Blood Gases, pH & Buffers (Acid-Base Balance) Slides

Blood Gases, ph & buffer systems(Bishop text – Chapter 12) Paul R. Nelson, M.S., MLS(ASCP) Associate professor, Medical Lab Sciences [email protected]  Lesson Objectives Describe chemical & physiological systems involved in acid/base balance,...

Blood Gases, ph & buffer systems(Bishop text – Chapter 12) Paul R. Nelson, M.S., MLS(ASCP) Associate professor, Medical Lab Sciences [email protected]  Lesson Objectives Describe chemical & physiological systems involved in acid/base balance, including bicarb, phosphate, protein & hemoglobin buffer systems along with role of kidneys & lungs Use Henderson-Hasselbalch equation to predict pH or bicarb of sample Define/describe & identify 4 major acid/base imbalances: metabolic acidosis & alkalosis, respiratory acidosis & alkalosis Explain compensation mechanisms for each acid/base imbalance Use Henderson-Hasselbalch to determine acid/base disorder & compensatory status Describe O2 transport & assessment mechanisms for patient O2 status Describe pH, pCO2, 2,3-DPG/BPG & temp affect on O2 dissociation curve shape Describe measurement principles of pH, pCO2, pO2 & various hemoglobins Explain clinical significance of: pH, pCO2, pO2, bicarb, carbonic acid, base excess, O 2 saturation, fractional oxyhemoglobin, hemoglobin oxygen (binding) capacity, oxygen content & , total CO2 Discuss collecting & handling samples for blood gas analysis: i.e. syringes, anticoagulants, mixing, icing, capillary vs venous vs arterial samples Describe methods used to measure various hemoglobins, pH & blood gas parameters Describe QA/QC processes, proficiency testing & delta checks to assess blood gas result quality Discuss reasons for discrepancies, given O2 saturation data calculated by blood gas analyzer & measured by CO-oximeter  ACID/BASE BALANCE Acid: substance donating hydrogen (H+) ions when dissolved in water Base: substance accepting H+ ions when dissolved in water pH: represents measurement of body’s regulation of H+ aka, measurement sample acidity pH represents negative logarithm of hydrogen ion concentration Henderson-Hasselbach equation mathematically shows: for every 1 pH unit decrease, hydrogen ion concentration increases 10- foldIn short, the more hydrogen ion concentration in the blood, the lower the pH: pH normal range: 7.35 – 7.45 Decreased pH = more hydrogen in sample (more acidic) Increased pH = less hydrogen in sample (more basic)  Buffer Systems: Buffers: resist pH changes; combination of weak acid & corresponding salt base Buffering systems = correct combinations of acid/base to produce desirable outcome Bicarbonate/Carbonic Acid buffering system is a principle body pH buffering systems Effectiveness of a buffering system depends on pKa of environmental pH Ka (Dissociation/Ionization Constant): describes relative strength of acid or base; tables of known dissociation constants exist for many acids/bases pKa (negative log of dissociation constant): describes point at which sample/solutions are at equilibrium (aka neutralization) if acid is spilled, how much base (amount & concentration) required to neutralize  Acid/base balance: Role of lungs & kidneys Transport of CO2 CO2 is waste product of most aerobic metabolic processes CO2 easily diffuses out of tissue & into surrounding capillary plasma/RBCs Dissociation of carbonic acid causes concentration gradient due to increased bicarb in RBCs At rate determined by needs of body: Lungs (respiratory): exchange CO2 for fresh O2 Most efficient when dissolved plasma CO2 (dCO2) = CO2 in lungs Kidneys (metabolic): reabsorption of needed bicarb & excretion of acids (primarily ammonium ions & hydrogen)  Acid/base balance: Role of lungs & kidneys (cont’d) Regulation of hydrogen ions (aka pH) Body produces ~150g of H+ per day Blood pH regulated via lungs (respiratory) & kidneys (metabolic) Body stores & excretes excess hydrogen to maintain homeostasis Extremely narrow pH Range required to sustain life: 7.35- 7.45 Acidosis: pH level below reference range (7.45)  Regulation of H+ (cont’d) Buffer systems: body’s 1st line of defense against extreme H+ concentration changes Bicarb/carbonic acid buffering system important for three reasons: 1. H2CO3 dissociates into CO2 & H2O allows CO2 to be eliminated by lungs hydrogen to be eliminated as water by kidneys 2. Changes in CO2 concentration can speed up/slow down breathing (respiratory) 3. HCO3− concentration can be altered by kidneys (metabolic)  pH: Respiratory regulation Lungs (respiratory pathway): Blood pH affected by respiratory ventilation O2 inhaled/diffuses from alveoli into blood then into tissue cells CO2 created during cellular metabolism Diffuses into blood then alveoli & eliminated via respiratory exhalation Minimal H+ concentration changes between venous & arterial circulations  pH: metabolic regulation Kidneys (metabolic pathway): Blood pH affected by hydrogen elimination Kidneys regulate acid/base balance by filtering/excreting acids & bases Bicarb (HCO3−) filtered then reabsorbed back into blood stream serves as blood buffer (base) to prevent excessive blood acid gain  Henderson-Hasselbach Equation Henderson-Hasselbach equation shows