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BLOOD-GAS SAMPLING ERRORS Objectives Venous Blood Gas

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BLOOD-GAS SAMPLING ERRORS Objectives Venous Blood Gas Objectives • List the five types of arterial blood sampling errors and describe the effect of BLOOD-GAS SAMPLING the error on the results of blood-gas values. ERRORS • State how pulse oximetry may be helpful in distinguishing arterial from venous Module C blood samples. • State the effect of increased or decreased body temperature on blood gas results. • Malley – Chapters 3 & 5 TOPICS TO BE COVERED – Venous Blood Gas Values MODULE C • Gas Laws. •PO2 35 - 45 mm Hg • Air in the Blood Sample. •PCO2 41- 51 mm Hg • Inadvertent venous sampling/venous • pH 7.32 – 7.42 admixture. •SO2 70 – 75% • Dilution due to anticoagulants. •CO2 12 - 15 vol% • Effects of metabolism. • Temperature effects. Dalton’s Law More on Dalton • John Dalton (1776-1844): “Total pressure of a mixture of gases is equal to • If we know the fractional concentration, we the sum of the partial can calculate the partial pressure. pressures of each constituent gas. (1802) •FGAS * (PBARO –PH2O) = PGAS in blood • If a gas comprises 25% of • .21 * (760-47) = 149.73 mmHg the total, it will exert 25% of • Water vapor pressure can be obtained for any the total pressure. temperature from tables (3-2 in Malley). •PBARO = PN2 + PO2 + PCO2 + ... • Effect of changes in barometric pressure. 1 Gas Laws – Boyle’s Law Gas Laws – Charles’s Law • Robert Boyle (1627- • Jacques Charles (1746 1691) – 1823) • Spring of Air • Balloon and Benjamin Franklin • Volume Inversely • Volume Directly related related to Pressure if to Temperature if Temperature is held Pressure is held constant. constant. •P1V1 = P2V2 •V1/T1 = V2/T2 • http://www.grc.nasa.gov • http://www.grc.nasa.gov /WWW/K- /WWW/K- 12/airplane/aglussac.ht 12/airplane/aboyle.html ml Gas Laws – Gay-Lussac’s Henry’s Law • Joseph Louis Gay- • William Henry (1774-1836) Lussac (1778-1850) • Predicts how much gas will dissolve in a liquid. • Pressure Directly • If the temperature of the liquid remains constant, the related to volume of a gas that dissolves in a liquid equals its Temperature if volume solubility coefficient times its partial pressure (that is the is held constant gaseous partial pressure above the liquid). •V = x Pgas •P1/T1 = P2/T2 • Solubility coefficient for oxygen is 0.023 ml/ml • http://www.grc.nasa.g • Solubility coefficient for carbon dioxide is 0.510 ml/ml ov/WWW/K- • http://hyperphysics.phy- 12/airplane/Animation/ astr.gsu.edu/hbase/kinetic/henry.html frglab2.html Graham’s Law EXPRESSIONS • Thomas Graham (1805- 1869) • BTPS – Body Temperature (37°C), Body • The rate of diffusion of a Pressure (Ambient, i.e. Barometric), Saturated gas is inversely with water vapor (PH2O = 47 mmHg) proportional to the square • FOUND IN THE BLOOD root of its gram molecular weight (GMW). • ATPS – Ambient Temperature (~22°C), Ambient 1 Pressure (Barometric), Saturated with water • Dgas vapor as determined by the Relative Humidity (PH2O = 19.6 mmHg * RH) GMW • STPD - Standard Temperature (0°C), Ambient • http://www.chem.tamu.ed Pressure (Barometric), Dry Gas (0% Relative u/class/majors/tutorialnote Humidity) - PH O = 0 mmHg files/graham.htm 2 2 5 Common Errors Air in the Blood Sample • Effect on PaO • Air in the Blood Sample. 2 • Primary parameter affected. • Venous sampling or admixture. • Effect on PaCO2 (and pH) • Excessive or improper anticoagulant use. • Relationship to time. • Rate of Metabolism. • Within 2 minutes without mixing. • Temperature disparities between machine • Very significant with the and patient. presence of frothy bubbles. • Air contamination during measurement. Venous Sampling or Admixture • Most common in femoral punctures or hypotensive patients because of difficulty in assessment. •Flash • Pulsation • Venous admixture – contamination of an arterial sample with venous blood. • Can be due to overshoot. • Femoral vein anomaly that is punctured. 1 th • /10 part leads to a 25% error. • 0.5 ml of venous blood with a PO2 of 31 mixed with 4.5 ml of arterial blood with a PO2 of 86, yields a mixed sample with a PO2 of 56. • Suspect whenever clinical status results Anticoagulant Effects Effect of Metabolism • Anticoagulants are necessary evil. • Metabolism continues after sampling. • Oxygen is consumed and Carbon Dioxide produced • Depends on type, concentration, & volume • Depends on temperature of sample ( temperature, metabolism) of anticoagulant. • pH: Decrease 0.05/hr •PaCO : Increase 5 mmHg/hr • Lithium heparin is preferred anticoagulant. 2 •PaO2: Decrease by 20 mmHg/hr (150 mmHg/hr if initial PaO2 • Sodium heparin can increase Na+. over 250 mmHg) • At room temperature (20 -24 °C) 50% reduction • Excess volume can lead to reduction in PaCO2 • Icing sample to 4°C results in a 10% reduction and other electrolytes (Ca+2). • Solution is to analyze quickly! • Stronger concentrations than 1000 /ml can • “Leukocyte Larceny” – The rapid decrease in PaO that was observed in blood samples with affect the pH. 2 high leukocyte counts (leukocytosis). • More prominent with samples from neonate. 