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Jan B. Wade · #1 (**permalink**) 07-19-2004, 06:35 AM

New Trends in Oxygen Delivery, Consumption and Debt Assessment: Global and Regional Indices

Jan Headley, RN, BS
Datex-Ohmeda, Inc.
Tewksbury, Mass

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Introduction

The concept of optimizing oxygen delivery to meet oxygen demand is not new to the critical care arena. Threats to the oxygen balance can lead to inadequate tissue oxygen utilization. Global assessment parameters may not be sufficient to evaluate where the patient lies in the balance. New trends in tissue oxygenation assessment include global parameters and variables that assess regional oxygenation. Detection of regional hypoperfusion has been available with gastric tonometry. The gut is one of the first organs to undergo redistribution of blood flow and subsequent dysoxia. The inclusion of gastric tonometry measurements for regional oxygenation assessment provides valuable early indication of regional hypoperfusion.

This article will focus on methods of global measurements of oxygen delivery (DO2), oxygen consumption (VO2), and the concept of oxygen debt. Pulmonary artery catheter (PAC) based variables, in addition to indirect calorimetry as another method of measuring VO2, will be presented. Use of volumetric parameters optimizes the preload status of the critically ill patient. Newer trends in combining global measurements with regional assessment show promise for optimization of fluid resuscitation. Identification of early changes in regional tissue oxygenation alerts the clinician to provide interventions to restore the imbalance before observation of changes in global parameters.

Threats to the Oxygen Balance

Threats to tissue oxygen balance can lead to inadequate oxygen utilization at the cellular level. Global and regional parameters provide valuable information regarding the patient's response to threats to the oxygen balance. Patient outcomes are enhanced with earlier monitoring and applying additional assessment strategies. The goal of the critical care practitioner is not only to optimize the global parameters, but also to assess and maximize regional perfusion. [1-3]

Maintaining tissue oxygen balance relies on a progressive and competent 3-step process: pulmonary gas exchange, DO2, and systemic gas exchange. Each step of the process must function appropriately to ensure that the patient has an adequate tissue oxygen balance. [2]

When a threat to oxygen balance occurs, the body can instantaneously activate compensatory mechanisms. The first compensatory response to an increased demand for oxygen at the cellular level is an increase in cardiac output (CO). The second is to redistribute the blood flow by recruiting underperfused capillary beds. A final compensatory response is increased oxygen extraction by the cells. [1,2]

Assessment techniques for the compensatory mechanisms include assessment of CO and oxygen extraction indices. Previously, assessment of redistribution of blood flow had been unavailable. Because of gastric tonometry for regional monitoring, this assessment is now available. [1,4,5]

Controversy regarding the impact of optimizing DO2 and VO2 continues. Strategies directed toward optimizing the relationship of DO2 to VO2 attempted to decrease morbidity, mortality, and shorten length of stay, thereby decreasing overall hospital costs. [3,6,7] Controversy exists regarding these strategies partly due to the heterogeneous patient populations and impact of therapeutic agents on regional perfusion. [3,6-8]

Heyland et al8 performed a critical review of the literature reporting on the outcomes of optimization of DO2. Seven randomized studies that met the study group criteria were included. Upon analysis of the studies, the overall impact of DO2 optimization was found neither favorable nor unfavorable on patient outcomes. [8] Boyd and Bennett [7] reviewed 14 studies on the impact of enhancing DO2 on mortality outcomes. The research was divided into 2 groups of 7 papers each. The investigators found a significant difference in the studies where "early interventions" were compared to those with "late interventions." The overall conclusion was that patients who underwent therapeutic interventions directed at optimizing DO2 earlier had better outcomes than those who had interventions implemented later in the care process. [7] Current research supports the efforts of optimizing DO2 in preoperative patients who have not developed an oxygen debt before therapeutic interventions. Lobo et al [3] recently reported that in a high-risk surgical patient population, treatment aimed at optimization of DO2 intraoperatively and for 24 hours postoperatively resulted in a 68% reduction in 60-day mortality and a significant reduction in prevalence of complications. [3] More research will continue in this area as newer therapeutic agents used to optimize DO2 and their impact on tissue perfusion is evaluated. [6]

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