Invasive vs Non-invasive BP assessments

Methods for Blood Pressure Measurements

1. Oscillometric (Automated Cuff) Method – Most Common Noninvasive Technique

• Principle: Detects oscillations in cuff pressure as blood flow returns during deflation; the point of maximum oscillation corresponds to the mean arterial pressure (MAP). Systolic and diastolic values are algorithmically derived.

• Site: Usually the right upper arm (pre-ductal); legs or other limbs can be used but may yield lower/damped values.

• Advantages: Easy, quick, and noninvasive; suitable for trend monitoring.

• Limitations: Accuracy depends on correct cuff size, placement, and patient stability. Motion, hypotension, or poor perfusion can cause errors. Algorithms are not validated for extreme neonatal BPs.

2. Doppler with Sphygmomanometer

• Principle: A Doppler probe detects flow distal to the cuff; the systolic pressure is identified when flow reappears during deflation.

• Use: Often used in small or hypotensive infants when oscillometric readings are unreliable.

• Advantages: Can detect systolic pressure even with weak pulses; useful for confirming perfusion.

• Limitations: Provides only systolic BP; operator-dependent; requires quiet environment and training.

3. Auscultatory Method (Manual with Stethoscope)

• Principle: Similar to older pediatric/adult methods—Korotkoff sounds detected with a stethoscope during cuff deflation.

• Use: Rarely feasible in neonates because sounds are faint and vessel compliance is high.

• Advantages: Historically the reference for validation.

• Limitations: Technically challenging and unreliable in very small or preterm infants. Detects systolic blood pressure only.

A few words on Oscillometric method 

Oscillometric method has several limitations in the neonatal population. The accuracy can be affected by the infant's movement, crying, or irregular heart rhythms, leading to unreliable readings. Furthermore, in very small or sick infants, especially those with low blood pressure (hypotension), oscillometric devices may overestimate the blood pressure, potentially masking a serious condition. The choice of cuff size is also critical; a cuff that is too large will give a falsely low reading, while one that is too small will produce a falsely high reading. The most common method for calculating systolic and diastolic blood pressure in non-invasive cuff devices is the fixed-ratio algorithm, which is based on the oscillometric method. 

• Measure Oscillations: The device inflates the cuff to stop blood flow and then slowly deflates it. During deflation, a pressure sensor detects oscillations (vibrations) in the artery wall under the cuff. 

• The algorithm first identifies the point of maximum oscillation. The cuff pressure at which this peak vibration occurs is the Mean Arterial Pressure (MAP). 

• Estimate Systolic and Diastolic: Systolic and diastolic pressures are not measured directly but are estimated as specific points on the oscillation curve relative to the peak. 

• Systolic Blood Pressure (SBP) is calculated at a point on the ascending part of the oscillation curve, where the vibration amplitude is a specific fraction (e.g., 55%) of the maximum amplitude. 

• Diastolic Blood Pressure (DBP) is calculated at a point on the descending part of the oscillation curve, where the vibration amplitude has dropped to a different, specific fraction (e.g., 75%) of the maximum amplitude. 

• The exact fractions or "ratios" used are proprietary, empirically determined by each manufacturer, and programmed into the device's software. 

Limitations in Newborns of Oscillometry

• Small vessel size and low compliance: Neonatal arteries are small and very compliant, which alters the transmission of oscillations and reduces accuracy. 

• Low perfusion or hypotension: Weak pulsations may be undetectable, especially in critically ill infants. 

• Cuff size and position: Inappropriate cuff size or placement can significantly over- or underestimate values. 

• Peripheral damping: Measurements taken on distal limbs may underestimate central pressures, particularly in conditions like shock or with ductal shunting, where pre- and post-ductal pressures differ.

Measurement in the newborn: 

"Blood pressure was measured using an oscillometric device with an appropriate-sized cuff (cuff width to the arm circumference ratio closest to 0.50) [14, 15]. We preferably used the right upper arm to measure the blood pressure. Calf blood pressure measurements were used in case of contraindications such as right arm tissue injury, peripheral cannula or PICC-line [16–18]. According to the literature, calf blood pressure can be used during the first 6 months of life as the results are similar to upper limb blood pressure measurements [18]." Reference: Kiss JK, Gajda A, Mari J, Nemeth J, Bereczki C. Oscillometric arterial blood pressure in haemodynamically stable neonates in the first 2 weeks of life. Pediatr Nephrol. 2023 Oct;38(10):3369-3378. doi: 10.1007/s00467-023-05979-x. Epub 2023 May 5. PMID: 37145184; PMCID: PMC10465666.)

