Why you should pay attention to the pulse waveform in asthma

Good morning — this week we have 20 new pages in bleep.guide, a new directory to store useful numbers at your hospital, and we're doing an acute asthma teaching session on Wednesday at 7pm. But first, a quick note about pulse oximetry in asthma.

Next time you're with a properly tight asthmatic, ignore the number on the sats monitor for a second and look at the waveform. In a severe attack the beats often don't stay the same height. They shrink with each breath in and recover with each breath out, so the whole trace rises and falls with the patient's breathing.

That swing is pulsus paradoxus, hiding in plain sight on a monitor you're already looking at.

It got its name in 1873, when Adolf Kussmaul described a pulse that vanished at the wrist every time his patient breathed in, even though the heart carried on beating. To him that was a paradox.¹ It isn't really. Everyone's systolic pressure dips a little on breathing in; pulsus paradoxus is that dip pushed past 10 mmHg.²

So why does a tight chest do it? To get air in through narrowed airways, the patient has to suck hard, and the pressure inside the chest drops much further than normal. That pulls blood into the right ventricle, which swells. The two ventricles share one pericardial sac, so the bigger right ventricle pushes the septum into the left, and the left fills and pumps less with that beat.² The oximeter is only reading pulsing blood in a finger, so it draws exactly that: a smaller beat with every hard breath in.

And in studies, it tracks how bad things are. In 1975, Rebuck found a systolic swing of more than 15 mmHg in asthmatic children only when their FEV1 was below 60% of predicted.³ In intensive care, Hartert's group showed the swing in the oximeter waveform tracked the paradoxus and the amount of trapped air.⁴ And in 684 patients with acute asthma, the swing on the oximeter tracked their FEV1, and changed with it as they were treated.⁵

One catch, which you'll recognise from the silent chest. The swing depends on how hard the patient is pulling. It follows that a patient who is tiring, and no longer generating those big pressure swings, may lose the swing just as things get worse. Movement and a cold finger will mess with the trace too.

So the waveform is worth a glance, but SIGN 158 says pulsus paradoxus "is an inadequate indicator of the severity of an attack and should not be used", so don't grade an attack on it. I've written about it because the physiology behind it is interesting. A swinging trace in a wheezy patient is telling you the work of breathing is high, but it doesn't replace speech, peak flow, sats and a gas, and SIGN 158 says any life-threatening feature means senior and ICU help now.⁶ It's all in our acute severe asthma checklist, and on Wednesday I'll go through it live.

20 new pages, from tension pneumothorax to a needlestick at 3am

• Emergencies: tension pneumothorax, thyroid storm, acute transfusion reaction, severe epistaxis, tracheostomy emergency
• Clerking: refeeding syndrome, rhabdomyolysis, hypernatraemia, hypocalcaemia, low platelets, hot swollen joint, Guillain-Barré syndrome, myasthenic crisis, glomerulonephritis or vasculitis, serotonin syndrome, first presentation of psychosis
• Ward cover: needlestick injury, high INR, asked for a sleeping tablet, a patient who may need detaining under the Mental Health Act

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Acute severe asthma, live on Wednesday

Wednesday 21 October at 7pm: half an hour, free and online. Severity, oxygen targets, the drugs, magnesium, and how to read serial gases without panicking. Bring questions.

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Signed up for the teaching but never opened the app? Have a look. Joining is free, and every page is open for your first 30 days. The directory, calculators, guides and teaching stay free after that.

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As always, reply and tell me what you'd want covered next. It comes straight to me, and I read every one.

— Oliver

Med reg in Cambridge; maker of bleep.guide :)

References

1. Kussmaul A. "Über schwielige Mediastino-Perikarditis und den paradoxen Puls." Berliner klinische Wochenschrift. 1873;10. ↩︎
2. Hamzaoui O, Monnet X, Teboul JL. "Pulsus paradoxus." European Respiratory Journal. 2013;42(6):1696–1705. Read the paper. ↩︎
3. Rebuck AS, Tomarken JL. "Pulsus paradoxus in asthmatic children." Canadian Medical Association Journal. 1975;112(6):710–711. Read the paper. ↩︎
4. Hartert TV, Wheeler AP, Sheller JR. "Use of pulse oximetry to recognize severity of airflow obstruction in obstructive airway disease: correlation with pulsus paradoxus." Chest. 1999;115(2):475–481. Read the paper. ↩︎
5. Arnold DH, Wang L, Hartert TV. "Pulse oximeter plethysmograph estimate of pulsus paradoxus as a measure of acute asthma exacerbation severity and response to treatment." Academic Emergency Medicine. 2016;23(3):315–322. Read the paper. ↩︎
6. British Thoracic Society and Scottish Intercollegiate Guidelines Network. SIGN 158: British guideline on the management of asthma. Revised November 2024, corrections May 2025. Read the guideline. ↩︎