What measurement assesses oxygen delivery and consumption in critically ill patients?

Measurements That Assess Oxygen Delivery and Consumption

The primary measurements that assess oxygen delivery and consumption in critically ill patients are cardiac output/cardiac index (CO/CI), oxygen consumption (VO2), and central/mixed venous oxygen saturation (ScvO2/SvO2), with ScvO2 >70% serving as the most practical bedside indicator that oxygen delivery is adequate to meet tissue metabolic demands. 1

Core Measurements for Oxygen Delivery Assessment

Mixed Venous Oxygen Saturation (SvO2) and Central Venous Oxygen Saturation (ScvO2)

• ScvO2 >70% (or SvO2 >65%) serves as an indirect indicator of whether cardiac output is adequate to meet tissue metabolic demand, with normal values approximately 70-75% reflecting adequate oxygen delivery when arterial saturation is 100%, hemoglobin is adequate, and cardiac output meets metabolic needs 1, 2

• ScvO2 values below 70% indicate inadequate oxygen delivery relative to consumption and warrant immediate intervention in critically ill patients 2

• The relationship is derived from the Fick equation: assuming hemoglobin of 10 g/dL and 100% arterial oxygen saturation, a cardiac index >3.3 L/min/m² with normal oxygen consumption of 150 mL/min/m² results in mixed venous saturation >70% 1

• ScvO2 (from superior vena cava) typically runs approximately 5% higher than true mixed venous saturation (SvO2 from pulmonary artery), which explains the different therapeutic targets 2

Cardiac Output and Cardiac Index

• Measurement of cardiac output and oxygen consumption are of benefit in patients with persistent shock because a cardiac index between 3.3 and 6.0 L/min/m² and oxygen consumption >200 mL/min/m² are associated with improved survival 1

• Low cardiac output is associated with increased mortality in pediatric septic shock, and children treated with goal-directed therapy (CI >3.3 and <6 L/min/m²) have improved outcomes 1

• Cardiac output determination requires pulmonary artery catheterization and is calculated using arterial-venous oxygen content difference 3

Oxygen Consumption (VO2)

• Oxygen consumption represents the best mirror of actual metabolic activities of tissues and can be measured either noninvasively by the difference between inspiratory and expiratory oxygen concentrations in connection with minute ventilation, or invasively using arterial-venous oxygen content difference and cardiac output 3

• The calculation is: O2 consumption = CI × [arterial O2 content – venous O2 content] 1

• Target oxygen consumption >200 mL/min/m² is associated with improved survival in persistent shock 1

Systemic Oxygenation Measurements

Arterial Blood Gas Parameters

• PaO2, SaO2, and SpO2 are indicators of systemic oxygenation and should be used to guide airway and ventilator management in patients requiring neurocritical care 1

• Arterial or arterialised capillary blood gases should be measured on arrival at hospital for most patients requiring emergency oxygen therapy, with the oxygen device and flow rate noted 1

• PaCO2 is a reliable measurement of hyper- or hypocapnia and is superior to end-tidal CO2 monitoring 1

Pulse Oximetry (SpO2)

• Pulse oximetry should be available to all healthcare staff managing patients receiving oxygen therapy, though clinicians must recognize it gives no information about PCO2 or pH 1

• Most pulse oximeters are unreliable when SpO2 falls below approximately 85%, and readings may be affected by shock, skin pigmentation, nail varnish, cold hands, and severe Raynaud's phenomenon 1

• SpO2 may not always be a reliable predictor of arterial oxygen saturation (SaO2) in hypoxemic, hemodynamically compromised, and critically ill patients 4

Specialized Measurements

Brain Tissue Oxygenation

• Brain parenchymal oxygen tension (PbtO2) with normal values of 23-35 mmHg, where values <20 mmHg represent compromised brain oxygen requiring intervention 1

• Jugular bulb oxygen saturation (SjvO2) can detect both ischemia and hyperemia, though positioning errors and clot formation make it less reliable than PbtO2 monitoring 1

• Near-infrared spectroscopy (NIRS) provides non-invasive bedside assessment of regional tissue oxygenation 1, 5

Special Populations

• In very low birth weight infants, superior vena cava (SVC) blood flow measurement is useful, with values >40 mL/kg/min associated with improved neurologic outcomes and survival 1

• ScvO2 saturation can be used in low birth weight infants but may be misleading in the presence of left-to-right shunting through the patent ductus arteriosus 1

Critical Clinical Caveats

• A normal or high ScvO2/SvO2 does not exclude tissue hypoxia, particularly in septic patients where impaired oxygen extraction is characteristic, with approximately 23% of septic patients presenting with elevated lactate despite ScvO2 >70% 2

• Low cardiac output is associated with increased oxygen extraction, making ScvO2 saturation a useful indirect indicator of adequacy 1

• Routinely measured hemodynamic parameters such as heart rate and systemic arterial pressure only poorly reflect oxygen transport to the tissues 3

• The diagnostic significance of oxygen variables increases in the order of: pO2 < sO2 < cO2, with oxygen content (cO2) being the most clinically significant as it determines mean capillary pO2 and oxygen availability 6