Acute changes of metabolic parameters after fluid challenge

Introduction

The detection of heart response to fluid administration is still a challenge in clinical practice. Changes in metabolic parameters may be useful to detect changes in cardiac output (CO) after fluid expansion.

Methods

This is a prospective observational study in adult critically ill patients. CO was measured either by echocardiography or by a thermodilution method (PiCCO, Swan-Ganz catheter). Hemodynamic measurements and blood gas analysis were obtained before and after a fluid challenge with either 1,000 ml crystalloid or 500 ml colloid. Arterial and central venous blood gas samples were taken simultaneously. Oxygen delivery (DO₂), oxygen consumption (VO₂) and carbon dioxide production (VCO₂) were calculated according to well-known formulas. Patients were divided into three groups (high responders, mild responders and nonresponders) according to their change in CO (>20%, 10 to 20%, <10%, respectively).

Results

We evaluated 27 patients, age 68 (95% CI: 61 to 74) and APACHE II score 22 (95% CI: 18 to 26). Seven patients were high responders, eight patients were moderate responders and 12 were nonresponders. DO₂ was significantly increased in high responders (37 ± 35%, P < 0.01) as compared with moderate responders or nonresponders. Furthermore, nonresponders had a decrease in their DO₂ (-10 ± 7%, P < 0.01), while moderate responders showed no change in their DO₂ (1.6% ± 10, P = 0.73) after fluid challenge. We found no differences in changes in lactate levels and central venous oxygen saturation (ScvO₂) between high responders, moderate responders and nonresponders. No differences in the changes of VCO₂ or VO₂/VCO₂ ratio were found between high responders, mild responders and nonresponders too. Changes in DO₂/VCO₂ ratio were found to be significantly increased only in high responders (47 ± 73% vs. -14 ± 31%, P = 0.02) and not in mild responders (15 ± 54% vs. -14 ± 31%, P = 0.15) as compared with nonresponders.

Conclusion

Only significant increases of CO (>20%), after fluid administration, lead to improved oxygen delivery; DO₂ may be decreased in nonresponders. The changes of ScvO₂ and lactate levels do not track the changes of CO after fluid challenge. The DO₂/VCO₂ ratio may be a useful index to identify significant increases of CO after fluid challenge in cases where CO measurement is not feasible.