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Jan B. Wade · #1 (**permalink**) 07-19-2004, 06:38 AM

New Trends in Oxygen Delivery, Consumption and Debt Assessment: Global and Regional Indices
Jan Headley, RN, BS

About the Author
By Jan M. Headley, RN, BS, Datex-Ohmeda, Inc, Tewksbury, Mass.

http://www.aacn.org/aacn/conteduc.ns...3?OpenDocument

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 patientâ€™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 Bennett⁷ 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 â€œearly interventionsâ€ were compared to those with â€œlate interventions.â€ 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 al³ 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.³More research will continue in this area as newer therapeutic agents used to optimize DO₂ and their impact on tissue perfusion is evaluated.⁶

Cardiac Output: A Global Parameter
Heart rate times stroke volume determines CO. Preload, afterload, and contractility influence stroke volume. In optimizing CO, each component should be assessed to evaluate its influence on global performance.²

Preload has a significant influence on stroke volume and subsequent CO. Traditional pressure-based parameters for preload assessment include central venous pressure and right atrial pressure to indirectly reflect right ventricular preload. Parameters, which provide indirect reflection of left ventricular preload, include pulmonary artery diastolic pressure, pulmonary artery wedge pressure, and left atrial pressure. Because of ventricular compliance changes, pressure measurements for preload assessment may be misleading. Key factors are those that cause a decrease in ventricular compliance, producing a higher pressure for a given volume.

Causative factors of decreased ventricular compliance are increased juxtacardiac pressure from increased intrathoracic pressure, increased intrapericardial pressure, and increased intra-abdominal pressure. Other factors such as positive inotropes and ischemia decrease compliance. Conversely, an increase in compliance generates a lower pressure for a given volume. Factors increasing compliance include the use of afterload reducing agents and dilated cardiomyopathies.^1,2,9,10 Pressure-based indices for preload assessment correlate poorly to the volume and are a poor way to assess the response to fluid administration for preload optimization. Volumetric parameters, rather than pressure-based indices, reflect the preload status of the critically ill patient more accurately. Targeting a right ventricular end-diastolic volume index (RVEDVI) of 90 to 130 mL/m² has been shown to produce an optimal increase in CO, regardless of the pressure based indices obtained.^1,9-11

Oxygen Consumption: Global or Regional?
Ensuring adequate DO₂ is only one side of the oxygen balance equation. Adequate use of oxygen at the tissue level must also occur to maintain optimal cellular function. Oxygen consumption is the amount of oxygen used by the cells to perform metabolic functions. Organs have varying oxygen requirements and therefore consume different amounts of oxygen.

The Fick principle describes the relationship of CO, VO₂, and arterial and venous oxygen content difference. CO is the product of the uptake of oxygen (VO₂) divided by the content of oxygen upstream, minus the content downstream (arterial oxygen content - venous oxygen content). Global VO₂ is evaluated by measuring global arterial DO₂ and global venous DO₂. The difference between the two is the amount of oxygen consumed (VO₂ = global arterial DO₂ - global venous DO₂). The modified Fick equation reflects global systemic global arterial DO₂ and global venous VO₂ when a PAC is used to obtain arterial and venous saturation values.²

Indirect calorimetry employs the measurement of gas exchangeâ€”specifically oxygen and carbon dioxide. Measuring the difference between inhaled and exhaled oxygen provides VO₂. Measuring the difference between exhaled and inhaled carbon dioxide provides carbon dioxide production (VCO₂), the by-product of cellular function. Clinically, the importance of measuring VO₂ and VCO₂ via indirect calorimetry is gaining significant recognition. VO₂ and VCO₂ can be measured at the patientâ€™s airway. Because there are no stores of oxygen requiring equilibrium before analysis, changes in VO₂ in the tissues are immediately reflected by measurements from the lungs. The use of gas exchange measurements includes the VO₂ from the lungs. The traditional PAC-modified Fick equation does not include pulmonary VO₂ since the site of measurement is before and after the lungs.^12-14

