Continuous Venous Oxygen Saturation Monitoring

Venous oxygen saturation monitoring is performed to measure the oxygen saturation of the venous blood. The value can be obtained either from the superior vena cava or from the pulmonary artery. Continuous assessment of the balance between a patient’s oxygen delivery and oxygen consumption can be monitored with a specialized fiberoptic central venous or pulmonary artery catheter and an associated computer or module.

PREREQUISITE NURSING KNOWLEDGE

• Anatomy and physiology of the cardiopulmonary system should be understood.

• Physiologic principles related to invasive hemodynamic monitoring should be understood.

• Technical aspects of central line placement and pressure monitoring should be understood.

• Technical aspects of pulmonary artery (PA) pressure monitoring should be understood.

• Physiologic concepts of oxygen delivery, oxygen demand, and tissue oxygen consumption should be understood.

• The percent of venous oxygen saturation as measured in the PA (SVO₂) is flow weighted and represents a true mixing of all venous blood: inferior vena cava (IVC), superior vena cava (SVC), and coronary sinus.

• Clinically, SVO₂ provides an index of overall oxygen balance because it is a reflection of the dynamic relationship between the patient’s oxygen delivery (DO₂) and oxygen consumption (VO₂). Whenever a threat to the oxygen balance occurs, the body’s primary compensatory mechanisms are to increase oxygen delivery by increasing cardiac output or to increase oxygen extraction at the tissue level.

• In a critically ill patient, if cardiac output is limited, increased extraction occurs to meet the demand for oxygen at the tissue level. The result is a decreased level of oxygen returning to the heart and a lower SVO₂ measurement. Many factors can affect the requirements for oxygen and subsequently SVO₂ (Table 16-1).^2,6,8,13

Table 16-1

Common Conditions and Activities That Affect Venous Oxygen Saturation Values

Decreased Central and Mixed Venous Oxygen Saturation

Decreased Oxygen Delivery

Decreased cardiac output
Decreased hemoglobin
Decreased arterial oxygen saturation
Decreased arterial partial pressure of oxygen

Increased Oxygen Consumption

Fever
Pain
Shivering
Seizures
Increased work of breathing
Agitation
Infection and sepsis
Vasoactive and beta-agonist medications
Multiple organ failure
Burns
Head injury
Increased musculoskeletal activities
Numerous nursing procedures (e.g., dressing changes, suctioning, turning, and chest physiotherapy)

Increased Central and Mixed Venous Oxygen Saturation

Increased Oxygen Delivery

Increased cardiac output
Increased hemoglobin
Increased arterial oxygen saturation
Increased arterial partial pressure of oxygen

Decreased Oxygen Consumption

Hypothermia

Hypothyroidism

Pharmacologic paralysis and sedation

Anesthesia

Cellular dysfunction

Decreased work of breathing

Decreased musculoskeletal activities

• SVO₂ does not correlate directly with any of the determinants of oxygen delivery or oxygen consumption. Because a critically ill patient is in a dynamic state with rapidly changing oxygen demand and oxygen consumption, SVO₂ must be viewed in the light of these changing determinants and considered an index of oxygen balance.1,2,6,8,13

• A normal SVO₂ generally is considered to be 60% to 80%,2,8 and a clinically significant change in SVO₂ (5% to 10%) can be an early indicator of physiologic instability.^2,8 SVO₂ values of less than 60% may result from either inadequate oxygen delivery or excess oxygen consumption. SVO₂ monitoring is used in critically ill patients for earlier detection of oxygenation instability than that obtained through traditional PA monitoring.^2,6,8,13