Covert oxygen supply failure measured using the LiDCO plus monitor

Introduction

Oxygen delivery represents one side of the global oxygen flux equation. Normal physiology suggests that cardiac output is linked to tissue metabolic requirement. In sick patients this linkage is frequently abnormal and the ability to maintain tissue oxygen delivery is a prognostic variable. An increasing number of randomised controlled clinical trials and meta-analyses have shown that early manipulation oxygen delivery in certain groups of critically ill patients can reduce morbidity and mortality [1]. Despite a growing bibliography supporting targeting and maintaining oxygen delivery, it is rarely calculated or its level appreciated, despite the fact that the cardiac output has been measured. We have formed a Haemodynamic Nursing/Technical team who institute calibrated CO monitoring and protocolised resuscitation of haemodynamically unstable patients using fluid challenges.

Hypothesis

This study was designed to assess the range and clinical appreciation oxygen delivery in patients admitted to the ICU.

Methods

The cardiac index and oxygen delivery index were measured using the LiDCO plus monitor in 106 critically ill adult patients admitted with a variety of diagnoses, to General Intensive Care at Southampton Hospital.

Results

For the purposes of this study the normal range was taken as being the normal value ± 30%. Values below the range would be considered as being low. Indexed results were used to compensate for patient size.

Conclusion

Two patients in the study group had a cardiac index below the calculated normal range; however, the DO₂I (Fig. 1) showed a fivefold variation across the group, with 31/106 (30%) patients having a DO₂I considered low. These findings were covert, coincident with anaemia and/or poor respiratory gas exchange. In the majority of these patients there had been a failure to appreciate the low DO₂I and clinical management changes were subsequently considered. The continuous measurement of the cardiac index and DO₂I enabled early identification and enhanced standard of care for patients with low DO₂I. These patients may represent the subgroup of critical care patients, cited by RCT evidence, whose survival may be enhanced by CO augmentation and optimisation of oxygen delivery. Because these patients were high risk with potentially serious management implications from DO₂ measurement errors, clinical confidence in the data was provided by calibration with lithium dilution.

Figure 1

Scatter plot: oxygen delivery index.