Clinical Chemistry I - Unit III: Blood Gases and Buffer Systems PDF

Clinical Chemistry I Unit III: Blood Gases and Buffer Systems Dr. Murtada Taha Prince Sultan Military Medical College for Health Sciences Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Blood Gases, pH, and Buffer Systems By Sharon S. Ehrmeyer...

Clinical Chemistry I Unit III: Blood Gases and Buffer Systems Dr. Murtada Taha Prince Sultan Military Medical College for Health Sciences Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Blood Gases, pH, and Buffer Systems By Sharon S. Ehrmeyer, John J. Ancy Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Unit Objectives  Upon completion of this Unit, the clinical laboratorian should be able to do the following:  Outline the interrelationship of the buffering mechanisms of bicarbonate, carbonic acid, and hemoglobin. - -  Explain the clinical significance of the pH and blood gas parameters.  Determine whether data are normal or represent metabolic or respiratory acidosis or - metabolic or respiratory alkalosis using the Henderson-Hasselbalch equation and blood gas data.  Identify some common causes of nonrespiratory acidosis and alkalosis, respiratory acidosis and alkalosis, and mixed abnormalities. S - acidosis & & alkalosis E  State how the body attempts to compensate (kidney and lungs) for the various conditions.  Describe the significance of the hemoglobin–oxygen dissociation curve and the impact of pH, 2,3-diphosphoglycerate (2,3-DPG), temperature, pH, and pCO2 on its shape and release of O2 to the tissues.  Describe the principles involved in the measurement of pH, pCO2 , pO2 , and the various hemoglobin species. -  Discuss problems and precautions in collecting and handling samples for pH and blood gas analysis.  Describe instrumental approaches to measuring various hemoglobin species and pH and blood gas parameters.  Describe approaches to quality assurance, including quality control and proficiency testing. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Introduction  An important aspect of clinical biochemistry is information on a patient’s 1 1 acid-base balance and blood gas homeostasis. //  Data are used to assess patients in life threatening situations. [Imbalances in the oxygen, carbon dioxide, and pH levels of your blood can indicate the presence of certain medical conditions. These may include: disease cause & change in that variant  kidney failure. ·  Heart failure.  Uncontrolled diabetes.  Hemorrhage.  Metabolic disease.  Head or neck injuries that affect breathing.  Chemical poisoning.  Drug overdose.  Shock. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Definitions: Acid, Base, Buffer  Basic concepts are important for understanding acid- base balance. meanProduce ↑ [  Acid: a substance that can yield a hydrogen ion (H+) or hydronium ion when dissolved in water.  Base: a substance that can yield hydroxyl ions (OH-). & & [ Dissociation constant (ionization constant K value): describes relative strengths of acids and bases. EpK: negative log of ionization constant and pH in  which protonated and unprotonated forms are present in equal concentrations.  pH=- log (H+) Less than 7 PH mean high Hio more =-- = Low Copyright © 2014 Wolters Kluwer Health Hion | Lippincott Williams & Wilkins  Definitions: Acid, Base, Buffer -g) , 4 - Buffer: combination of weak acid or weak base and its salt; a system that resists & changes in pH. In plasma, the bicarbonate-carbonic acid system, having a pKa of 6.1, is one of the principal buffers. H2CO3 HCO3- + H+ The reference value for blood plasma pH is 7.40. & - Weisberg example to demonstrate the --- effectiveness of the pH (water and blood). Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Acid–Base Balance: Maintenance of H+ -1 8  Normal concentration of H+ in extracellular body fluid ranges from 36 to 44 nmol/L (pH, 7.34-7.44), but body produces much greater quantities of H+.  Intracellular pH is approximately 7.1 in virtually every tissue.  Body resists the change in blood pH by different mechanism:  Blood buffers, via lungs and kidneys, body controls and excretes H+ to maintain pH homeostasis.  Any H+ value outside this range can lead to alteration in chemical - reactions and metabolism which lead to alterations in consciousness, neuromusclar irritability, tetany, coma, and death.  The reference value for arterial blood pH is 7.40 (40 nmol/L)- & pH = - log [H+]. - -  Acidosis/acidemia: a pH level below reference range (7.44).  (-osis is the cause of the -emia). Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Acid–Base Balance: Maintenance of H+  Extreme metabolic alkalemia has been associated with a high risk of mortality of up to 45% with a pH of 7.55 and 80% when pH is greater than 7.65. &  Appropriate intervention and correction is warranted when arterial blood pH exceeds 7.55 (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2856150/?). &  According to the American Association for Clinical Chemistry (AACC), acidosis is characterized by a pH of 7.35 or lower. Acidosis can lead to numerous health issues, and it can even be life-threatening. -  Severe metabolic or mixed acidemia defined by a plasma pH level lower than & 7.20 within the first 24 hours of ICU admission.  Severe metabolic or mixed acidemia was associated with an ICU mortality rate of 57%. &  Rapidity of acidemia correction was associated with mortality. (https://ccforum.biomedcentral.com/articles/10.1186/cc10487).  The arterial pH is controlled by systems that regulate the production and retention of acids and bases. -  These include buffers, the respiratory center and lungs, and the kidney. & & Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Balance Buffer Systems: Regulation of H+  Buffer systems are body’s first line of defense against extreme changes in H+ concentration.  All buffers consist of a weak acid and its salt or conjugate base.  When acid or bases added to the buffer system the following will happen: Carbonic acid & -  H+ + HCO3- H2CO3 + OH- HCO3- + H2O  Bicarbonate-carbonic acid system has low buffering capacity, but is still important buffer for 4 reasons: & [  1. H2CO3 dissociates into CO2 and H2O, allowing CO2 to be eliminated by lungs and H+ as water.  2. Changes in CO2 modify ventilation (respiration) rate. E  3. HCO3- concentration can be altered by kidneys.  4. Counters the effects of fixed nonvolatile acids.  Other buffers: phosphate system and plasma protein. - -  Lungs and kidneys play important roles in regulating blood pH. & & Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance: Lungs and Kidney  & Carbon dioxide, easily diffuses out of the tissue into the plasma and red cells.  In tissues CO2 is produced by the cellular metabolic activity. &  In the plasma small amount of CO2 is physically dissolved or & combined with proteins to form carbamino compounds.  Most of the CO2 combines with water to form H2CO3 in & presence of the enzyme carbonic anhydrase, which dissociate to HCO3- and H+. & HCO3- diffuses out of the RBC into the plasma.   To maintain electroneutrality, chloride diffuses into the cell & (chloride shift). 1111  Plasma proteins and plasma buffers combine with the freed H+ to maintain stable pH. // Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance: Lungs and Kidney X Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance: Lungs and Kidneys Carbonic Anhydrase  CO2 + H2O & 5 H2CO3  H2CO3 & HCO3- + H+  H+ ↑ is buffered by hemoglobin. &  Increase in H+ ions (decrease in pH) causes the Eis deoxygenation of hemoglobin. -  This ensures the supply of O2 to tissue. & HbO2 + H+ - HHb + O2 Hemog'obi & HCO3 - is carried to lung through Blood. > - , 8 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance: Lungs  Ventilation affects pH of blood. unlike kidney &  Respiratory system provides a rapid mechanism & for the maintenance of acid base balance. & -  In the lungs the process is- & reversed.  O2 is inspired and diffuses from alveoli & into blood and is bound to HHB. & by carbonic  CO2 diffuses into alveoli from blood in the form of HCO3-. Anhydrase & -Hi -  H+ from reduced HHB combines with HCO3- to form H2CO3 , which & & is dissociated into CO2 and H2O that is eliminated via ventilation.  Result in minimal change in H+ concentration between venous and arterial circulations. ? what the organ respond rapid  The lungs by responding within seconds, together with the buffer systems, provide the first line of defense to changes in - acid base status. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance: Lungs &  At lungs the high partial pressure of oxygen causes & the oxygenation of hemoglobin with the release of H+ - ions.  HHb + O2 & HbO2 + H+  H+ combines with HCO3 - to form H2CO3 which then split into CO2 and H2O in presence of carbonic anhydrase.  [Carbonic anhydrase 3  H2CO3 H2O + CO2  CO2 is removed through the exhaled air. - & Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Regulation of Acid–Base Balance 1 Interrelationship of bicarbonate and hemoglobin buffering systems. 24/ I The net effect of the interaction of - these two buffering systems is a minimal change in H+ concentration between Venous the venous and arterial circulation.  I Ventilation affects the pH of the blood in both states. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Renal Mechanism for pH regulation & Kidneys play a significant role in the maintenance of blood pH.  & & Regulate acid–base balance by excreting acids or bases.  