Transcutanous Blood Gas Monitoring


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Transcutanous Blood Gas Monitoring

  1. 1. Transcutaneous Blood Gas Monitoring Hutch, A., Acta Anaesthesiologica Scandinavica. Supplementum. 107:87-90,1995 Journal Arcticle. Review Transcutaneous Oxygen Measurements: Implications for Nursing Rich, Kathleen, Journal of Vascular Nursing. 19(2):55-9, 2001 June. Journal Article. Review AARC Clinical Practice Guideline. Transcutaneous Blood Gas Monitoring for Neonatal & Pediatric Patients. Anonymous, Respiratory Care. 39(12):1176-9, 1994 December. Journal Article. Practice Guideline Saif K. Ahmedi Claudia Amaya Resc 3621 Physiologic Monitoring Spring 2006
  2. 2. Introduction Transcutaneous blood gas measurement is a noninvasive, diagnostic technique that records the partial pressure of oxygen(Ptc0 2 ) and carbon dioxide (PtcCO 2 ) electrochemically at the skin surface. Its advantage over direct measurement of arterial blood gas is that it is noninvasive and is appropriate for continuous and prolonged monitoring. However, because it is an indirect measure of arterial values, arterial blood gas values using bench level co-oximeters still remain the gold standard. Unlike pulse oximeters, which measure hemoglobin saturation, TCMs measure partial pressure directly. A slight delay in warm-up and response time make it better suited for monitoring slow trends rather than instant changes. Due to the oxygen hemoglobin saturation/dissociation curve, observing saturation alone is unreliable when hyperoxemia must be closely monitored, as in premature infants. In these instances, use of a TCM is indicated when frequent arterial blood sampling may be harmful or unfeasible. PtcO 2 and PaO 2 have a 1:1 correlation in neonates due to their skin composition. In older children and adults, the PtcO 2 /PaO 2 ratio decreases as skin becomes more impermeable to O 2 . For this reason, RTs normally do not use TCMs to monitor adults and children.
  3. 3. History The possibility of skin surface measurement for estimating central PO 2 was suggested by Bramberger and Goodfriend in 1951. They showed that when a finger was immersed in an electrolyte solution maintianed at 45 o C, the PO 2 of the solution equilibrated with that of arterial blood. In 1972, Huch and associates developed a miniaturized, heated electrode that measured the skin surface PO 2 . This is the basis behind the transcutaneous oxygen monitors in use today.
  4. 4. The Elelctrodes <ul><li>The CO 2 Severinghaus electrode(1) Consists of a pH </li></ul><ul><li>sensitive glass electrode in contact with a thin </li></ul><ul><li>film of sodium bicarbonate solution separated </li></ul><ul><li>from the test solution by a gas permeable </li></ul><ul><li>membrane. The pH of the bicarbonate solution </li></ul><ul><li>is proportional to the log carbon dioxide tension. </li></ul><ul><li>Carbon dioxide diffuses into the bicarbonate </li></ul><ul><li>solution and the hydrogen ions produced results </li></ul><ul><li>in a potential difference between the test </li></ul><ul><li>solution in the electrode. This is measured by the </li></ul><ul><li>voltmeter and reported as partial pressure of carbon </li></ul><ul><li>dioxide. </li></ul><ul><li>The O 2 Clark electrode. Oxygen diffuses from </li></ul><ul><li>the skin into the (4) electrolyte chamber. </li></ul><ul><li>oxygen reacts with water to produce hydroxyl </li></ul><ul><li>ions. The electrons consumed results in a </li></ul><ul><li>current difference between the platinum and </li></ul><ul><li>silver anode(7). The current difference is </li></ul><ul><li>measured by a current meter and reported as </li></ul><ul><li>partial pressure of oxygen. </li></ul><ul><li>Heating element (9) and thermistor (a resistor that </li></ul><ul><li>measures temperature changes)(8) heat the skin to </li></ul><ul><li>induce active hyperemia and to change the diffusion </li></ul><ul><li>conditions in the skin. </li></ul>
  5. 5. Skin Structure <ul><li>The skin structure comprises three distinct </li></ul><ul><li>Layers: </li></ul><ul><li>The capillary network. </li></ul><ul><li>The stratum basale, stratum spinosum and stratum granulosum where oxygen is consumed due to cellular respiration. </li></ul><ul><li>The stratum corneum, the layer of dead cells where oxygen consumption is 0. </li></ul>
  6. 6. Physiology Behind Use <ul><li>Three interdependent factors determine PtcO 2 and PtcCO 2 : </li></ul><ul><li>Skin Perfusion </li></ul><ul><li>Skin Respiration </li></ul><ul><li>Skin Diffusion </li></ul><ul><li>Normally, the measured PtcO 2 at the epidermal layer is 0mmHg. When the dermal temperature is raised between 42 o C to 45 o C there are several changes in the three factors. </li></ul>
