Related Column: Stabilizing a Trauma Patient Beyond the “Golden Hour”
My previous column focused on key indicators of effective resuscitation. I suggested that adequate resuscitation depends on maintenance of aerobic cellular metabolism. In order for this to occur, there must be adequate blood flow to the tissues, and oxygen delivery must meet or exceed tissue requirements. We focused on which parameters should be observed to assess success in tissue oxygenation.
A reasonable assumption could be made that if clinicians could ensure that oxygen delivery meets or exceeds consumption in the setting of shock, then resuscitative efforts have a good chance of being successful regardless of etiology. With that in mind, assessment of oxygen delivery (DO2) and oxygen consumption (VO2) become valuable parameters. A fundamental understanding of the factors we can manipulate to improve the delivery to consumption substrate is critical in assessing delivery and consumption.
Oxygen Delivery
Oxygen delivery is a product of cardiac output (CO), hemoglobin (Hb) and oxygen saturation (SaO2). Manipulation of preload, afterload, cardiac contractility, fraction of inspired oxygen (fiO2), and the number of circulating red blood cells all affect the amount of oxygen being delivered to the tissues. Oxygen delivery is typically measured in milliliters per minute. .
Oxygen delivery equals cardiac output (CO) multiplied by oxygen content (CO2) and is measured utilizing the following equation:
DO2 = 10 × CO × (1.34 × Hb × SaO2 + 0.003 × PaO2)
Where:
- 10 is a conversion factor used to convert units to ml/min
- 1.34 ml of O2 can be carried per gram of Hb
- 3 ml of oxygen dissolved per liter of blood (almost a negligible amount, which explains why hemoglobin is a larger factor in delivery of oxygen to tissues)
Many textbooks agree that a “normal” DO2 is approximately 950 to 1150 ml/minute.
Oxygen Consumption
Oxygen consumption (VO2) is the amount of oxygen being extracted by body tissues. It is a function of cardiac output and the difference in arterial (CaO2) versus venous (CvO2) oxygen content. Oxygen content is a product of hemoglobin, oxygen saturation and partial pressure of dissolved oxygen (see the oxygen delivery equation above). Consumption can be calculated in milliliters per minute. More practically speaking, consumption is quantified based on “extraction” from tissues. This percentage is calculated by taking the difference between CaO2 and CvO2 and dividing this difference by the CaO2. Normal oxygen “extraction ratio” ranges from 2530 percent.
Utilizing Extraction to Determine Adequacy of Resuscitation
Mixed Venous Saturation
Measuring the oxygen saturation of blood returning from the body’s tissues can give valuable clues as to adequacy of delivery versus consumption. In clinical practice, this value is measured by pulling a sample off of the distal lumen of the pulmonary artery catheter (PAC) and obtaining a saturation. This “mixed venous” saturation (SvO2) is reflective of oxygen extraction by the tissues. It can be assumed that based on a normal extraction ratio of 25-30 percent subtracted from a normal arterial saturation (SaO2) of 95-100 percent that a “normal” SvO2 would range from 60-80 percent. This would indicate equilibrium between oxygen delivery and consumption. Under stress -- such as in shock states -- an increased metabolic rate requires more oxygen extraction by the tissues, causing SvO2 to drop, indicating the need for reduced consumption or increased delivery. Therefore, this is a very valuable parameter in determining response to therapy.
Central Venous Saturation (ScvO2)
This value is increasingly utilized in ICUs across the country as it is far less invasive than inserting a PAC and it accurately trends with SvO2. A blood sample from the distal lumen of a central venous catheter inserted in either the subclavian or internal jugular vein is used to measure ScvO2. ScvO2 monitoring utilizing a fiber optic oximetric catheter and monitoring device provides continuous readings, eliminating the need for periodic blood sampling and lab use. Normal value for ScvO2 ranges from 70-90 percent. Again, this value provides “real time” indication of oxygen consumption and can give valuable information with respect to adequacy of support.
Are “Swans” on the way out?
Obtaining venous saturation and cardiac output traditionally required the insertion of a pulmonary artery (PA) catheter. Delivery and consumption can now be assessed with esophageal doppler technology. A transducer probe, often as small as a post-pyloric feeding tube, is placed in the esophagus. Utilizing doppler technology, waveforms can help the clinician determine cardiac contractility and volume status. Interpretation of these waveforms will be the topic of another discussion. Doppler technology is less invasive and potentially safer compared to PAC placement. Additionally, resuscitation endpoints can be quickly determined.
Transport Considerations
Unless your transport times are rather lengthy, much of this information will pertain more towards bedside stabilization prior to transport. Having said that, when properly utilized, this information can greatly affect patient outcomes. While calculating oxygen delivery in transport is not yet practical, use of these data in your treatment plan is invaluable. Additionally, understanding parameters that affect oxygen delivery can help you maximize perfusion during transport.
References
- Goodrich C. Endpoints of Resuscitation: What Should We Be Monitoring? AACN Advanced Critical Care 17(3). 2006. pp. 306-316.
- Prentice D and Sona C. Esophageal Doppler Monitoring for Hemodynamic Assessment. Critical Care Nursing Clinics of North America 18(2). pp. 189-193.
