Transcutaneous Blood Gas MonitoringTranscutaneous Blood Gas MonitoringHutch, A., Acta Anaesthesiologica Scandinavica. Supplementum. 107:87-90,1995Hutch, A., Acta Anaesthesiologica Scandinavica. Supplementum. 107:87-90,1995

Journal Arcticle. ReviewJournal Arcticle. Review

Transcutaneous Oxygen Measurements: Implications for NursingTranscutaneous Oxygen Measurements: Implications for NursingRich, Kathleen, Journal of Vascular Nursing. 19(2):55-9, 2001 June.Rich, Kathleen, Journal of Vascular Nursing. 19(2):55-9, 2001 June.

Journal Article. ReviewJournal Article. Review

AARC Clinical Practice Guideline. Transcutaneous Blood GasAARC Clinical Practice Guideline. Transcutaneous Blood GasMonitoring for Neonatal & Pediatric Patients.Monitoring for Neonatal & Pediatric Patients.

Anonymous, Respiratory Care. 39(12):1176-9, 1994 December.Anonymous, Respiratory Care. 39(12):1176-9, 1994 December.Journal Article. Practice GuidelineJournal Article. Practice Guideline

Saif K. AhmediClaudia Amaya

Resc 3621 Physiologic MonitoringSpring 2006

IntroductionIntroductionTranscutaneous blood gas measurement is a noninvasive, diagnostic technique that records the partial pressure of oxygen(Ptc02) and carbon dioxide (PtcCO2) 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 bloodgas 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 timemake 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.

PtcO2 and PaO2 have a 1:1 correlation in neonates due to theirskin composition. In older children and adults, the PtcO2/PaO2 ratiodecreases as skin becomes more impermeable to O2. For this reason, RTsnormally do not use TCMs to monitor adults and children.

HistoryHistory

The possibility of skin surface measurement for estimating central PO2 was suggested by Bramberger and Goodfriend in 1951. They showed that when a finger was immersed in an electrolyte solutionmaintianed at 45oC, the PO2 of the solutionequilibrated with that of arterial blood. In 1972, Huchand associates developed a miniaturized, heatedelectrode that measured the skin surface PO2. This isthe basis behind the transcutaneous oxygen monitorsin use today.

The ElelctrodesThe ElelctrodesThe COThe CO22 Severinghaus electrode(1) Consists of a pH Severinghaus electrode(1) Consists of a pHsensitive glass electrode in contact with a thinsensitive glass electrode in contact with a thinfilm of sodium bicarbonate solution separatedfilm of sodium bicarbonate solution separatedfrom the test solution by a gas permeable from the test solution by a gas permeable membrane. The pH of the bicarbonate solutionmembrane. The pH of the bicarbonate solutionis proportional to the log carbon dioxide tension.is proportional to the log carbon dioxide tension.Carbon dioxide diffuses into the bicarbonate Carbon dioxide diffuses into the bicarbonate solution and the hydrogen ions produced resultssolution and the hydrogen ions produced resultsin a potential difference between the testin a potential difference between the testsolution in the electrode. This is measured by thesolution in the electrode. This is measured by thevoltmeter and reported as partial pressure of carbonvoltmeter and reported as partial pressure of carbondioxide.dioxide.

The OThe O22 Clark electrode. Oxygen diffuses from Clark electrode. Oxygen diffuses fromthe skin into the (4) electrolyte chamber. the skin into the (4) electrolyte chamber. oxygen reacts with water to produce hydroxyloxygen reacts with water to produce hydroxylions. The electrons consumed results in aions. The electrons consumed results in acurrent difference between the platinum and current difference between the platinum and silver anode(7). The current difference issilver anode(7). The current difference ismeasured by a current meter and reported as measured by a current meter and reported as partial pressure of oxygen.partial pressure of oxygen.

Heating element (9) and thermistor (a resistor thatHeating element (9) and thermistor (a resistor thatmeasures temperature changes)(8) heat the skin to measures temperature changes)(8) heat the skin to induce active hyperemia and to change the diffusioninduce active hyperemia and to change the diffusionconditions in the skin. conditions in the skin.

Skin StructureSkin StructureThe skin structure comprises three distinctThe skin structure comprises three distinctLayers:Layers:

The capillary network.The capillary network. The stratum basale, stratum spinosum and stratum granulosum where oxygen is The stratum basale, stratum spinosum and stratum granulosum where oxygen is

consumed due to cellular respiration.consumed due to cellular respiration. The stratum corneum, the layer of dead cells where oxygen consumption is 0.The stratum corneum, the layer of dead cells where oxygen consumption is 0.

