannula: the easy-going roommate

• Comfortable, lets you sip coffee, gossip with grand-kids
• FiO₂ swings with every sniff, mouth-breath or moustache density
• Flow meters usually scaled 0–15 L/min; below 1 L the little ball barely hovers, so we round up “just in case”

Stable COPD patients often refuse the Venturi mask at home; can you blame them? Try watching Netflix with a plastic aeroplane on your cheeks. Yet sending them home on 2 L/min “because the sat looked better” is like dispensing insulin in a teaspoon because the syringe is scary.

The tyranny of the 1-to-15 scale

Most wall-mounted flow meters are factory-graded for full-throttle adult oxygen—1 to 15 L/min. Lovely for the 80 kg pneumonia patient, but for the 45 kg lady with pink puffers, 1 L is already a jump from 21% to ~24%. If your smallest increment is 1 L, you’ve lost the ability to fine-tune. It’s a piano with only bass keys.

Clinical pearl: ask respiratory to swap in paediatric-style flow meters (0–6 or 0–8 L) on COPD wards. You’ll be amazed how often you’ll run at 0.5 L and still hit 88–92% SpO₂.

A practical oxygen road-map for COPD

Acute exacerbation (ED, first 24 h)

1. ABG in 15 min
2. Venturi 28% (or 24% if CO₂ > 60 mmHg) – target 88–92%
3. Repeat ABG at 30 min; if CO₂ rises > 8 mmHg, time to think NIV

Transition phase (day 2–3, improving but still in hospital)

1. Switch to low-scale flow meter (0–6 L)
2. Nasal cannula, start 0.5 L, titrate by 0.5 L every 15 min
3. SpO₂ target remains 88–92%; document litre-for-litre

Discharge home

1. Prescribe exact L/min, not “keep sat > 88%”
2. Supply low-scale regulator for concentrator; teach patient to count clicks (“one click = 0.5 L”)
3. Arrange oximetry spot-checks at week 1 and month 1; bring them back to 88–92% range

My quick-and-dirty nasal cannula cheat-sheet

Remember: mouth breathing, tachypnoea, or a moustache can drop or jump these numbers faster than hospital cafeteria coffee cools.

Patient story: when “just a bit more” turned into “way too much”

Mrs. G, 71, COPD GOLD stage 3, came in for an ankle fracture. Ortho cleared her, but the night nurse saw SpO₂ 87% and bumped nasal O₂ from 1 to 3 L. By morning she was obtunded, pH 7.18, pCO₂ 86. We intubated, she spent six days in ICU, and her first understandable scribble on the communication board was “took oxygen away?” That one extra turn of the knob cost her a week of life and the hospital north of 30 grand. The kicker? Her ABG on arrival had already shown CO₂ retention; we simply forgot to flag the exact litre limit.

Three take-home tricks you can start tomorrow

1. Stick a bright label above the flow meter: “COPD – target 88–92% – max ___ L/min” and write the number in Sharpie
2. If you don’t have paediatric low-scale meters, ask respiratory for a “portable O₂ conserver”; most have 0.5-L increments
3. Teach patients the “tingle test”: if their fingertips, lips or old man’s carpal tunnel scar start to vibrate (myoclonus), it’s CO₂ narcosis—time to call 911

The future: digital flow meters and smart cannulas

Start-ups are now selling Bluetooth flow sensors that ping your phone if the cannula kinks or the patient wanders above target SpO₂. Until they’re as cheap as a stethoscope, the best tech remains knowledge: know the litres, know the target, write it down, and tell the next shift.

Signing off: keep the pizza, dial down the boom-box

Oxygen is the most common inpatient drug we never chart accurately. In COPD, precision trumps generosity. Swap the 15-L scale for a 6-L one, start low, go slow, and remember: the flow meter’s little ball is a tiny crystal ball—read it carefully, or the future will read you the riot act.

See you at the next conference—hopefully with better coffee and fewer mumbled answers.