Cardiac output and pulse pressure respiratory variation in hemorrhagic shock

Background

Arterial pressure values frequently decline later after intravascular volume contraction, or when this hypovolemia becomes severe. On the other hand, cardiac output (CO) may decline early in hypovolemic states. CO is strongly coupled with systolic volume (SV). SV presents cyclic variations with intrathoracic pressure changes during mechanical ventilation, and pulse pressure variations (ΔPp) are SV dependent. Thereby, ΔPp has been recommended to guide fluid replacement.

Objective

To test the hypothesis that ΔPp and CO alter with a similar precocity during intravascular volume contraction.

Methods

In 12 ventilated and anesthetized dogs (19 ± 2.5 kg), the CO was measured with a Swan–Ganz catheter while the mean arterial pressure (MAP) and ΔPp were measured through an intra-arterial catheter. After baseline measurements (BL), a graded hemorrhage (20 ml/min) was promoted. Measurements were performed every 5 min during hemorrhage. Digital recordings of arterial pressure waves by multiparametric monitor (Datex-Ohmeda, Finland) were obtained. ΔPp was defined by the following formula: ΔPp (%) = 100 × (Pp_ins - Pp_exp) / [(Pp_ins + Pp_exp) / 2]. Pp_ins, inspiratory pulse pressure and Pp_exp, expiratory pulse pressure.

Results

See Table 1. ΔPp amplification (P = 0.006) and CO decline (P = 0.02) occurred significantly at 10 min of hemorrhage (15 ± 2% estimated volume contraction). The MAP drop occurred by 20 min.

Table 1

Conclusion

We conclude that ΔPp and CO have the same precocity to detect intravascular volume depletion, and that arterial pressure decreases only in later phases of hypovolemia.