PROCEDURE 16 • 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 Increased Oxygen Consumption Fever Increased Central and Mixed Venous Oxygen Saturation Increased Oxygen Delivery Increased cardiac output 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 • The percent of venous oxygen saturation as measured in the superior vena cava (ScVO2) reflects the mixing of venous blood from the superior half of the body. It does not include blood from the IVC and coronary sinus. ScVO2, right atrium (RA), and SVO2 do not correlate absolutely. ScVO2 does trend with SVO2 in a variety of hemodynamic states. In normal conditions, ScVO2 is slightly less than the RA oxygen saturation and lower than SVO2. In septic or shock states, ScVO2 is higher than SVO2, with a difference that ranges from 5% to 7% and up to 18% in severe shock. This difference is in part because of a redistribution of blood flow caused by the various pathophysiologies. Therefore, ScVO2 overestimates SVO2 in shock conditions; a low ScVO2 likely indicates an even lower SVO2.4,9–11 • Small French size and shorter oximetry catheters have pediatric patient applications for assessment of venous saturation.9 • Some common proper setup and maintenance steps for the catheters and bedside computer or module are necessary for accurate monitoring of both ScVO2 and SVO2. • Continuous venous saturation monitoring is performed with a three-component system (Fig. 16-1)2,5–7:
Continuous Venous Oxygen Saturation Monitoring
PREREQUISITE NURSING KNOWLEDGE
Decreased hemoglobin
Decreased arterial oxygen saturation
Decreased arterial partial pressure of oxygen
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 hemoglobin
Increased arterial oxygen saturation
Increased arterial partial pressure of oxygen
Related posts:
39: Automated External Defibrillation
139: Calculating Doses and Flow Rates and Administering Continuous Intravenous Infusions
130: Fecal Containment Devices and Bowel Management System
20: Chest Tube Placement (Perform)
95: Lumbar Puncture (Perform)
35: Invasive Mechanical Ventilation (Through an Artificial Airway): Volume and Pressure Modes
Full access? Get Clinical Tree
16: Continuous Venous Oxygen Saturation Monitoring
