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Third Compensation: Anaerobic Metabolism iIncreases If this is not sufficient to meet tissue energy needs, we move to our second compensation. This results in a reduced amount of oxygen remaining in the blood as it returns to the right side of the heart (decreased SvO2). Tissues begin to remove or extract a higher percentage of oxygen from the arterial blood. Second Compensation: Tissue oxygen extraction increases. The cardiac output is increased in an effort to increase the amount of oxygen being delivered to the tissues as shown below. If the amount of oxygen being received by the tissues falls below the amount of oxygen required (because of an increased need, or decreased supply), the body attempts to compensate as follows:įirst Compensation: Cardiac Output increases Tissues require oxygen in order to make ATP (energy). How do I use this information clinically?ĪTP (energy) is needed for all cell function and survival. Normal SvO2 60-80%. Normal ScvO2 (from an internal jugular or subclavian vein) is > 70%. It can be very useful if measured before and after changes are made to cardiac medications or mechanical ventilation, particularly in unstable patients. Mixed venous oxygen saturation (SvO2) can help to determine whether the cardiac output and oxygen delivery is high enough to meet a patient's needs. We usually assume (possibly incorrectly at times) that a blood gas sample obtained from the internal jugular or subclavian (which reflects only head and upper extremities) will have the same meaning as an SvO2. It may be used to identify changes in a patient's tissue oxygen extraction. An ScvO2 measurement is a surrogate for the SvO2. An ScvO2 refers to a central venous sample. The mixed venous sample also captures the blood before it is re-oxygenated in the pulmonary capillary.īecause pulmonary artery catheter use has declined dramatically, ScvO2 measurements obtained from internal jugular or subclavian catheters are often used and interpreted in the same manner. By the time the blood reaches the pulmonary artery, all venous blood has "mixed" to reflect the average amount of oxygen remaining after all tissues in the body have removed oxygen from the hemoglobin. An increase in extraction is the bodies way to meet tissue oxygen needs when the amount of oxygen reaching the tissues is less than needed.Ī true mixed venous sample (called SvO2) is drawn from the tip of the pulmonary artery catheter, and includes all of the venous blood returning from the head and arms (via superior vena cava), the gut and lower extremities (via the inferior vena cava) and the coronary veins (via the coronary sinus). It is used to help us to recognize when a patient's body is extracting more oxygen than normally. This refects the amount of oxygen "left over" after the tissues remove what they need. Mixed venous oxygen saturation (SvO2) is the percentage of oxygen bound to hemoglobin in blood returning to the right side of the heart.