The right upper arm is the standard (pre-ductal) location, as it reflects blood pressure proximal to the ductus arteriosus and most closely approximates central aortic pressure. Alternative sites: The left arm or leg (post-ductal) may be used if the right arm is unavailable, but readings can differ—especially in the presence of a patent ductus arteriosus (PDA), where post-ductal pressures may be lower due to ductal runoff into the pulmonary circulation. Avoid very distal sites (e.g., wrists, ankles) because smaller arteries and higher damping produce less reliable readings. How to Measure Cuff size: Use a cuff with a bladder width ≈ 40% and length ≈ 80% of the limb circumference. Placement: Place the cuff snugly around the limb with the bladder centered over the artery. Positioning: Keep the limb at heart level to prevent hydrostatic differences. Device: Use a validated neonatal oscillometric monitor; repeat at least two measurements and report the average (preferably the mean arterial pressure [MAP]). Context: Document whether the measurement is pre-ductal or post-ductal, especially in infants with duct-dependent lesions or suspected PDA.

Although numerous studies show a significant correlation between IBP and NIBP measurements, the clinical accuracy remains under debate. A systematic review concluded that oscillometric methods are less accurate and precise compared with intra-arterial methods. Furthermore, studies often report wide 95% limits of agreement between the two methods, indicating a poor level of agreement that can lead to clinically significant overestimation or underestimation of BP.

Agreement between invasive and non-invasive BP

The agreement between invasive and non-invasive BP methods varies depending on the specific neonatal population and their hemodynamic stability. One prospective study focusing on preterm neonates found that good agreement was maintained between NIBP and IBP measurements regardless of birth weight or gestational age. This study included groups ranging from extreme preterm infants to larger preterm infants. However, findings from systematic reviews emphasize that the accuracy and precision of oscillometric methods decrease substantially in neonates with low BP. Specifically, oscillometric devices are less accurate when the mean arterial pressure (MAP) is <30 mmHg, often resulting in the NIBP method overestimating the actual BP. Similarly, a study on preterm infants showed that the bias between invasive and non-invasive MAP was smaller in normotensive infants compared with hypotensive infants. For term or near-term neonates, such as those undergoing therapeutic hypothermia (TH) for hypoxic ischemic encephalopathy (HIE), NIBP measurements demonstrated good positive correlation with IABP, but DBP and MBP were significantly underestimated compared to IABP, both during TH and normothermia.

The location of the invasive BP measurement also factors into the assessment, contrasting measurements taken centrally via an Umbilical Arterial Catheter (UAC) against those taken peripherally via a radial or peripheral arterial line (PAC). The UAC typically measures BP in the descending aorta (a central artery), while PACs measure in peripheral arteries (e.g., radial or tibial posterior). Some early studies found comparable results when measuring BP in the umbilical artery or peripheral arteries. Correspondingly, one study in preterm infants concluded that the correlation and degree of agreement between IBP and NIBP were not affected by the position of the catheter (umbilical vs. peripheral). However, differences in arterial characteristics between central and peripheral sites can contribute to discrepancies. A separate study that focused exclusively on peripheral arterial cannulation in preterm infants, comparing peripheral IBP and NIBP in the same limb to mitigate bias due to different arterial paths, noted that there was less bias between invasive MAP and non-invasive MAP measurements in the upper extremities (radial PAC) as compared with the lower extremities (posterior tibial PAC).

Regarding the correlation between cuffed evaluations and the specific BP components, MAP is consistently reported as the most accurate BP value derived from oscillometric devices when compared to intra-arterial measurements. Because the oscillometric method directly measures MAP (the point of maximum oscillations), and subsequently calculates SBP and DBP using algorithms, clinicians are generally recommended to use MAP as the primary BP value to compare with normative data. In studies analyzing the bias between IBP and NIBP, the bias was found to be less for mean BP comparisons compared with systolic BP comparisons. The systematic review indicated that oscillometric SBP measurements tend to overestimate intra-arterial SBP. Conversely, in term/near-term infants undergoing HIE management, SBP measurements were deemed robust during therapeutic hypothermia (TH), whereas DBP and MBP were underestimated. Overall, NIBP is typically used only for screening purposes, and when clinical concerns arise, particularly concerning hypotension or unstable hemodynamics requiring vasoactive agents, IBP monitoring is considered for more continuous evaluations.

