Factsheet Neonatal haemodynamics Preterm
We all learned a number for neonatal hypotension. The trouble is that there are several numbers, none of them was built on outcome data, and a baby can sit below every one of them and still be perfusing perfectly well.
A low blood pressure reading is a prompt to look harder, not a trigger to treat. Put the number next to the baby: central capillary refill, heart rate trend, urine output, lactate, base deficit and, where you can, targeted echo.
Three numbers, one problem
Every pressure-based definition asks the same question — is this number low? — when the question that matters is whether the tissues are getting enough blood. Here is how the three common definitions compare.
The BAPM rule: mean BP below gestational age
The rule first appeared in a 1992 joint BAPM and Royal College of Physicians report on the management of respiratory distress syndrome.1 It stuck because it is simple: an invasive mean pressure is objective, continuous, and lets you watch the effect of whatever you do. Its statistical logic is that, in the first days after birth, the lower end of the normal range for a preterm baby sits roughly at its gestation in weeks. expert consensus
It ignores time
Blood pressure rises spontaneously over the first days of life. A fixed threshold that ignores this has no empirical support, and more than half of very immature infants sit below it on day 1.5
It is set a little high on day 1
In 4,907 very low birthweight infants in the German Neonatal Network, the median lowest mean BP in the first 24 hours for babies ≤29 weeks was 1–2 mmHg below gestational age.7
It does not track flow
Blood pressure correlates poorly with superior vena cava flow on the first day, and is an imperfect bedside test for low systemic blood flow.613
It does not track outcome
In a matched cohort of 66 dopamine-treated infants, mean BP below GA was not associated with lower cerebral oxygenation or poorer scores at 18–24 months. Low cerebral oxygenation was.8
Read the German data carefully. A lower day-1 minimum pressure was still independently associated with IVH, BPD and death in that cohort.7 Low pressure is a marker of a sicker baby; that is not the same as proof that raising the number helps.
The 30 mmHg cut-off
The 30 mmHg line comes from the idea of protecting the brain: below a certain pressure, cerebral autoregulation fails and flow becomes pressure-passive. physiological rationale Its clinical footing is a single small study. Miall-Allen and colleagues followed 33 infants under 31 weeks and found that a mean pressure below 30 mmHg for more than an hour was associated with severe haemorrhage, ischaemic lesions or death within 48 hours; no severe lesions developed at 30 mmHg or above.9
Why it does not hold up
- Tiny, older cohort with no control for modern care, sedation or vasoactive drugs9
- The smallest babies are most likely to be below 30 and most likely to bleed anyway — the association is confounded by immaturity expert opinion
- Applied rigidly, it labels almost every 23-weeker hypotensive at birth expert opinion
- For detecting low SVC flow, mean BP <30 mmHg had only 59% sensitivity and 77% specificity6
What the brain can actually tolerate
- Critical closing pressure — the pressure at which cerebral flow stops — had a median of 22 mmHg (IQR 19–25) in 179 infants of 23–31 weeks10
- It rises by about 1.4 mmHg per week of gestation10
- Directly measured closing pressure in extremely low birthweight infants: median 20 mmHg (IQR 18–25)11
- This low closing pressure may explain why very preterm babies tolerate low pressures without global injury10
Centile charts
Here, hypotension means a reading below a chosen centile — anywhere from the 3rd to the 10th, sometimes higher — for gestation, birthweight and postnatal age. The advantage is obvious: charts move with the baby as pressure rises. The practical problem is that nobody agrees which centile to use. In the Indian survey, 38% used centile charts, with a spread of different thresholds behind that figure.3
The charts themselves carry baggage. Many were built from small or retrospective cohorts, some included infants with brain injury, and many relied on oscillometric cuffs, which tend to over-read in exactly the hypotensive babies you worry about. expert opinion Treat a centile as a signal for a fuller assessment, not a treatment trigger.
Clinical criteria: capillary refill and heart rate
Ask neonatal doctors how they judge perfusion and the top three answers are capillary refill, blood pressure and heart rate; 79% considered a CRT over 3 seconds a reliable sign of poor perfusion.3 Popular is not the same as accurate.
