Correlation of Noninvasive Blood Pressure and Invasive Intra-arterial Blood Pressure in Patients Treated with Vasoactive Medications in a Neurocritical Care Unit.

Background: The correlation between noninvasive (oscillometric) blood pressure (NBP) and intra-arterial blood pressure (IAP) in critically ill patients receiving vasoactive medications in a Neurocritical Care Unit has not been systematically studied. The purpose of this study is to examine the relationship between simultaneously measured NBP and IAP recordings in these patients.

Methods: Prospective observational study of patients (N = 70) admitted to a neurocritical care unit receiving continuous vasopressor or antihypertensive infusions. Paired NBP/IAP observations along with covariate and demographic data were abstracted via chart audit. Analysis was performed using SAS v9.4.

Results: A total of 2177 paired NBP/IAP observations from 70 subjects (49% male, 63% white, mean age 59 years) receiving vasopressors (n = 21) or antihypertensive agents (n = 49) were collected. Paired t test analysis showed significant differences between NBP versus IAP readings: ([systolic blood pressure (SBP): mean = 136 vs. 140 mmHg; p < 0.0001], [diastolic blood pressure (DBP): mean = 70 vs. 68 mmHg, p < 0.0001], [mean arterial blood pressure (MAP): mean = 86 vs. 90 mmHg, p < 0.0001]). Bland-Altman plots for MAP, SBP, and DBP demonstrate good inter-method agreement between paired measures (excluding outliers) and demonstrate NBP-IAP SBP differences at extremes of blood pressures. Pearson correlation coefficients show strong positive correlations for paired MAP (r = 0.82), SBP (r = 0.84), and DBP (r = 0.73) recordings. An absolute NBP-IAP SBP difference of > 20 mmHg was seen in ~ 20% of observations of nicardipine, ~ 25% of observations of norepinephrine, and ~ 35% of observations of phenylephrine. For MAP, the corresponding numbers were ~ 10, 15, and 25% for nicardipine, norepinephrine, and phenylephrine, respectively.

Conclusion: Despite overall strong positive correlations between paired NBP and IAP readings of MAP and SBP, clinically relevant differences in blood pressure are frequent. When treating with vasoactive infusions targeted to a specific BP goal, it is important to keep in mind that NBP and IAP values are not interchangeable.