return back - & HCO3- is reclaimed from glomerular filtrate (proximal tubules) to prevent  excessive acid gain in blood from loss of HCO3- in urine. -  Under normal conditions, the body produces a net excess (50–100 mmol/L) of acid (H) each day that must be excreted by the kidney. - - - & Kidneys regulate the blood pH by:  D Reabsorption of bicarbonate. 3 Excretion of H+. s  Along with reabsorption of bicarbonate.  As phosphate.  As ammonium ions. 29 - & In alkalotic conditions, the kidney excretes HCO3- to compensate for the elevated blood pH. & Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Reabsorption of Bicarbonate The process is not a direct transport of HCO3- across the tubule membrane & into the blood. -  Sodium (Na) in the glomerular filtrate is exchanged for H+ in the tubular cell. - Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins. Excretion of H+ ions Under normal conditions, the body produces a net excess (50–100 mmol/L) of acid (H) each day that must be excreted by the kidney. Sodium (Na) in the glomerular filtrate is exchanged for H+ in the tubular cell. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Excretion of H+ ions as phosphate  H+ ions are buffered off by phosphate buffer system in the tubular lumen.  The amount of HPO42- available for combining with H+ is fairly constant. - - Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Excretion of Ammonium  Occurs at the distal convoluted tubules. -  Daily excretion of H + in urine largely depends on the amount of NH4+ formed. - Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Assessment of Acid–Base Homeostasis  Bicarbonate Buffering System and the Henderson- Hasselbalch Equation:  Measurement of components of bicarbonate buffering system provides information on other buffers and systems that regulate production,D 3retention, excretion of acids and bases.3 - -  Inferences can be made from these results about the systems produce, - retain, and excrete the acids and bases.  Henderson-Hasselbalch equation expresses acid–base relationships: &1 & 5- > same equation  pH = pK’ + log cA-/cHA - > - PH = -Log (H)  In the plasma and at body temperature the pK’ of the bicarbonate. buffering system is- 6.1. -  The concentration of H2CO3 is proportional to the partial pressure exerted by the dissolved CO2. & & & D & Pressure &  Both pH and the pCO2 are measured in blood gas analysis, and the pK’ is # constant; therefore HCO3- can be calculated. -  Knowing any of the three variables allows for the calculation of the fourth. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Assessment of Acid–Base Homeostasis  Bicarbonate Buffering System and the Henderson- 11 Hasselbalch Equation.  pH= pK’ + log cHCO-3/0.0307 x pCO2 H Cos Zo times greater  - In health, when the kidney and lungs are functioning properly,than a 20:1 ratio of HCO3- to H2CO3 will be maintained (pH of 7.40). H 2 Co --  This is illustrated by substituting normal values (table 17.1) for HCO3- and pCO2 into the preceding equation.  24 mmol/L = 24/1.2= 20/1  0.0307 mmol/L.mm Hg-1 x 40 mm HG &  pH = 6.1 + log 20= 6.1+ 1.3= 7.4  - 96154 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  Result from a variety of pathologic conditions.  Acidemia: & excess acid or H concentration (blood pH < reference range).  Alkalemia: excess base (blood pH > reference 2 range). A - disorder caused by ventilatory dysfunction is termed primary respiratory acidosis or alkalosis. L A disorder resulting from a change in the bicarbonate level is termed a nonrespiratory disorder (renal or metabolic).  Mixed respiratory and nonrespiratory disorders X occasionally arise from more than one pathologic process and represent the most serious conditions as compensation for the primary disorder is failing. - Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Compensatory Mechanisms Because the body’s cellular and metabolic & activities are pH dependent, the body tries to restore acid-base homeostasis whenever an imbalance occurs by physiological mechanisms. These efforts & are called compensatory mechanisms. The body accomplishes this by altering the & factor not primarily affected by the pathologic process. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Compensatory Mechanisms  If there is alteration in pH due to metabolic/renal & cause; compensation is by respiratory system. I If alteration of pH is due to respiratory cause;  compensation by kidneys. I By compensation,HCO -/H CO ratio is brought back to  3 2 3 20:1 so that pH is maintained normal.  