Mixed Venous Oxygen Saturation: A Global Indicator
Mixed venous oxygen saturation (SVO₂) has been described as a valuable index of the dynamic relationship between the patientâ€™s oxygen balanceâ€”DO₂ and VO₂. SVO₂ does not correlate directly with any of the determinants of DO₂ or VO₂; it closely correlates to CO only if the arterial oxygen saturation, hemoglobin, and VO₂are constant. The value of SVO₂ is that it will have an inverse relationship with oxygen extraction indices. Since SVO₂ is measured in the pulmonary artery via a PAC, the value reflects global oxygenation status and does not reflect regional changes.^1,9

Oxygen Consumption and Delivery Relationships
Studying the relationship between DO₂, VO₂, and oxygen extraction assists in identifying the patientâ€™s risk of developing tissue hypoxia and oxygen debt. Normally, VO₂ is 25% of DO₂. This relationship has been conceptualized in a biphasic model (Figure 1). In the delivery-independent region, DO2 is in abundance to meet the metabolic needs of the tissues; aerobic metabolism occurs in this region. With a decrease in DO₂, VO₂ will remain relatively stable through increased oxygen extraction.⁹

Oxygen extraction as a compensatory response is limited in the critically ill patient. Once extraction has been maximized, if DO₂declines further, VO₂ becomes dependent on DO₂. This is the supply-dependent region characterized by demands greater than consumption and a resulting tissue hypoxia. Critical DO₂ is the level at which VO₂ becomes dependent upon DO₂. Anaerobic metabolism occurs in the supply-dependent region (Figure 1).⁹

The clinical use of PAC-derived measurements of VO₂and DO₂ has been questioned due to the potential mathematical coupling that occurs when 2 variables are included in the same measurement. Strategies to reduce the influence of mathematical coupling include reducing the variability of CO determinations, increasing CO determination from 3 to 5 per set, randomize the CO determination to DO₂ and VO₂ calculations, ensure that DO₂ is increased by a minimum of 100 mL/min/m², and to obtain VO₂ values as measured by indirect calorimetry.¹⁵

Implications of Oxygen Debt
Clinically, the accumulation of oxygen debt occurs when oxygen demand is greater than VO₂. This can be reflected by the supply-dependence region of the VO₂-DO₂ relationship curve. Increasing demands, decreasing oxygen delivery, or decreasing cellular extraction place a patient at risk for developing an oxygen debt. The accumulation of oxygen debt influences morbidity and mortality.^9,13,14

Assessing for oxygen debt requires evaluation of metabolic function. Because of inadequate DO₂ or utilization, the cells resort to anaerobic metabolism producing elevated lactate levels. Other anaerobic markers such as increased base deficit and decreased pH reflect tissue hypoxia in the supply-dependence region. Metabolic markers are global indicators of potential oxygen debt. However, they do not reflect the degree of the debt nor the organ location.^1-3,13

The relationship of VO₂ to VCO₂ provides information about the patientâ€™s oxygen balance. The ratio of VO₂ to VCO₂ is termed the respiratory quotient (RQ) and reflects the relationship of VO₂ to carbon dioxide production. Oxygen debt conditions will lower a VO₂ value since there is little consumption occurring. Due to the shifting of carbon dioxide stores in the body, VCO₂ may be normal or elevated. This produces an RQ value that is either high or increasing over time. When an oxygen debt condition is resolving, the patient begins to consume more oxygen and the VO₂ value will increase. The ratio of VCO₂ to VO₂ then decreases with the RQ decreasing.^12,13,16

Regional Assessment of Tissue Hypoperfusion
Gastric tonometry as a means to assess regional tissue hypoperfusion has been available clinically for decades. The gut as an organ sensitive to alterations in perfusion is well documented.^4,5,17-19 The value of a regional assessment versus global assessment has gained more significance. However, the challenge has lied in the limitations of the technology and possibly the timing of the measurement. Tonometry employs the use as a special nasogastric tube that houses a gas permeable silicone balloon near the tip. When properly placed, the nasogastric catheter lies in the stomach. Carbon dioxide diffuses from the intermucosal wall into the balloon for sampling. Previously, saline has been used as the medium diffusion. A sample of the saline is then measured for carbon dioxide values. Currently, an automated air tonometry method is available. This method employs capnography as the measurement technology and has eliminated the need to draw gastric samples from a saline filled balloon (Figure 2).^1,4,5,20

Early indices of gastric perfusion included the calculation of intramucosal pH (pHi) based on the Henderson-Hasselbalch equation. Arterial bicarbonate does not always equal regional bicarbonate and therefore may produce errors in the pHi equation. This effect has caused questioning of the value of pHi as a specific indicator of regional hypoperfusion. A more direct perfusion assessment is obtained by measuring the partial pressure of gastric carbon dioxide (PgCO₂) diffused from the mucosal lining of the stomach into the gastrointestinal balloon. However, arterial PCO₂ that is impacted by ventilation influences PgCO₂ values. Therefore, measuring the gap between PgCO₂ and arterial PCO₂ has been described as a more sensitive indicator of gastric and splanchnic hypoperfusion.^1,4,5,20

Direct therapies aimed at increasing regional perfusion to the gut have been inconclusive. The value of PgCO₂ as a parameter becomes important when used as an early indicator of regional hypoperfusion. Pestel and Uhlig¹⁷ recently reported on the use of gastric tonometry as an early warning sign. Upon review of global and regional parameters after an adverse event (a decrease in mean arterial pressure of >20 mm Hg, the change in PgCO₂ was noted up to 4 hours before a change in global parameters such as SVO₂, lactate, and base deficit. In addition, patients who sustained an elevated PgCO₂-arterial gap for longer periods lead to longer stays in the intensive care unit. Gastric tonometry may not provide information regarding regional tissue hypoxia directly, it does however provide information that gut perfusion is compromised and alerts the clinician to institute therapies directed at restoring perfusion.¹⁷

Combining regional assessment parameters such as RVEDVI and PgCO₂ brings a new perspective to the patientâ€™s response to fluid resuscitation. Some studies have shown that volume replacement globally coupled with regional perfusion indices, such as PgCO₂, provide a better indicator of the adequacy of resuscitation efforts. Targeting RVEDVI values above 120 mL/m² while maintaining a normal pHi (PgCO2) produced less complications and subsequent organ failure. Patients resuscitated at higher levels of preload have significantly better visceral perfusion than those resuscitated at normal preload with the addition of inotropes.^1,18,19

Clinical Implications
Accurate assessment of the adequacy of the 3 components of oxygen utilization is necessary to ensure that the critically ill patient is not placed at risk for developing an oxygen debt. The primary event leading to oxygen debt is tissue hypoxia. Factors that can cause tissue hypoxia are insufficient pulmonary gas exchange, decreased VO₂, and altered oxygen extraction. Normal compensatory mechanisms increase CO, redistribution of blood flow, and increase oxygen extraction.

Assessment is directed at identifying potential inadequate DO₂states. Inadequate DO₂ may occur as a result of absolute reduction in CO, relative reduction in relation to tissue needs, or maldistribution of blood flow. If the patient has clinical evidence of hypoperfusion, then DO₂ is inadequate. Evaluating the metabolic indices of oxygen debt, such as lactate levels >2 mmol/L, SVO₂ <50%, or VO₂ index <110 mL/min/m², may not provide an accurate assessment of the degree of debt.

However, it can alert the clinician to a situation of oxygen imbalance. By adding regional values such as the PgCO₂ - arterial PCO₂ gap, RVEDVI, and RQ, a more complete assessment of global and regional indices can be made.^18,19 Monitoring the variables described in this article and evaluating their relationships may be useful in interpreting responses to interventions, decreasing the level of oxygen debt accumulation, and enhancing survival.

Acknowledgements
The author and AACN wish to thank the following individuals for providing their extensive peer review services: Lisa Chapman, RN, BSN, charge nurse, the Heart Institute of Spokane, Spokane, Wash; Katie Schatz, RN, MSN, FNP-C, clinical practice specialist, AACN, Aliso Viejo, Calif; and Jacalyn West, RN, BSN, radiology nurse, Sacred Heart Medical Center, Spokane, Wash.

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