In a critically ill patient, which blood pressure measurement should be relied on when there is a discrepancy between non-invasive blood pressure (NIBP) and invasive blood pressure (IBP)?

When to Rely on Invasive vs Non-Invasive Blood Pressure in Critically Ill Patients

In critically ill patients with discrepancy between NIBP and IBP, invasive arterial blood pressure (IBP) should generally be relied upon as the gold standard, but clinicians must actively verify the accuracy of the IBP signal using the fast-flush test and confirm with NIBP when resonance/underdamping artifacts are suspected. 1, 2

Understanding the Discrepancy

The relationship between NIBP and IBP is not straightforward and varies with the actual blood pressure level:

• During hypotension (systolic BP <111 mmHg): NIBP typically reads higher than IBP, potentially masking the severity of hypotension 3
• During hypertension: NIBP typically reads lower than IBP, potentially underestimating the degree of hypertension 3
• In critically ill patients, the median difference in mean arterial pressure is approximately 6 mmHg, but this can be much larger in specific clinical scenarios 1

When IBP May Be Inaccurate: The Resonance Problem

A critical pitfall is that IBP can be systematically inaccurate in approximately 30% of critically ill patients due to underdamping/resonance artifacts. 2 This is a frequently overlooked problem that can lead to inappropriate treatment decisions.

Identifying Resonance/Underdamping

Perform the fast-flush Gardner's test on the arterial line to assess dynamic response 2:

• A pressure-time ratio >1.67 mmHg/msec suggests underdamping with 97% specificity and 77% sensitivity 2
• When underdamping is present, systolic IBP overestimates actual pressure by an average of 28.5 mmHg 2
• Diastolic and mean arterial pressures remain relatively accurate even with underdamping 2

High-Risk Scenarios for IBP Inaccuracy

IBP is more likely to be inaccurate in patients with: 2

• Polydistrectual arteriopathy (OR 2.82)
• History of arterial hypertension (OR 2.09)
• Chronic obstructive pulmonary disease (OR 2.61)
• 20-gauge arterial catheters (vs 18-gauge; OR 0.35)
• Diffuse atherosclerosis - can cause IBP-NIBP differences >40 mmHg 4

When NIBP May Be Inaccurate

NIBP using oscillometric devices has important limitations: 5, 6

• Oscillometric devices measure mean arterial pressure directly and algorithmically derive systolic and diastolic values, which may be inaccurate 5
• Not validated for atrial fibrillation - manual auscultatory measurement is preferred in this setting 5
• Unreliable with increasing vasopressor doses - accuracy decreases with the number of inotropes/vasopressors the patient receives 6
• In patients on inotropes, only 54.2% of systolic and 74.1% of diastolic NIBP measurements were clinically acceptable (within 10 mmHg of IBP) 6

Practical Algorithm for Managing Discrepancies

Step 1: Verify Proper Measurement Technique

• Ensure 5 minutes of rest before NIBP measurement 5
• Use appropriately sized cuff (bladder 75-100% of arm circumference) 5
• Position arm at heart level with back and arm supported 5
• For IBP: perform fast-flush test to check for underdamping 2

Step 2: Assess Clinical Context

Rely primarily on IBP when: 1, 2

• Patient is hemodynamically unstable requiring beat-to-beat monitoring
• Vasopressor doses are low to moderate
• Fast-flush test shows appropriate damping
• Patient does NOT have diffuse atherosclerosis or severe peripheral vascular disease

Consider NIBP as more reliable when: 2, 4

• Fast-flush test reveals underdamping/resonance (pressure-time ratio >1.67 mmHg/msec)
• IBP shows unexpectedly high systolic values inconsistent with clinical picture
• Patient has severe atherosclerosis or peripheral vascular disease
• IBP-NIBP difference is >40 mmHg 4

Step 3: Use Confirmatory Measurements

• Measure NIBP in both arms AND one leg to identify peripheral vascular disease 5
• Compare with carotid stump pressure during vascular surgery if available 4
• Take multiple NIBP measurements and average them, especially in atrial fibrillation 5

Impact on Clinical Decision-Making

The choice of monitoring method has significant therapeutic implications: 3

• Use of NIBP alongside IBP (vs IBP alone) was associated with:
  • 60% reduction in blood transfusions (27% vs 43%)
  • 61% reduction in vasopressor infusion use (7% vs 18%)
  • 73% reduction in antihypertensive medication use (12% vs 44%)

This suggests that relying solely on potentially inaccurate IBP may lead to overtreatment of apparent hypotension or hypertension 3.

Key Clinical Pitfalls to Avoid

• Never assume IBP is always accurate - verify with fast-flush test regularly 2
• Do not ignore large IBP-NIBP discrepancies - investigate the cause rather than arbitrarily choosing one 2, 4
• Avoid using NIBP alone in patients on multiple vasopressors - accuracy decreases significantly 6
• Do not use oscillometric NIBP in atrial fibrillation - use manual auscultatory method instead 5
• Recognize that NIBP may falsely reassure during hypotension - it tends to read higher than actual pressure when BP is low 3