  7. 7. Results of Epidermal Warming
  8. 8. Results of Epidermal Warming (continued) <ul><li>OXYGEN </li></ul><ul><li>Heat causes a rise in the capillary partial pressure of oxygen. </li></ul><ul><li>Oxygen consumption and the diffusion barrier together lower the capillary partial pressure of oxygen </li></ul><ul><li>Net result – transcutaneous partial pressure of oxygen approximates arterial partial pressure of oxygen </li></ul><ul><li>CARBON DIOXIDE </li></ul><ul><li>Heat causes a rise in the capillary partial pressure of carbon dioxide. </li></ul><ul><li>Heat causes a rise in cellular metabolism which in turn increases carbon dioxide production, raising the partial pressure of carbon dioxide. </li></ul><ul><li>Net result – transcutaneous partial pressure of carbon dioxide is greater than arterial partial pressure of carbon dioxide. </li></ul>
  9. 9. Oxygen Pressure Profiles and Temperature Changes in Skin <ul><li>Under normal conditions of skin temperature, a major decrement in PO 2 occurs along the capillary network. (A) </li></ul><ul><li>Nicotinic acid is used as a vasodilator to increase perfusion without active induction of hyperemia. (B) </li></ul><ul><li>Ideal curve (C) </li></ul><ul><li>Electrode heated skin curve (D), is deviated upward from the ideal curve in layer one due to several reasons: </li></ul>
  10. 10. Oxygen Pressure Profiles and Temperature Changes in Skin (continued) <ul><ul><li>skin perfusion increases and the PO 2 gradient from capillary bed to skin surface is negligible. </li></ul></ul><ul><ul><li>Oxyhemoglobin curve shift to right and more oxygen is unloaded. </li></ul></ul><ul><ul><li>Oxygen permeability in the skin is increased. </li></ul></ul><ul><ul><li>In layer 2 the curve falls back down due to tissue respiration in the epidermal layer and because the Clark electrode consumes oxygen in the reaction. </li></ul></ul>
  11. 11. Due to variations in skin characteristics, ratios between ABG and TCM values differ depending on age, sex, and skin site: Premature infants: 1.41 : 1.00 Neonates: 1.00 : 1.00 Children: 0.68 : 1.00 Adults: 0.79 : 1.00 Older Adults: 0.68 : 1.00 There is a tendency to measure infra-arterial values in male adults and supra-arterial values in premature infants. However, consistency is more important than absolute value, and consistency is only assured if TCM measurements are independent of flow…
  12. 12. Transcutaneous-Arterial Ratios Change With Flow Region 2 – partial pressure is effected by changes in flow. Region 3 – partial pressure does not change in a region of hyperemia. Blood pressure changes in this region also have no effect on partial pressures.
  13. 13. Transcutaneous-Arterail Ratios Change With Flow (continued) Since the energy required to maintain a constant electrode core temperature (for active hyperemia to occur) changes as blood flow increases, this change in power consumption (displayed on some devices) is sometimes used as a measure of perfusion status. A decrease in PtcCO 2 as opposed to PaCO 2 is also an indicator of poor tissue perfusion.
  14. 14. Indications <ul><li>Adults </li></ul><ul><li>Wound evaluation (PtcO 2 >40mmHg) </li></ul><ul><li>Hyperbaric therapy (PtcO2 > 50mmHg at 2.5atm is indicative of successful therapy) </li></ul><ul><li>Plastic Surgery </li></ul><ul><li>Determination of amputation level </li></ul><ul><li>Adjunct in peripheral artery disease and gangrene diagnosis (PtcO 2 = 0-30mmHg) </li></ul><ul><li>Evaluation of injuries and vascular surgeries. </li></ul><ul><li>Pediatric & Neonatal </li></ul><ul><li>Adjunct to ABG </li></ul><ul><li>Continuous and prolonged monitoring during mechanical ventilation, CPAP, and supplemental oxygen administration. </li></ul><ul><li>Assessment of functional shunts, PPHN, persistent fetal circulation(PDA), or congenital heart disease. </li></ul><ul><li>CONTRAINDICATION – patients with poor skin integrity and/or adhesive allergy. </li></ul>
