The effect of a higher blood pressure-target on volume and venous return in comatose out of hospital cardiac arrest patients

Cardiac output is dependant of intrinsic cardiac pump function, afterload and the venous return of blood to the heart. The latest guidelines from the European Resuscitation council suggest a more individualized approach to interventions in post-resuscitation care. This individualization might be achieved by a physiological view focusing on venous return hemodynamics, incorporating both the volume state and the pressure gradient for venous return.

Hypothesis

We hypothesized that a mean arterial blood pressure (MAP) target of 77 mmHg would influence the volume state and venous return differently compared to a MAP target of 63 mmHg.

This post-hoc study is based on the BOX-trial, which randomized comatose patients resuscitated after out of hospital cardiac arrest to either a MAP of 77 mmHg (MAP77) or 63 mmHg (MAP63).

The 730 out of 789 patients included in this study were all equipped with a pulmonary artery catheter (PAC).

The volume state was calculated by the analogue mean systemic filling pressure (Pmsa), a measurement for the intravascular volume:

Pmsa = CVP • 0.96 + MAP • 0.04 + CO • c (anthropometric and age adjusted resting resistance).

The venous return driving pressure was calculated as: Pmsa – CVP (mmHg).

Parameters was compared between the MAP targets in the first 36 hours from admission.

Patients in MAP77 received a significantly higher accumulated dosage of noradrenaline (NA) p<0.0001. compared to those in MAP63. A difference between groups was also found regarding administration of fluids: 6009 (IQR: 4726-7495) ml vs 5526 (IQR: 4408-6744) ml for MAP77 vs. MAP63 p<0.0001.

The intravascular volume state (calculated by Pmsa) was higher in MAP77 at all timepoints compared to MAP63. While CVP was similar between both groups, the heart rate was higher in MAP77. Venous return driving pressure of blood to the heart, and therefore cardiac output, increased continuously in both groups during the initial 36h but was significantly higher in MAP77 compared to MAP63 at all timepoints p<0.0001, see figure 1.

By increasing MAP with predominantly NA, the stressed vascular volume is increased providing a greater pressure gradient for venous return to the heart. Besides the increased chronotropic effect from NA, the heart adapted to the increased filling by increasing the heart rate and thus CO. By assessing hemodynamic through this holistic perspective, clinicians could be guided more effectively to improve perfusion compared to the traditional cardio centric approach.

Figure 1. Venous return driving pressure in patients randomized to a high (MAP of 77 mmHg) or a low (MAP of 63 mmHg) blood pressure.Figure 1