Capillary refill time
Normal newborn CRT is under 3 seconds. Measure centrally: forehead and chest values are normally distributed and reproducible between observers, while heel values are widely scattered.12 Even so, central CRT ≥3 s picked up low SVC flow with only 55% sensitivity.6 Incubators, radiant warmers and cooling all distort it. expert opinion
Heart rate
The preterm heart has limited ability to raise stroke volume, so it leans on rate to hold cardiac output. But heart rate also moves with gestation, pain, temperature and drugs such as morphine. The trend over time tells you far more than any single value. expert consensus
Biochemical markers: lactate, pH and base deficit
Lactate is the bedside marker of anaerobic metabolism. In very low birthweight babies, initial lactate was markedly higher in those who died (median 9.2 vs 3.6 mmol/L), and early hyperlactataemia predicted death.14 On day 1, a lactate above 2.8 mmol/L was 100% sensitive but only 60% specific for low SVC flow.13
Know the confounders. Lactate is often raised at birth after a stressful delivery, can spike when flow returns to under-perfused tissue (“washout”), and can be raised by metabolic disease or a haemolysed sample. Regional hypoperfusion can exist with a normal blood lactate. Extremely preterm babies also develop a benign, transitional metabolic acidosis in the first days of life. expert consensus
Putting it together
On their own, each of these signs is a weak test. Combined, they become useful. In Miletin’s day-1 cohort of very low birthweight infants, a CRT over 4 seconds together with a lactate over 4 mmol/L had 97% specificity for a low SVC flow state.13 Combining mean BP <30 mmHg with central CRT ≥3 s raised sensitivity for low flow to 78% in Osborn’s cohort.6
Numeric hypotension, well baby
- Pink, central CRT <3 s, settled heart rate
- Passing urine, normal and falling lactate
- Consider watchful waiting — “permissive hypotension” expert consensus
Low pressure with poor perfusion
- Prolonged central CRT, rising heart rate trend
- Rising lactate, worsening base deficit, poor urine output
- Get an echo, work out the physiology, and treat the cause
Do not start an inotrope because a number crossed a line. There is no outcome evidence that treating a blood pressure threshold on its own helps preterm babies — confirm poor perfusion first.58
References
- Joint Working Group of the British Association of Perinatal Medicine and the Research Unit of the Royal College of Physicians. Development of audit measures and guidelines for good practice in the management of neonatal respiratory distress syndrome. Arch Dis Child. 1992;67:1221–7. doi:10.1136/adc.67.10_spec_no.1221
- Stranak Z, Semberova J, Barrington K, et al. International survey on diagnosis and management of hypotension in extremely preterm babies. Eur J Pediatr. 2014;173:793–8. doi:10.1007/s00431-013-2251-9
- Das R, Nagpal R, Deshpande S, et al. A survey on management practices of hypotension in preterm neonates: an Indian perspective. Front Pediatr. 2024;12:1411719. doi:10.3389/fped.2024.1411719
- Dempsey EM, Barrington KJ. Diagnostic criteria and therapeutic interventions for the hypotensive very low birth weight infant. J Perinatol. 2006;26:677–81. doi:10.1038/sj.jp.7211579
- Barrington KJ. Time for pressure tactics. Pediatrics. 2007;119:396–7. doi:10.1542/peds.2006-3456
- Osborn DA, Evans N, Kluckow M. Clinical detection of low upper body blood flow in very premature infants using blood pressure, capillary refill time, and central-peripheral temperature difference. Arch Dis Child Fetal Neonatal Ed. 2004;89:F168–73. doi:10.1136/adc.2002.023796
- Faust K, Härtel C, Preuß M, et al. Short-term outcome of very-low-birthweight infants with arterial hypotension in the first 24 h of life. Arch Dis Child Fetal Neonatal Ed. 2015;100:F388–92. doi:10.1136/archdischild-2014-306483
- Alderliesten T, Lemmers PMA, van Haastert IC, et al. Hypotension in preterm neonates: low blood pressure alone does not affect neurodevelopmental outcome. J Pediatr. 2014;164:986–91. doi:10.1016/j.jpeds.2013.12.042
- Miall-Allen VM, de Vries LS, Whitelaw AG. Mean arterial blood pressure and neonatal cerebral lesions. Arch Dis Child. 1987;62:1068–9. doi:10.1136/adc.62.10.1068
- Rhee CJ, Fraser CD, Kibler KK, et al. Ontogeny of cerebrovascular critical closing pressure. Pediatr Res. 2015;78:71–5. doi:10.1038/pr.2015.67
- Thaba AA, et al. Observed and calculated cerebral critical closing pressure are highly correlated in preterm infants. Pediatr Res. 2019;86:242–6. doi:10.1038/s41390-019-0403-y
- Strozik KS, Pieper CH, Cools F. Capillary refilling time in newborns — optimal pressing time, sites of testing and normal values. Acta Paediatr. 1998;87:310–2. doi:10.1080/08035259850157372
- Miletin J, Pichova K, Dempsey EM. Bedside detection of low systemic flow in the very low birth weight infant on day 1 of life. Eur J Pediatr. 2009;168:809–13. doi:10.1007/s00431-008-0840-9
- Phillips LA, Dewhurst CJ, Yoxall CW. The prognostic value of initial blood lactate concentration measurements in very low birthweight infants and their use in development of a new disease severity scoring system. Arch Dis Child Fetal Neonatal Ed. 2011;96:F275–80. doi:10.1136/adc.2010.185793
For education only. Points marked expert consensus, expert opinion or unit guideline reflect current practice rather than trial evidence. Thresholds are for orientation only — follow your local guidelines and senior advice.