Physiology Behind UsePhysiology Behind Use

Three interdependent factors determine PtcOThree interdependent factors determine PtcO22 and PtcCO and PtcCO2 2 ::

Skin PerfusionSkin Perfusion

Skin RespirationSkin Respiration

Skin DiffusionSkin Diffusion

Normally, the measured PtcONormally, the measured PtcO22 at the epidermal layer is at the epidermal layer is 0mmHg. When the dermal temperature is raised 0mmHg. When the dermal temperature is raised

between 42between 42ooC to 45C to 45ooC there are several changes in the C there are several changes in the three factors.three factors.

Results of Epidermal WarmingResults of Epidermal Warming

Results of Epidermal Warming (continued)Results of Epidermal Warming (continued)

OXYGENOXYGEN

Heat causes a rise in the Heat causes a rise in the capillary partial pressure of capillary partial pressure of oxygen.oxygen.

Oxygen consumption and Oxygen consumption and the diffusion barrier the diffusion barrier together lower the together lower the capillary partial pressure of capillary partial pressure of oxygenoxygen

Net result – Net result – transcutaneous partial transcutaneous partial pressure of oxygen pressure of oxygen approximates arterial approximates arterial partial pressure of oxygenpartial pressure of oxygen

CARBON DIOXIDECARBON DIOXIDE

Heat causes a rise in the Heat causes a rise in the capillary partial pressure of capillary partial pressure of carbon dioxide.carbon dioxide.

Heat causes a rise in cellular Heat causes a rise in cellular metabolism which in turn metabolism which in turn increases carbon dioxide increases carbon dioxide production, raising the production, raising the partial pressure of carbon partial pressure of carbon dioxide.dioxide.

Net result – transcutaneous Net result – transcutaneous partial pressure of carbon partial pressure of carbon dioxide is greater than dioxide is greater than arterial partial pressure of arterial partial pressure of carbon dioxide.carbon dioxide.

Oxygen Pressure Profiles and Temperature Changes in SkinOxygen Pressure Profiles and Temperature Changes in Skin

Under normal Under normal conditions of skin conditions of skin temperature, a temperature, a major decrement in major decrement in POPO22 occurs along occurs along the capillary the capillary network. (A)network. (A)

Nicotinic acid is Nicotinic acid is used as a used as a vasodilator to vasodilator to increase perfusion increase perfusion without active without active induction of induction of hyperemia. (B)hyperemia. (B)

Ideal curve (C)Ideal curve (C) Electrode heated Electrode heated

skin curve (D), is skin curve (D), is deviated upward deviated upward from the ideal from the ideal curve in layer one curve in layer one due to several due to several reasons:reasons:

Oxygen Pressure Profiles and Temperature Changes in Skin Oxygen Pressure Profiles and Temperature Changes in Skin (continued)(continued)

– skin perfusion skin perfusion increases and the increases and the POPO22 gradient from gradient from capillary bed to capillary bed to skin surface is skin surface is negligible. negligible.

– Oxyhemoglobin Oxyhemoglobin curve shift to right curve shift to right and more oxygen and more oxygen is unloaded.is unloaded.

– Oxygen Oxygen permeability in the permeability in the skin is increased.skin is increased.

– In layer 2 the In layer 2 the curve falls back curve falls back down due to tissue down due to tissue respiration in the respiration in the epidermal layer epidermal layer and because the and because the Clark electrode Clark electrode consumes oxygen consumes oxygen in the reaction.in the reaction.

Due to variations in skin characteristics, ratiosbetween ABG and TCM values differ depending on age, sex, and skin site:

Premature infants: 1.41 : 1.00Neonates: 1.00 : 1.00Children: 0.68 : 1.00Adults: 0.79 : 1.00Older 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…

Transcutaneous-Arterial Ratios Change With Transcutaneous-Arterial Ratios Change With FlowFlow

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.

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 PtcCO2 as opposed to PaCO2 is also an indicator of poor tissue perfusion.

Transcutaneous-Arterail Ratios Change With Flow (continued)Transcutaneous-Arterail Ratios Change With Flow (continued)

Indications Indications

AdultsAdults Wound evaluation (PtcOWound evaluation (PtcO22

>40mmHg)>40mmHg) Hyperbaric therapy (PtcO2 Hyperbaric therapy (PtcO2

> 50mmHg at 2.5atm is > 50mmHg at 2.5atm is indicative of successful indicative of successful therapy)therapy)

Plastic SurgeryPlastic Surgery Determination of Determination of

amputation levelamputation level Adjunct in peripheral Adjunct in peripheral

artery disease and artery disease and gangrene diagnosis (PtcOgangrene diagnosis (PtcO22 = 0-30mmHg)= 0-30mmHg)

Evaluation of injuries and Evaluation of injuries and vascular surgeries.vascular surgeries.

