National Early Warning Score 2 (NEWS2) to identify inpatient COVID-19 deterioration: The importance of pO2:FiO2 ratio
Editor – We read with great interest the early Newcastle experience of using National Early Warning Score 2 (NEWS2) to identify hospitalised COVID-19 patients at risk of clinical deterioration.1 The study highlights an average >30 hour lag between triggering the NEWS2 ≥5 threshold and first ‘serious clinical event’ (defined as initiation of respiratory support, intensive care unit (ICU) transfer, end-of-life care or death).
Baker et al confirm that COVID-19 patients frequently present with isolated hypoxaemia without significant disturbance of other physiological parameters.1 Low fractional inspired oxygen (FiO2; ∼0.3) oxygen therapy with 91% oxygen saturation and otherwise normal physiology are sufficient to trigger the NEWS2 ≥5 threshold (Table 1; ‘Time 0–12 hours’). The extremely low probability of clinical escalation with less severe physiological deterioration (ie arterial oxygen partial pressure (pO2):FiO2 ratio >26) would explain the reported high sensitivity (0.98) of NEWS2 ≥5 in this population.1
The NEWS2 ≥5 threshold had an excellent ability to identify deterioration, albeit typically >24 hours before a ‘serious clinical event’. We believe the important NEWS2 ≥7 threshold (immediate ICU review trigger) would have been a better test of practical clinical utility, albeit with lower sensitivity.
The authors acknowledge a potential weakness of NEWS2, namely the binary response to oxygen therapy.2 We suggest the relatively wide ‘dividing bins’ used by NEWS2 for individual parameters including oxygen saturations are also suboptimal, particularly in this patient cohort.3
The failure of NEWS2 to recognise increasing FiO2 from 0.3 to 0.5 is well recognised.1,2 Failure to recognise worsening desaturation from 91% to ≥90% is also problematic, and often overlooked.3 The combined net effect is a risk that clinically significant reduction in pO2:FiO2 ratio may go unnoticed in the event of failure to escalate any increase in FiO2 or mild desaturation (Table 1).
By 2022 when NEWS is next due for review, the vast majority of acute NHS hospitals will have switched from ‘pen and paper’ to ‘electronic’ observation charts. We believe this considerable investment should be accompanied by the introduction of more sophisticated early warning scoring systems which include patient trajectory and the capability to detect clinically relevant deteriorations in pO2:FiO2 ratio among other key parameters.4,5
- © Royal College of Physicians 2021. All rights reserved.
References
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- Baker KF
- ↵Royal College of Physicians. NEWS2 and deterioration in COVID-19. RCP, 2020. www.rcplondon.ac.uk/news/news2-and-deterioration-covid-19
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- Chiu YD
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- Pimentel MAF
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- Zhu Y
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