3 AARC CPG on ABG Sampling Icing the Sample (ABS) • 7.1.7 Specimens held at room temperature must be • All samples should be analyzed immediately. analyzed within 10-15 minutes of drawing; iced samples • If a delay of greater than 30 minutes is anticipated, a should be analyzed within 1 hour. The PaO2 of samples drawn from subjects with elevated white cell counts may glass syringe should be used and the sample should be decrease very rapidly. Immediate chilling is placed in an Ice/Water slush solution capable of necessary.(12-13) Some dual-purpose electrolyte/blood maintaining a temperature of 1-5 °C. gas analyzers stipulate immediate analysis without • Barrel should be immersed within slush solution. chilling because of possible elevations in potassium from • If sample is in a plastic syringe and can be analyzed chilling;(14) however, the accuracy of the blood gas results should not be affected by the chilling. within 10-15 minutes, icing the sample is not necessary. • Plastic syringes have been shown to allow for an increase in PaO if analysis is delayed more than 30 minutes. • 10.1.3 Container of ice and water (to immerse syringe 2 barrel if specimen will not be analyzed within l5 min) • AARC CPG states that iced samples should be analyzed within 1 hour CPG), however it isn’t stated (but implied) that those samples are in a glass syringe. • What isn’t stated for highlighted portion is that these samples should be collected in a glass syringe! Effect of Temperature Assessment of Internal • Gay-Lussac’s law – Pressure and Temperature Consistency react directly with each other. aO • Does the pH correlate with the PaCO2 and • Effect on P 2. - HCO3 ? • Effect on PaCO2. • pH: 7.32 PaCO : 23 mmHg HCO -: 31 mEq/L • Effect on pH. 2 3 • Does the degree of pH change match the PaCO2 • Correction takes into account physical - relationship between pressure & temperature. or HCO3 change? • pH: 7.60 PaCO : 30 mmHg HCO -: 23 mEq/L • Does not take into account the change in 2 3 metabolic activity at different temperature. • Use one of the following four methods to help assess internal consistency: • What is the “normal” PaO2 at 39°C? • Instrumentation temperature must be maintained • Indirect Metabolic Assessment at 37°C + 0.1°C. • Rule of Eights • Modified Henderson Equation •Acid-Base Map Metabolic Assessment Rule of Eights • Premise: pH change is due to a respiratory • Used to predict plasma - bicarbonate when the pH pH Factor (PaCO2) or metabolic (HCO3 ) component. and PaCO2 are known. • Acute decrease in PaCO2 by 10 mmHg yields a 0.10 •Factor x PaCO2 = increase in pH. Predicted Bicarbonate 7.60 8/8 • Acute increase in PaCO2 by 10 mmHg yields a 0.06 • Compare actual with decrease in pH. predicted (difference 7.50 6/8 should be less than 4 • If the PaCO2 rises from a normal of 40 to 50 mmHg, the pH should fall by 0.06. mEq/L) 7.40 5/8 • The “expected pH” should then be compared with the • pH: 7.50 •PaCO : 25 “measured pH”. If the variation is greater than 0.03 2 PaCO is 15 mmHg, 7.30 4/8 (error factor), a metabolic alteration is present. 2 therefore pH should be - 7.55. It isn’t, so some • pH: 7.60 PaCO2: 30 mmHg HCO3 : 23 mEq/L 7.20 2.5/8 loss of bicarbonate is •30 mm Hg PaCO yields a pH of 7.50. Some additional 2 present. metabolic alkalosis must be present. The HCO - is 3 • 25 * 6/8 = 18.75 mEq/L 7.10 2/8 actually 28.5 mEq/L. 4 Modified Henderson Equation ACID-BASE MAP •[H+] in nanequivalents pH [H+] 7.80 16 per liter to PaCO2 & 7.70 20 - 7.60 25 [HCO3 ] 7.55 28 + •[H] = 24 x 7.50 32 - (PaCO2/[HCO3 ]) 7.45 35 • Need to convert pH to 7.40 40 7.35LINEAR 45 + [H ] 7.30 50 • Linear between 7.20 & 7.25 56 7.50. 7.20 63 7.15 71 0.01 pH = 1 nEq/L 7.10 79 • pH 7.40 = 40 nEq/L 7.00 100 6.90 126 6.80 159 External Congruity Metabolic Acid-Base Indices • Ensure that all laboratory tests and observations • Metabolic indices may lead to are in harmony with blood gas results. - inaccurate conclusions in the • [HCO3 ] = Total CO2 from Electrolytes - •Total CO2 = (PaCO2 x 0.03) + [HCO3 ] presence of hypo- or hypercapnia. • Patient-Laboratory Congruity • EXAMPLE • Appearance of patient & results • BE: -4 mEq/L •FIO -PaO Incongruity • pH: 7.16 2 2 • Std [HCO -]: 20 mEq/L • Dalton’s Law Estimate: PaO < 130 mmHg on RA 3 2 •PaCO2: 80 mmHg •FIO2 * 5 on higher FIO2s • [BE]ecf: 0 mEq/L •PaO2: 80 mmHg aO pO - •S 2-S 2 Incongruity •T Std [HCO ]: 24 • [HCO -] : 28 mEq/L 40 3 • Compare invasive to non-invasive 3 mEq/L Plasma Bicarbonate and Internal Congruity Buffering for Respiratory Acidosis - • Indirect Metabolic Assessment • Hydrolysis Effect: Some rise in HCO3 will • Predicts normal HCO3- of 24 mEq/L, not 28 occur because of excess PaCO2 in the • Rule of Eights system.
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