(see also ScVO2 Monitoring; Swan-Ganz Pulmonary Artery Catheter; ABGs)
Figure 2.71
Mixed venous oxygen saturation reflects the body's ability to provide adequate O2 in response to tissue oxygen demands. The SVO2 specifically measures the percentage of oxygen that returns to the right side of the heart, bound to hemoglobin. This value reflects the amount of oxygen that was “taken up” by the tissues and is now “left over,” returning to the heart. It is affected by cardiac output, hemoglobin, arterial oxygen saturation, delivery, and tissue oxygen consumption (metabolic demand). When this number is out of range, it can alert the clinician to investigate why the body may be using more or less oxygen than expected.
SVO2 readings are obtained via the fiberoptics housed within the oximetry lumen (optical module connector) of a specialized pulmonary artery catheter. The fiberoptics transmit light directly to the pulmonary artery by reflectance spectrophotometry (absorption, refraction, and reflection). The light from this catheter then scatters off the oxygenated and deoxygenated hemoglobin it “sees,” and the measurement is converted to a number and seen on the monitor.
Generally, the SVO2 value is >ScVO2. Blood for the ScVO2 is sampled from the upper body venous return, via either a subclavian or internal jugular vein catheter. Conversely, the mixed venous SVO2 sample is drawn from the pulmonary artery catheter tip and samples blood from the entire body, namely, from the superior and inferior vena cava and the coronary veins
The normal range for SVO2is 60 to 80%. As a quick “rule of thumb,” if the SVO2 is low, consumption, demand, or both are increased. If the SVO2 is high (>80%), this may indicate a high-flow state (e.g., sepsis, liver disease, hyperthyroid, etc.)
Effects of SVO2below normal for >3 minutes:
- Below 60% → O2 reserve being used; cardiac decompensation
- Below 50% → O2 reserve depleted; anaerobic metabolism (lactic acidosis begins)
- Below 30% → O2 reserve depleted; insufficient tissue O2; coma
Management of Decreased SVO2/Low Cardiac Output
- Look for correctable causes that are noncardiac (e.g., acidosis, electrolyte imbalance, respiratory causes, pain, anxiety, fear, shivering).
- Optimize preload.
- Optimize heart rate with pacing if necessary (90 to 100 beats/min).
- Control arrhythmias.
- Assess cardiac output and give inotropic agents if cardiac index <2.0 L/min/m2.
- Give vasodilating agents if the calculated SVR is >1,500 dynes/sec/cm5.
- Blood transfusions if hemoglobin level is <7.0 to 8.0 g/dL (hematocrit of 21 to 24 g/dL).
Oximetry Calibration
The specific procedure for calibrating an oximetry system may vary depending on both the specific type/manufacturer of the catheter and the monitor. Generally, oximetry calibration may be done via two methods, either in vitro or in vivo.
In vitro calibration (prior to catheter insertion)
- Key points:The catheter lumens should NOT be flushed (primed) prior to in vitro calibration. Calibrate the catheter with air in the line. Flush (prime) the line only after the in vitro calibration has been completed (and before insertion). Do not perform in vitro calibration after catheter in situ.
- Connect the oximetry cable to the monitor via the port indicated.
- Carefully insert the catheter optical connector into the oximetry cable, “TOP” side “UP,” and close the clasp for a snug fit.
- Select “oximetry calibration.”
- Indicate “SVO2 or ScVO2” and then select “in vitro” on the monitor.
- Enter the most recent hemoglobin or hematocrit value in the field indicated
- Press “calibrate” and a reading indicating calibration success should appear. The catheter may now be inserted. Press “start” to begin monitoring once catheter is placed.
In vivo calibration (after catheter insertion, catheter in situ)
- The catheter should already be placed in the patient. If not already connected, carefully insert the catheter optical connector into the oximetry cable, “TOP” side “UP,” and close the clasp for a snug fit.
- Verify signal quality index = SQI level 1 or 2 optimal.
- Select “oximetry calibration.”
- Indicate “SVO2 or ScVO2” and then select “in vitro calibration”:
- A warning message may appear indicating an issue with improper position of the catheter, or unstable signal. If present, attempt to troubleshoot the problem, and then select “Recalibrate” or “Continue” to begin the blood sampling procedure.
- Select the “Draw” button and obtain sample by accessing the distal port. First, draw a waste (clearing volume), then slowly draw blood (2 mL over 30 seconds recommended), and send promptly for analysis.
- When results are obtained, in the fields indicated, enter the hemoglobin or hematocrit and the mixed venous O2 saturation value (normal 60% to 80%). Do NOT confuse this with the mixed venous PO2 value (normal 25 to 40 mm Hg).
- Select “calibrate.”
- Key points:In vivo calibration every 24 hours is recommended for optimal accuracy. Do not permit the oximetry cable connection to lie on the patient or become wrapped up in sheets of blankets for >10 minutes. This may result in an “exceeded temperature limit” alarm due to the warming of the connector.
Transport
- The catheter and optical module (while remaining connected) can be disconnected from the monitor for transport, with data recall upon reconnection to the monitor (data must be <24 hours old).
- Upon arrival, simply plug in the optical module into the monitor, and select “oximetry calibration,” followed by “recall oximetry data” and then “yes” (if <24 hours of stored data) to resume monitoring.Key Point: Prior to selecting “yes,” be sure to verify that the historical data are that of the current patient.
- If “no” is selected at this point, an “in vivo” calibration will be required.