When the Numbers Lie: Why Your NICU Baby’s Blood Pressure Might Be Lying to You

Why Blood Pressure Isn’t Enough?

Blood pressure tells us about driving pressure in the arterial system, but it reveals almost nothing about what’s happening at the tissue level. Think of it like checking the water pressure in your pipes while ignoring whether water is actually reaching the taps.

In the NICU, this matters enormously. A preterm baby can maintain adequate blood pressure through compensatory vasoconstriction even as cerebral, renal, and splanchnic perfusion tanks. That 26-weeker with a textbook MAP of 28 mmHg might be achieving it by clamping down peripheral vessels so hard that blood flow to vital organs has dropped dangerously low. Conversely, a baby with a PDA might have a low diastolic pressure but excellent systemic blood flow.

Baby J’s situation reflects this disconnect. His BP is acceptable, but his clinical picture (cool peripheries, rising lactate, oliguria) suggests his tissues aren’t getting what they need. His body is doing everything it can to defend that blood pressure number we’re fixated on, but he’s paying for it with organ perfusion. We need to look deeper.

NIRS: Windows into Tissue Oxygenation

Near-infrared spectroscopy gives us something beautifully simple: real-time measurement of regional tissue oxygen saturation. NIRS uses light wavelengths that penetrate tissue to measure the ratio of oxygenated to deoxygenated haemoglobin in the microcirculation beneath the sensor. It’s non-invasive, continuous, and can be placed over different organs.

In practice, we most commonly monitor cerebral NIRS (placing sensors on the forehead) and sometimes splanchnic NIRS (over the abdomen) or renal NIRS (over the flanks). What you are looking for is not the absolute number but the trend and the relationship to systemic saturations. The cerebral-to-systemic oxygen saturation ratio (the fractional tissue oxygen extraction, or FTOE) indicates whether the brain is working harder to extract oxygen (a sign that oxygen delivery might be difficult).

Back to Baby J: You place cerebral NIRS sensors on his forehead. His cerebral saturation reads 40% while his pre-ductal SpO₂ is 92%. That is FTOE of 0.57 calculated through the following Formula :(SpO2-rSO2)/SpO2.

Normoxia FTOE is defined as FTOE >0.1 and ≤0.4, hypoxia FTOE as FTOE > 0.4, and Hyperoxia FTOE as FTOE ≤ 0.1. From the above, this means that baby J has high FTOE (his brain is extracting more oxygen than it should, suggesting oxygen delivery is compromised). Despite that “normal” blood pressure, his brain isn’t getting adequately perfused. Now you have objective data that aligns with your clinical instincts.

Electrical Cardiometry: The Flow Behind the Pressure

Electrical cardiometry measures cardiac output non-invasively by analysing changes in thoracic electrical conductivity during the cardiac cycle. As blood is ejected from the heart, the orientation of red blood cells changes from random to aligned, altering electrical conductivity across the chest. By measuring these changes, EC calculates stroke volume and, combined with heart rate, gives you cardiac output.

In neonates, this technology has been validated and gives you beat-to-beat cardiac output trends. You’re seeing litres per minute per kilogram and variations in stroke volume. Suddenly, you’re not just measuring pressure, you’re measuring flow.

The real power comes from trending. Is cardiac output dropping despite stable blood pressure? That tells you systemic vascular resistance is rising to compensate. Is cardiac output high but pressure low? You might be looking at a significant PDA or septic vasodilation.

Pros:

• Completely non-invasive with just four ECG-style electrodes
• Provides continuous cardiac output monitoring
• Helps differentiate between cardiac and vascular causes of instability
• Can guide fluid versus inotrope decisions

Cons:

• Accuracy can be affected by movement and poor electrode contact
• Less reliable with significant arrhythmias
• Requires training to interpret trends properly
• Not a direct measure (relies on algorithms)

You attach EC monitoring to Baby J. His cardiac output drops from 240 to 150 mL/min (lower than expected for a day-two preterm baby).

In this case, his cardiac output dropped, and his systemic vascular resistance is markedly elevated. Now the picture becomes clearer: he’s maintaining blood pressure by constricting his vessels, but his cardiac output is inadequate. His heart isn’t generating enough flow, and his body is compensating by squeezing down the pipes. That’s why his perfusion is poor despite acceptable pressures.

Below is a table for normal ranges of cardiac output for different gestational age groups.

N.B. Although it would be preferred to measure cardiac output accurately, it might be more useful to categorise cardiac output (low, normal, or high) to understand the underlying pathophysiology and to focus on trends rather than absolute numbers.

Functional Echocardiography: The Physiological Detective

Functional echocardiography at the bedside takes monitoring to another level by answering the “why” questions. fECHO isn’t about diagnosing structural heart disease; it’s about understanding haemodynamic physiology in real time.

A focused scan can quickly assess whether the left ventricle is contracting well. Is there a haemodynamically significant PDA? What does the right ventricle look like? Is it dilated, suggesting pulmonary hypertension? What’s the IVC doing? Is it plump (suggesting volume overload) or collapsing (suggesting hypovolaemia)? How are the systemic and pulmonary flows relating to each other?

These answers directly guide management. Low cardiac output with good contractility but a collapsing IVC? That baby might need volume. Low output with poor contractility? That’s an inotrope scenario. High output with a massive PDA? Time for targeted PDA management.

Pros:

• Provides direct visualisation of cardiac function and structure
• Can identify specific causes of haemodynamic compromise
• Guides precise, targeted interventions
• Repeatable for monitoring treatment response

Cons:

• Requires significant training and skill
• Time-consuming (though focused protocols help)
• Not continuous, gives snapshots rather than real-time trending
• Variability between different practitioner capabilities and interpretation

You grab the portable echo machine for Baby J. His left ventricle shows mildly reduced function with a shortening fraction of 22% (normal >28%). His IVC is small and collapsing more than 50% with respiratory variation, suggesting he’s relatively hypovolaemic despite the urine output. There’s no significant PDA, and his right ventricle looks reasonable.

The picture is complete: he’s hypovolaemic with borderline cardiac function, compensating with vasoconstriction to maintain blood pressure at the cost of tissue perfusion.

Pulling It All Together: The Multimodal Monitoring Revolution

Baby J’s story illustrates why multimodal monitoring matters. Blood pressure alone told us he was “fine.” Every other signal told us he wasn’t.

The NIRS showed us his brain was struggling to extract enough oxygen. The electrical cardiometry revealed his cardiac output was inadequate and his systemic vascular resistance was sky-high. The functional echo showed us why: relative hypovolaemia with compromised ventricular function. Each tool added a piece of the puzzle that blood pressure couldn’t provide.

Here’s a practical framework: In a haemodynamically unstable neonate, blood pressure should be just one data point in a constellation of monitoring. If BP is acceptable but NIRS is low, cardiac output is dropping, and echo shows poor filling, you’ve got a problem, regardless of that blood pressure number.

For Baby J, management changed dramatically. Instead of being reassured by his “normal” blood pressure or perhaps blindly starting inotropes, the team gave a careful volume bolus of 10 mL/kg and started low-dose dobutamine to support contractility.

Within an hour, his cerebral NIRS climbed to 62%, cardiac output improved to 200 then 240 mL/kg/min, SVR normalised, and his lactate started trending down. His peripheries warmed up, urine output picked up, and although the blood pressure number barely changed, everything else improved.

This is precision neonatal haemodynamic care. Not treating numbers, but treating babies by understanding their underlying physiology.

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