Selected References

1. Liu J, Hahn JO, Mukkamala R. Error mechanisms of the oscillometric fixed-ratio blood pressure measurement method. Ann Biomed Eng. 2013 Mar;41(3):587-97. doi: 10.1007/s10439-012-0700-7. Epub 2012 Nov 21. PMID: 23180030.

2. Agreement of invasive versus non-invasive blood pressure in preterm neonates is not dependent on birth weight or gestational age Authors: Sascha Meyer, Julia Sander, Stefan Gräber, Sven Gottschling and Ludwig Gortner Publication Details: Journal of Paediatrics and Child Health 46 (2010) 249–254. DOI: doi:10.1111/j.1440-1754.2009.01679.x

3. Method of Blood Pressure Measurement in Neonates and Infants: A Systematic Review and Analysis Authors: Janis M. Dionne, Stephen A. Bremner, Simin K. Baygani, Beau Batton, Ebru Ergenekon, Varsha Bhatt-Mehta, Eugene Dempsey, Martin Kluckow, Luana Pesco Koplowitz, Dina Apele-Freimane, Hiroko Iwami, Agnes Klein, Mark Turner, and Heike Rabe, on behalf of the International Neonatal Consortium Publication Details: J Pediatr 2020;221:23-31.e5. DOI: https://doi.org/10.1016/j.jpeds.2020.02.072

4. Comparison of Invasive Arterial Blood Pressure Monitoring vs. Non-Invasive Blood Pressure Monitoring in Preterm Infants < 37 Weeks in the Neonatal Intensive Care Unit— A Prospective Observational Study Authors: Sachin Shah, Amita Kaul, Jayant Khandare and Saleha Dhalait Publication Details: Journal of Tropical Pediatrics, 2021, 67(6), 1–9. DOI: https://doi.org/10.1093/tropej/fmab109

5. Comparison of non-invasive vs invasive blood pressure measurement in neonates undergoing therapeutic hypothermia for hypoxic ischemic encephalopathy Authors: J Zhou, O Elkhateeb and K-S Lee Publication Details: Journal of Perinatology (2016) 36, 381–385. DOI: doi:10.1038/jp.2015.198

Clinical and Hemodynamic Significance of the Dicrotic Notch

• Systemic Circulation (Aortic Pressure Waveform) 

  • Aortic Regurgitation: The dicrotic notch may be blunted or absent due to the backflow of blood into the left ventricle during diastole. With significant AR, there is no IVRT because the LV continues to fill during diastole from the onset of diastole. 

  • Aortic Stenosis: The upstroke is delayed, and an anacrotic notch may be seen instead. The dicrotic notch may be less prominent due to prolonged ejection time and reduced stroke volume. 

  • Low Systemic Vascular Resistance (SVR): Seen in vasoplegia, sepsis, or with vasodilator therapy, the dicrotic notch may be diminished or shifted downward due to reduced arterial recoil. Vasopressin, norepinephrine and phenylephrine, or other agents increasing SVR, may re-establish or re-increase the dicrotic notch - when effective in increasing SVR. 

  • Increased Arterial Compliance: In conditions like aging or arteriosclerosis, the notch may be less distinct due to dampened wave reflection. 

• Pulmonary Circulation (Pulmonary Artery Pressure Waveform) 

  • Pulmonary Valve Regurgitation: Similar to aortic regurgitation, the dicrotic notch may be absent or blunted. 

  • Pulmonary Arterial Hypertension (high PVR): The notch may be accentuated due to increased pulmonary vascular resistance and stiffness. 

  • Pulmonary Stenosis: Causes a delayed systolic upstroke and may obscure the dicrotic notch. 

  • Right Ventricular Dysfunction: May alter the timing and appearance of the dicrotic notch due to impaired ejection dynamics. 

• Summary:

  • Prominent dicrotic notch: Often seen with increased afterload or vasoconstriction. 

  • Low or absent notch: May indicate semilunar or truncal valvular insufficiency, low vascular tone, or high diastolic runoff (e.g., AV fistulas, aortopulmonary shunts, left to right PDA, MAPCAs). 

  • Therapeutic Implications: The appearance of the dicrotic notch can guide vasoactive therapy—for instance, vasoconstrictors may restore a diminished notch in vasodilatory states.

Blood Pressure Values by Gestational Age (at birth) for Day One of Age

Blood Pressure Values by Corrected Post Conceptional Age - To be used after Day 1 of life

Calculator by NeoPeds Academy: https://neopeds.academy/bp/ 

Oscillatory Blood Pressure Values in Newborn Infants by Gestational Ages