Respiratory compensation is immediate but short and & partial. I Renal compensation is slow but almost complete and a  long term. & Permanent/complete correction is possible only by  correction of cause. ↳ PH- (2) Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis 5 and Alkalosis ] is due to a  Nonrespiratory acidosis [(Metabolic acidosis): decrease in bicarbonate content which leads to a fall in blood pH. - causes of decrease PH  Bicarbonate concentration may be decreased due to: D The direct administration of acids producing substances. 8 Excessive formation of organic acids which combine with bicarbonate and deplete alkali reserve. Diabetic keto acidosis, starvation keto acidosis, lactic acidosis. & Reduce excretion of acids (renal tubular acidosis). ⑪ Excessive loss in urine or GI tract. alkali loss HCos 8  Its utilization in buffering H+ ions.  Failure to be regenerated. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Compensation of Metabolic Acidosis:  Acute metabolic acidosis is usually compensated by & respiratory mechanism i.e. hyperventillation. &  Hyperventillation leads to increased elimination of CO2 from the body and hence H2CO3. ↑ Kussmaul’s breathing: Pattern of increased breathing  rate found in diabetic Keto acidosis.  Respiratory compensation is shortlived. &  Secondary compensation occurs when the original organ & (kidney) begins to correct the ratio by retaining the bicarbonate. & ↑ Renal compensation occurs within 3-4 days by increasing  excretion of NH4+ ions by kidneys. B Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  Respiratory acidosis: decrease in alveolar ventilation, causing A decreased elimination of CO2 by lungs leading to increased concentration of H2CO3. &  Causes: 8  Pulmonary disorders:  Broncho pneumonia,chronic obstructive lung diseases- bronchial asthma, emphysema, pulmonary oedema.  Hypoventilation caused by drugs e.g barbiturates, morphine, or alcohol.  Decreased cardiac output e.g CHF.  Compensatory mechanism:  Respiratory acidosis is compensated by renal mechanisms. & &  More bicarbonate is regenerated and retained by the kidneys. &  Excretion of titratable acids and NH4+ is elevated in urine. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  As D with acidosis, alkalosis can result from the nonrespiratory and respiratory causes. D  Nonrespiratory (Metabolic Alkalosis):  The primary abnormality in metabolic alkalosis is & increase in HCO3- concentration.  Causes:  Excessive ↑ intake of alakali for therapeutic purpose. D Severe vomiting resulting in loss of H+ ions. ↑ Nasogastric suction.  Prolonged use of diuretic. ↑ Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  Nonrespiratory Alkalosis Compensation: -  Metabolic alkalosis is compensated by respiratory mechanism-respiratory center depression. -  Hypoventillation retains CO2 leading to an increase in concentration of H2CO3.  Followed slowly by renal mechanism which excretes & more HCO3- and retains H+ ions. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  Respiratory Alkalosis: I  The primary abnormality in respiratory alkalosis is due to prolonged hyper ventilation, causes excessive elimination of CO2. &  Increase exhalation of CO2 by the lungs result in a decreased concentration of H2CO3.  Causes: g  Hypoxia. &  Hysteria. &  & Respiratory centre stimulation by drug e.g salicylates. & Increase in environmental temperature e.g fever. & Patient on ventilator when over ventilated. &  Pulmonary emboli and fibrosis. &  Respiratory center lesions. S Anxiety. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Acid–Base Disorder: Acidosis and Alkalosis  Compensation:  Compensation of respiratory alkalosis is by renal & mechanisms. excretion ↓ Decreased H+ secretion/increase reabsorption.  - &  Decreased reabsorption of HCO3-/increase secretion. excretion Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Oxygen and Carbon Dioxide  2 The role of oxygen in metabolism is crucial to all life. & Y  In mitochondria, electrons are transferred to molecular oxygen for ATP synthesize.3 &Sisi - & I Evaluation of a patients oxygen status is possible using the pO2  measured along with pH and pCO2 in the blood gas analysis. & & & - Se=/  7 conditions necessary for adequate tissue oxygenation: & I  Available atmospheric oxygen. 72i) Z  Adequate ventilation. + Su · & 3  Gas exchange between lungs & arterial blood. ↑ Loading of O onto hemoglobin. 2 S  Adequate hemoglobin. 6  Adequate transport. 7  Release of O2 to tissue.  Any disturbances could lead to poor tissue oxygenation. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Oxygen and Gas Exchange  Oxygen and Carbon Dioxide:  Common factors influencing amount of O2 that moves through alveoli into blood and then to tissue: I Destruction of alveoli. · Pulmonary edema. Airway blockage. Inadequate blood supply. Diffusion of CO2 and O2. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Oxygen and Gas Exchange  Oxygen Transport:  & Most O2 in arterial blood is transported to tissue by hemoglobin. &  Each hemoglobin can combine with up to four molecules of O2. - &  The actual amount of oxygen loaded onto hemoglobin depends on: I  Availability of O2. 2  Concentration and types of hemoglobin present. & 3  Presence of the interfering substances. & 4  pH and the temperature of the blood. - S  Levels of pCO2 and 2,3-diphosphoglycerate. Y 3- DPG 2, - Pressure Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Oxygen and Gas Exchange  Oxygen Transport: Blood hemoglobin exists in one of four conditions: &  1. Oxyhemoglobin (O2Hb): O2 reversibly bound to hemoglobin -  2. Deoxyhemoglobin (HHb): hemoglobin not bound to O2 but capable of forming a bond when O2 is available &  3. Carboxyhemoglobin (COHb): hemoglobin bound to CO 2 &  4. Methemoglobin (MetHb): hemoglobin unable to bind O2 because iron (Fe) is in an oxidized rather than reduced state 7 are  Dedicated spectrophotometers (cooximeters), HGBES used to determine the relative concentration of each of these species of hemoglobin. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Oxygen and Gas Exchange  Quantities associated with assessing a patient’s oxygen status. ·  Four parameters are commonly used: &  Oxygen saturation (SO2): ratio of O2 bound to carrier protein, hemoglobin, compared with total amount of hemoglobin capable of binding to O2: SO2 = cO2Hb/ (cO2Hb + cHHb) X 100. -  Fractional oxyhemoglobin (FO2Hb): ratio of concentration of oxyhemoglobin to concentration of total hemoglobin (ctHb):  FO2Hb= cO2Hb/ctHb= cO2Hb/(cO2Hb + cHHb + cdysHb). --  Trends in oxygen saturation: assessed by transcutaneous, pulse oximetry (SpO2) -  Oxygen content: total O2 in blood; sum of O2 bound to hemoglobin (O2Hb) and amount dissolved in plasma (PO2).  Normally 97% to 99% of the available hemoglobin is saturated with O2. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Oxygen and Gas Exchange & /  Hemoglobin–Oxygen Dissociation:  & O2 must be released at tissues from its carrier, hemoglobin.  & Oxygen dissociates from adult (A1) hemoglobin in a characteristic fashion (S-shaped curve). D Shape of oxygen-dissociation curve and affinity of hemoglobin  for O2 are affected by: factor affect a finity - &  Hydrogen ion activity. of & PCO and CO levels. HGB 2 & Body temperature. ⑪ 2,3-DPG. &  Dyshemoglobins (COHb or MetHb), can also S affect oxyhemoglobin dissociation.  Hemoglobin unique structure allows it to act as both an acid- D base buffer and O2 buffer. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement  How is it used? &  Blood gas measurements are used to evaluate a person's lung function and acid/base balance. 1-  Ordered if someone is having worsening symptoms of a respiratory problem, such as difficulty breathing or shortness of breath, and a condition such as asthma or chronic obstructive pulmonary disease (COPD) is suspected. D May also be used to monitor treatment for lung diseases and to evaluate the effectiveness of supplemental oxygen therapy. & Are used to detect an acid-base imbalance, such as can occur with kidney failure, heart failure, uncontrolled diabetes, severe infections, and drug overdose. & They may be ordered along with other tests, such as electrolytes to determine if an electrolyte imbalance is present, glucose to evaluate blood sugar concentrations, and BUN and creatininetests to evaluate kidney function. 2  Blood gases may also be ordered when someone has head or neck trauma, which may affect breathing, and when someone is undergoing prolonged anesthesia – particularly for cardiac bypass surgery or brain surgery – to monitor blood gases during, and for a period after, the procedure. & Checking blood gases from the umbilical cord of a newborn may uncover respiratory problems as well as determine acid/base status. Testing is usually only done if a newborn is having difficulty breathing.  https://labtestsonline.org/understanding/analytes/blood-gases/tab/test/ Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement  Spectrophotometric Determination of Oxygen Saturation: [  Actual percent oxyhemoglobin (O2Hb) can be determined using cooximeter] designed to measure various hemoglobin species. I Each species has a characteristic absorbance curve.   Number of wavelengths incorporated into instrument determines number -// S of species that can be measured, from 4 to hundreds.  4 most common hemoglobin species: HHb, O2Hb, COHb, MetHb.----  Potential sources of error: faulty instrument calibration and spectral- & interfering substances. !  Stabilize the patient’s ventilation status before blood sample collection.  All blood samples should be collected under anaerobic conditions and mixed immediately with heparin or other appropriate anticoagulant. & All samples should be analyzed promptly due to the consumption of oxygen by metabolizing cells. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Blood Gas Analyzers: pH, PCO2, and PO2  Blood gas analyzers measure pH, PCO2, and PO2 with electrodes & (macroelectrochemical or microelectrochemical sensors) as sensing devices. 12 28 &. - &  Amperometric: Amount of current S & flow indicates oxygen present (PO2). -  Potentiometric: Change & S in voltage indicates analyte activity (PCO2, pH). & &  Cathode: 1)[negative electrode; ] 2) site to which cations tend to travel; 3) site at which reduction occurs.  Anode: 1)[positive electrode; & 3 2) site to which anions tend to travel; 3) site at which oxidation occurs.  Electrochemical cell: formed when two opposite electrodes & are immersed in a liquid that will conduct current.  The blood gas analyzer can calculate several additional - parameters: bicarbonate, total CO2, base excess, and SO2. - - Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement  Measurement of PO2:  & PO2 (Clarke) electrodes measure amount of current flow in circuit related to amount of O2 being reduced at cathode. ( Electrons are taken up at the cathode and the current generated is  proportional to oxygen tension & cathode I Sources of error: buildup of protein material on surface of membrane, bacterial  contamination within measuring chamber and sample collection. I Continuous measurements for PO2 are possible using transcutaneous electrodes  placed directly on skin.  Measurement of pH and PCO2: - - Y  Ion force measurement requires 2 electrodes and a voltmeter.  Potential difference is related to concentration of ion of interest by Nernst equation.  To measure pH, a glass membrane sensitive to H+ is placed around an internal Ag– I AgCl electrode to form a measuring electrode. selectrode  pCO2 is determined with a modified pH electrode, called a Severinghaus electrode. I &  An outer semipermeable membrane that allows CO2 to diffuse into a layer of electrolyte, usually a bicarbonate buffer, covers the glass pH electrode. & Sources of error: buildup  of protein material on surface of membrane and calibration Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement  Types of Electrochemical Sensors: T a Macroelectrode sensors: used in blood gas instruments since beginning of clinical measurement of blood gases. ·  Microelectrodes: miniaturized - macroelectrodes.  Thick and thin film technology: Sensors are S tiny wires embedded in printed circuit card that & are disposable. ⑪ Optical sensors: -  Use fluorescent dyes, into which sample diffuses.  Have been applied to indwelling blood gas systems. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement  Calibration: & pH & blood gas measurements are extremely sensitive  to temperature. & Electrode sample chamber must be maintained at constant temperature. I pH electrode is calibrated with 2 buffer solutions, traceable to standards prepared by NIST.  Two gas mixtures are used to calibrate for PCO2 and PO2. X  Most instruments are self-calibrating and are programmed to indicate a calibration error if electronic signal from electrode is inconsistent with programmed expected value. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Measurement --  Calculated Parameters: & & (4) D  HCO3-: based on Henderson-Hasselbalch equation; can be calculated when pH and PCO2 are known. ⑤  Carbonic acid concentration: can be calculated using solubility coefficient of CO2 in plasma at 37C. &  Total carbon dioxide content: bicarbonate plus dissolved CO2 plus associated CO2 with proteins.  Some clinicians use the calculated base excess or deficit to assess the patient’s acid-base disorders. #  Correction for Temperature:  By convention, pH, PCO2, and PO2 are all measured at 37C.  If patient’s body temperature differs from 37C, blood gas instrument can “correct” values; results at 37C should be reported, too, however, for reference.20 Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins  Quality Assurance  Preanalytic Considerations:  Introduced during the collection and transport of samples before analysis. ·  Proper patient identification.  Correct labeling of specimen and accurate information provided.  Experienced, knowledgeable personnel.  Proper collection and handling of blood gas specimens.  Transport time.  The best practice in avoiding many of the preanalytic errors is to analyze the sample as quickly as possible. -  Analytic Assessments: QC and Proficiency Testing.  Surrogate liquid control materials, tonometry, duplicate assays, non-surrogate QC. after analytical  Interpretation of Results. Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins Quality Assurance  Blood gas analysis quality assurance cycle: Copyright © 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins