Monitoring Oxygen Delivery in the Critically Ill

doi:10.1378/chest.128.5_suppl_2.554S

Chest

Volume 128, Issue 5, Supplement 2, November 2005, Pages 554S-560S

https://doi.org/10.1378/chest.128.5_suppl_2.554S Get rights and content

An accurate assessment of regional tissue oxygen delivery (Do₂) may help the intensivist to attenuate end-organ damage in critically ill patients. Transport of oxygen from the ambient air to the mitochondria occurs by convection and diffusion, and is tightly regulated by neural and humoral factors. This article reviews the basic principles of Do₂ and the abnormal oxygen supply-demand relationship seen in patients with shock. It also discusses approaches to monitoring Do₂, including clinical symptoms/signs, acid-base status, and gas exchange, which provide global assessment, as well as gastric tonometry, which may reflect regional Do₂. Some new experimental methods, such as near-infrared spectroscopy and positron emission tomography, are still in development but may in the future provide useful clinical devices for quantifying the adequacy of regional tissue oxygenation in critically ill patients.

Learning Objectives

1. To understand the basics of oxygen delivery, particularly as it pertains to critically ill patients. 2. To summarize the standard and investigational methods used to measure regional oxygen delivery, including gastric tonometry, near-infrared spectroscopy, and metabolic positron emission tomography.

Section snippets

Do₂ and Oxygen Consumption

Transport of oxygen from the atmosphere to the cells of organs follows a relatively simple physical pathway involving convection (bulk flow), diffusion, and chemical combination with hemoglobin. During inspiration, oxygen is transported from the atmosphere by convective flow (ventilation) to the alveoli, where it diffuses into the blood and binds rapidly and reversibly to hemoglobin within the RBC (oxygen uptake). Oxygen bound to hemoglobin is then transported in the RBC by a convective process

Shock

As noted above, in the normal state Do₂ is more than sufficient to meet $\dot{V} O_{2}$ demands of all tissues and organs (Fig 3). Even in states of moderate reductions in Do₂, the OER increases to satisfy $\dot{V} O_{2}$ . This biphasic Do₂- $\dot{V} O_{2}$ relationship is shown in Figure 3.² The level of oxygen transport at which the $\dot{V} O_{2}$ begins to decline has been termed the critical Do₂. When oxygen transport is reduced to below the critical Do₂, $\dot{V} O_{2}$ becomes supply dependent. The tissues begin to

Monitoring Do₂

Because recovery from irreversible shock is difficult, it is important to diagnose and treat shock early so that Do₂ to the organs can be preserved. Clinical indications of shock and inadequate Do₂, such as increased heart rate, decreased BP, reduced urine output, and reduced skin temperature, are neither sensitive nor specific. These parameters are slow to change during the compensation phase, and abnormal values may be seen only in the late stages of diminished Do₂. Serum lactate, anion gap,

Measurement of Systemic Do₂

The adequacy of systemic or global Do₂ is most commonly assessed directly by measuring the oxygen content of arterial and mixed venous blood, estimating CO, and calculating Do₂ and $\dot{V} O_{2}$ using 2, 3. Mixed venous blood is sampled using a pulmonary artery catheter. CO is most often estimated by the thermal dilution technique, and arterial and venous oxygen contents are determined optically with a CO oximeter. Besides the fact that Do₂ and $\dot{V} O_{2}$ are global measurements, there may be errors

Measurement of Regional Do₂

Because of the regional differences in the circulatory response during shock, measurement of global Do₂ and $\dot{V} O_{2}$ may not adequately reflect oxygenation status in individual tissues and organs. For example, splanchnic organ blood flow may be disproportionately reduced during shock without affecting global tissue oxygenation.6, 10 During periods of hypoperfusion, gut barrier functions may become compromised, increasing its permeability and resulting in the introduction of endotoxin,

Conclusions

Maintaining adequate Do₂ to the peripheral tissues in critically ill patients is essential in preventing shock-related multisystem organ failure. Measuring Cao₂ and CO to calculate Do₂ remains the most common method for assessing global Do₂. This method is invasive and is not sensitive enough to detect oxygenation deficiencies in important tissues and organs. Although methods such as gastric tonometry exist for the evaluation of regional Do₂, they offer only intermittent monitoring and have yet

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Postoperative ScvO₂ and arterial lactate were obtained on arrival to the intensive care unit (ICU). ScvO₂ and lactate were drawn again at 8 and 24 hours, respectively, after ICU admission. Moderate global tissue hypoxia (GTH) was defined as ScvO₂ <70% and lactate ≥2 to <4 mmol/L, and severe GTH was defined as ScvO₂ <70% and lactate ≥4 mmol/L. Occult hypoperfusion was defined as moderate-to-severe GTH with mean arterial pressure ≥65 mmHg, central venous pressure ≥8 mmHg, and urine output ≥0.5 mL/kg/h. ScvO₂ on ICU admission negatively correlated with postoperative 24-hour lactate (p = 0.009), which was a strong predictor of time on mechanical ventilation, total complications, and ICU and hospital lengths of stay (p < 0.001 for all comparisons). On admission to the ICU, 19 patients (32%) exhibited occult hypoperfusion. Patients with severe GTH (n = 8) had longer ICU lengths of stay (p = 0.04) and a trend toward longer length of mechanical ventilation (p = 0.17) and number of complications per patient (p = 0.09) compared with those without GTH (n = 10).
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This publication is supported by an educational grant from Ortho Biotech Products, L.P.

The following authors have indicated to the ACCP that no significant relationships exist with any company/organization whose products or services may be discussed in their article: Yuh-Chin T. Huang, MD, FCCP.

Reproduction of this article is prohibited without written permission from the American College of Chest Physicians (www.chestjournal.org/misc/reprints.shtml).

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Chest

Monitoring Oxygen Delivery in the Critically Ill

Learning Objectives

Section snippets

Do2 and Oxygen Consumption

Shock

Monitoring Do2

Measurement of Systemic Do2

Measurement of Regional Do2

Conclusions

Chest

Br J Anaesth

Br J Anaesth

J Surg Res

Chest

Nucl Med Biol

Respiratory functions of the lung

The pulmonary physician in critical care: 2. Oxygen delivery and consumption in the critically ill

Thorax

Oxygen uptake, transport and utilization in the human body

Identification of the critical oxygen delivery for anaerobic metabolism in critically ill septic and nonseptic humans

JAMA

Oxygen delivery

Crit Care Med

Errors in the measurement of cardiac output by thermodilution

Can J Anaesth

Assessment of splanchnic oxygenation by gastric tonometry in patients with acute circulatory failure

JAMA

Relation of arterial blood lactate to oxygen delivery and hemodynamic variables in human shock states

Circ Shock

Do₂ and Oxygen Consumption

Monitoring Do₂

Measurement of Systemic Do₂

Measurement of Regional Do₂