Relationship of changes in cardiac output to changes in heart rate in medical ICU patients.

Objective: To determine whether changes in cardiac output are correlated with changes in other commonly measured covariables (heart rate, respiratory rate, mean arterial pressure, mean pulmonary artery pressure, pulmonary artery occlusion pressure, and temperature).

Design: Case series.

Setting: Medical intensive care unit (ICU) in a Veterans Administration Medical Center.

Patients: Twenty-three patients with Swan-Ganz catheters placed by the primary care team were studied on 25 occasions. Patients were managed by the primary team as clinically indicated.

Interventions: Thermodilution cardiac output and covariables were determined at baseline and at hourly intervals for the next 5 h. Each cardiac output measurement was calculated by averaging the last four of five individual measurements at each time point.

Results: The mean cardiac output (9.21/min), heart rate (107/min), and pulmonary artery occlusion pressure (19 mmHg) were elevated. The hourly mean change in cardiac output was 10.2%. Using least-squares linear regression analysis, we found clinically significant changes in cardiac output (> 6.4%) to be most closely correlated with changes in heart rate (R2 = 0.29, p < 0.001). Stepwise linear regression analysis showed that none of the other covariables added significantly to this relationship. No significant relationship was found between changes in cardiac output and changes in pulmonary artery occlusion pressure. Despite these correlations clinically significant changes in cardiac output were accompanied by changes in heart rate in the same direction only 62% of the time.

Conclusion: Changes in cardiac output were best correlated with changes in heart rate. Changes in pulmonary artery occlusion pressure were not correlated with changes in cardiac output in this population of medical ICU patients. A change in any of the covariables (alone or in combination) cannot be reliably used to indicate a simultaneous change in cardiac output.