Pediatric & NeonatalPediatric & Neonatal Adjunct to ABGAdjunct to ABG Continuous and Continuous and

prolonged monitoring prolonged monitoring during mechanical during mechanical ventilation, CPAP, and ventilation, CPAP, and supplemental oxygen supplemental oxygen administration.administration.

Assessment of functional Assessment of functional shunts, PPHN, persistent shunts, PPHN, persistent fetal circulation(PDA), or fetal circulation(PDA), or congenital heart congenital heart disease.disease.

CONTRAINDICATION – CONTRAINDICATION – patients with poor skin patients with poor skin integrity and/or adhesive integrity and/or adhesive allergy.allergy.

Clinical Uses in Neonatal CareClinical Uses in Neonatal Care

Continuos monitoringContinuos monitoring– Monitor oxygenation and prevent hyperoxemia.Monitor oxygenation and prevent hyperoxemia.– Monitor COMonitor CO22

High PtcCOHigh PtcCO22 may indicate inadequate ventilation. may indicate inadequate ventilation. While trending, PtcCOWhile trending, PtcCO2 2 < PaCO< PaCO22 may indicate poor perfusion. may indicate poor perfusion.

TrendingTrending– Factors that may cause TCOM to read lower:Factors that may cause TCOM to read lower:

ShockShock Severe AcidosisSevere Acidosis HypothermiaHypothermia Severe cyanosisSevere cyanosis Heart diseaseHeart disease Severe anemiaSevere anemia Skin edemaSkin edema PaOPaO22 greater than 100mHg greater than 100mHg Vasodilator drugs delivered (Tolazoline)Vasodilator drugs delivered (Tolazoline)

Detection of shunting bloodDetection of shunting blood– Using two electrodes to detect shunts through ductus Using two electrodes to detect shunts through ductus

arteriosus.arteriosus. Placement – right arm, left legPlacement – right arm, left leg

Indicator of Skin perfusionIndicator of Skin perfusion– The monitor can track the power required to heat the sensor, The monitor can track the power required to heat the sensor,

showing change in perfusion.showing change in perfusion.

Procedure for UseProcedure for Use

Allow to warm-up while placed at bedside. Check membranes (free of bubbles and scratches) Evaluation site should be free of bony prominences. Prepare sensor with adhesive ring and electrolyte gel. Set appropriate temperature:

– Adults: 44oC to 45oC– Neonates: 43oC to 43.5oC

Clean skin, remove excess hair if necessary. Attach probe and allow calibration time (10 to 20

minutes) Schedule site change time

– 3 to 4 hours Set high and low alarms.

ComplicationsComplications

False positives and false negatives.False positives and false negatives. Improper calibration.Improper calibration. Lower operating temperature.Lower operating temperature. Tissue injury at measuring site:Tissue injury at measuring site:

– Erythema (redness of skin)Erythema (redness of skin)– BlistersBlisters– Thermal injury (burns)Thermal injury (burns)– Epidermal stripping (skin tear)Epidermal stripping (skin tear)

Device LimitationsDevice Limitations PtcO2 does not reflect oxygen delivery or oxygen content. PtcO2 does not reflect cardiac output during hypoxemia. PtcCO2 does not reflect PaCO2 during shock. Technical Limitations

– Labor intensive– Prolonged stabilization time

Clinical: Discrepancy between ABG and TCM values– Presence of hyperoxemia hyperoxemia(PaO2>100mmHg)

(However, neonate must be kept at PaO2 between 60-90mmHg.)

– Presence of shock or acidosis– Improper electrode placement– Vasoactive drugs– Nature of patient’s skin (skinfold, thickness, edema)– Room temperature too low (shivering and vasoconstriction cause

increased oxygen consumption and decreased blood flow to epidermis)

– Excess solution leaking out (allows ambient air to flow to sensor)

Minimizing DiscrepancyMinimizing Discrepancy ABG values should be compared to TCM values taken at the

time of arterial sampling in order to validate the TCM values. Validation should be performed initially and periodically to

assure consistency. (If patient’s clinical state permits) The following should be periodically verified by the practitioner:

– High and low limits alarms are set.– Proper electrode temperature is set– Electrode placement is appropriate and systematic electrode-site

changes occur.

ConclusionConclusionPtcO2 and PtcCO2 have become routine clinical procedures in neonatal intensivecare, firstly because of almost ideal conditions obtained in this age group, whenperipheral skin perfusion can be maximally increased by hyperthermia. This permitsnoninvasive monitoring or arterial blood gases in the newborn. The second reason isthat in this specialty, unlike in adults, hyperoxemia has fatal consequences – such asROP - and must be prevented at all costs. Since this can be monitored only via thepartial pressure in the blood, pulse oximetry – whose simplicity has led to wide acceptance – cannot replace transcutaneous blood gas measurements in this phase

of life.