  15. 15. Clinical Uses in Neonatal Care <ul><li>Continuos monitoring </li></ul><ul><ul><li>Monitor oxygenation and prevent hyperoxemia. </li></ul></ul><ul><ul><li>Monitor CO 2 </li></ul></ul><ul><ul><ul><li>High PtcCO 2 may indicate inadequate ventilation. </li></ul></ul></ul><ul><ul><ul><li>While trending, PtcCO 2 < PaCO 2 may indicate poor perfusion. </li></ul></ul></ul><ul><li>Trending </li></ul><ul><ul><li>Factors that may cause TCOM to read lower: </li></ul></ul><ul><ul><ul><li>Shock </li></ul></ul></ul><ul><ul><ul><li>Severe Acidosis </li></ul></ul></ul><ul><ul><ul><li>Hypothermia </li></ul></ul></ul><ul><ul><ul><li>Severe cyanosis </li></ul></ul></ul><ul><ul><ul><li>Heart disease </li></ul></ul></ul><ul><ul><ul><li>Severe anemia </li></ul></ul></ul><ul><ul><ul><li>Skin edema </li></ul></ul></ul><ul><ul><ul><li>PaO 2 greater than 100mHg </li></ul></ul></ul><ul><ul><ul><li>Vasodilator drugs delivered (Tolazoline) </li></ul></ul></ul><ul><li>Detection of shunting blood </li></ul><ul><ul><li>Using two electrodes to detect shunts through ductus arteriosus. </li></ul></ul><ul><ul><ul><li>Placement – right arm, left leg </li></ul></ul></ul><ul><li>Indicator of Skin perfusion </li></ul><ul><ul><li>The monitor can track the power required to heat the sensor, showing change in perfusion. </li></ul></ul>
  16. 16. Procedure for Use <ul><li>Allow to warm-up while placed at bedside. </li></ul><ul><li>Check membranes (free of bubbles and scratches) </li></ul><ul><li>Evaluation site should be free of bony prominences. </li></ul><ul><li>Prepare sensor with adhesive ring and electrolyte gel. </li></ul><ul><li>Set appropriate temperature: </li></ul><ul><ul><li>Adults: 44 o C to 45 o C </li></ul></ul><ul><ul><li>Neonates: 43 o C to 43.5 o C </li></ul></ul><ul><li>Clean skin, remove excess hair if necessary. </li></ul><ul><li>Attach probe and allow calibration time (10 to 20 minutes) </li></ul><ul><li>Schedule site change time </li></ul><ul><ul><li>3 to 4 hours </li></ul></ul><ul><li>Set high and low alarms. </li></ul>
  17. 17. Complications <ul><li>False positives and false negatives. </li></ul><ul><li>Improper calibration. </li></ul><ul><li>Lower operating temperature. </li></ul><ul><li>Tissue injury at measuring site: </li></ul><ul><ul><li>Erythema (redness of skin) </li></ul></ul><ul><ul><li>Blisters </li></ul></ul><ul><ul><li>Thermal injury (burns) </li></ul></ul><ul><ul><li>Epidermal stripping (skin tear) </li></ul></ul>
  18. 18. Device Limitations <ul><li>PtcO 2 does not reflect oxygen delivery or oxygen content. </li></ul><ul><li>PtcO 2 does not reflect cardiac output during hypoxemia. </li></ul><ul><li>PtcCO 2 does not reflect PaCO 2 during shock. </li></ul><ul><li>Technical Limitations </li></ul><ul><ul><li>Labor intensive </li></ul></ul><ul><ul><li>Prolonged stabilization time </li></ul></ul><ul><li>Clinical: Discrepancy between ABG and TCM values </li></ul><ul><ul><li>Presence of hyperoxemia hyperoxemia(PaO 2 >100mmHg) (However, neonate must be kept at PaO 2 between 60-90mmHg.) </li></ul></ul><ul><ul><li>Presence of shock or acidosis </li></ul></ul><ul><ul><li>Improper electrode placement </li></ul></ul><ul><ul><li>Vasoactive drugs </li></ul></ul><ul><ul><li>Nature of patient’s skin (skinfold, thickness, edema) </li></ul></ul><ul><ul><li>Room temperature too low (shivering and vasoconstriction cause increased oxygen consumption and decreased blood flow to epidermis) </li></ul></ul><ul><ul><li>Excess solution leaking out (allows ambient air to flow to sensor) </li></ul></ul>
  19. 19. Minimizing Discrepancy <ul><li>ABG values should be compared to TCM values taken at the time of arterial sampling in order to validate the TCM values. </li></ul><ul><li>Validation should be performed initially and periodically to assure consistency. (If patient’s clinical state permits) </li></ul><ul><li>The following should be periodically verified by the practitioner: </li></ul><ul><ul><li>High and low limits alarms are set. </li></ul></ul><ul><ul><li>Proper electrode temperature is set </li></ul></ul><ul><ul><li>Electrode placement is appropriate and systematic electrode-site changes occur. </li></ul></ul>
  20. 20. Conclusion <ul><li>PtcO 2 and PtcCO 2 have become routine clinical procedures in neonatal intensive </li></ul><ul><li>care, firstly because of almost ideal conditions obtained in this age group, when </li></ul><ul><li>peripheral skin perfusion can be maximally increased by hyperthermia. This permits </li></ul><ul><li>noninvasive monitoring or arterial blood gases in the newborn. The second reason is </li></ul><ul><li>that in this specialty, unlike in adults, hyperoxemia has fatal consequences – such as </li></ul><ul><li>ROP - and must be prevented at all costs. Since this can be monitored only via the </li></ul><ul><li>partial pressure in the blood, pulse oximetry – whose simplicity has led to wide </li></ul><ul><li>acceptance – cannot replace transcutaneous blood gas measurements in this phase of </li></ul><ul><li>life. </li></ul>