Noninvasive continuous hemodynamic monitoring

Abstract

Monitoring of continuous blood pressure and cardiac output is important to prevent hypoperfusion and to guide fluid administration, but only few patients receive such monitoring due to the invasive nature of most of the methods presently available. Noninvasive blood pressure can be determined continuously using finger cuff technology and cardiac output is easily obtained using a pulse contour method. In this way completely noninvasive continuous blood pressure and cardiac output are available for clinical use in all patients that would otherwise not be monitored. Developments and state of art in hemodynamic monitoring are reviewed here, with a focus on noninvasive continuous hemodynamic monitoring form the finger.

Fig. 1

The differences in pressure levels and wave shape in the radial (red) and finger (blue) arteries. A physiological model can reconstruct the brachial artery pressure (green) from the finger arterial pressure. The staircase is the result of an automatic calibration. (Color figure online)

Fig. 2

The cZ pulse contour method, Modelflow and Nexfin CO-trek. The corrected Z (“cZ”, top) method uses an impedance Z to calculate stroke volume (SV) [121]. The time-integral of the arterial pressure wave above diastolic pressure and between valve opening and closing (as determined by upstroke and incisura) is called the pulsatile systolic area (PSA, hashed area in the Figure) of the pressure. This PSA together with the characteristic impedance Z_c, which is estimated from age and depending on mean arterial pressure (MAP) and heart rate (HR, the inverse of the heart period of the pressure wave), gives SV. Cardiac output (CO) is calculated by multiplying SV and HR. The method was developed for tracking of changes in CO and for correct absolute values a calibration of Z was necessary. In the Modelflow method (middle) the 3-element Windkessel was implemented. Z_c and Windkessel compliance C_w depend on age, gender, height and weight and are nonlinearly related to pressure as shown in the pressure—area relation [97]. Thus, patient data and arterial pressures are both needed to determine these Windkessel parameters [99]. The time varying R_p is the ratio of MAP and CO and iteratively determined. To calculate CO, Modelflow imposes arterial pressure on the Windkessel model. A flow curve is produced of which the time-integral gives SV. The Nexfin CO-trek method (bottom) divides the PSA by the input impedance of the Windkessel to instantaneously give SV. In contrast to Modelflow, there is no need for constructing a flow curve. Moreover while Modelflow was developed to be used on invasive pressures, Nexfin CO-trek was developed to work with noninvasive BP as measured with Nexfin [58].