pH balancing relationship of lungs & kidneys pH is negative log of hydrogen ion concentration pKa is pH at which equal concentrations of protonated & unprotonated solutions exist For blood pH this is constant at 6.1 A- is concentration of hydrogen proton acceptor aka weak base (~bicarb) HA is concentration of hydrogen proton donor aka weak acid (~carbonic acid) Short (~4 mins) clip re: Henderson-Hasselbach: https://www.youtube.com/watch?v=ww6-jlr72Dw  Henderson-Hasselbach Knowing any 3 variables allows calculation of 4th Plug in pKa: 6.1 for bicarb/carbonic acid buffer (constant/known) Plug in results from lab testing (see example below) Normal HCO3 is ~24 Normal pCO2 is ~40 - then Multiply pCO2 result by 0.03 (solubility constant in plasma) Ratio of Bicarb to Carbonic Acid is ~20:1  Oxygen & carbon dioxide Exchange Seven conditions necessary for adequate tissue oxygenation: 1. Available atmospheric oxygen 2. Adequate ventilation 3. Gas exchange between lungs & arterial blood 4. Loading of O2 onto hemoglobin 5. Adequate hemoglobin 6. Adequate transport 7. Release of O2 to tissue Factors influencing amount of O2 moving thru lungs, blood, tissue: Destruction of alveoli Pulmonary edema Airway blockage Inadequate blood supply Diffusion of CO2 & O2  Oxygen Transport Disturbances in O2 & CO2 conditions can result in poor tissue oxygenation (hypoxia) pO2 (along w/ pH & pCO2) measured to evaluate patient’s O2 status Most O2 in arterial blood is transported to tissue via hemoglobin: 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 (aka reduced hemoglobin) 3. Carboxyhemoglobin (COHb): hemoglobin bound to CO  Assessing Oxygen Status Four parameters used to assess a patient’s oxygen status: 1. Oxygen saturation (SO2 or 02 sat) 2. Amount of O2 dissolved in plasma (pO2) 3. Fractional (percent) oxyhemoglobin (FO2Hgb) 4. SO2 trends assessed via transcutaneous (TC) pulse oximetry (SpO2)  Hemoglobin/Oxygen Dissociation O2 releases/dissociates from carrier hemoglobin into tissues Dissociation of O2 Oxygen from adult (A1) Hgb in known, characteristic S-shaped curve Dissociation curve shape & Hgb affinity for O2 affected by: pH (Hydrogen ion activity) PCO2 levels Body Temp O2 dissociation curves: A = increased affinity 2,3-Diphosphoglycerate (2,3DPG) levels B = Normal C = Decreased affinity  Oxygen Saturation Measurement O2 Saturation: ratio of hemoglobin already bound to oxygen compared to all hemoglobin available to carry O2 (normal = 95- 100%) Actual % oxyhemoglobin (O2Hb) determined via co-oximeter spectrophotometer Number of wavelengths incorporated into instrument determines number of different hemoglobins that can be measured (from four to hundreds) Sources of error: faulty instrument calibration & spectral- interfering substances  pH, PCO2 & PO2 Measurement Blood Gas Analyzers measure pH, PCO2 & PO2 via electrodes Amperometry (Clarke Electrode): amount of current flow indicates amount of PO2 Potentiometry: change in voltage indicates analyte (PCO2, pH, etc.) activity Electrochemical cell: opposing electrodes immersed in sample to conduct current Cathode End: (1) negative electrode; (2) site to which cations are attracted; (3) site at which reduction occurs Anode: (1) positive electrode; (2) site to which anions are attracted; (3) site at which oxidation occurs  pH, PCO2 & PO2 Measurement (cont’d) Measurement of PO2 PO2 (Clarke) electrodes measure amount of current flow in circuit related to amount of O2 being reduced at cathode Sources of Error: buildup of protein material on surface of membrane, bacterial contamination within measuring chamber Continuous measurements for PO2 are possible using transcutaneous electrodes placed directly on skin Measurement of pH & PCO2 Ion force measurement requires two electrodes & voltmeter Potential difference is related to concentration of ion of interest by Nernst equation  Types of Measurement Devices Electrochemical Sensors 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 tiny wires embedded in printed circuit card that are disposable Optical Sensors Use fluorescent dyes, into which sample diffuses Have been applied to indwelling blood gas systems Blood Gas Instrument Calibration pH & blood gas measurements are extremely sensitive to temperature pH electrode is calibrated with two buffer solutions Two gas mixtures with known pCO2 and pO2 levels are used First one used to calibrate lower end gases (mixture is 0% O2 & 5% CO2) Second used to calibrate upper end gases (mixture is 20% O2 and 10% CO2) Most instruments self-calibrating & will have alarm to indicate any calibration error Temperature Correction By convention, pH, PCO2 & PO2 are all measured at 37°C  Calculated Parameters Directly Measured Results: pH, PCO2, PO2, O2 sat Calculated Results: HCO3−: Henderson-Hasselbalch; calculated when pH & PCO2 results are available H2CO3 (Carbonic acid): calculated using solubility coefficient of CO2 in plasma (37°C) Total Carbon Dioxide (TCO2): bicarbonate + dissolved CO2 + CO2 bound to proteins Base Excess (BE): Used to quickly assess metabolic pathway base imbalance Base Excess refers to the excess amount of base (bicarb) present in blood Positive base excess results = excess bicarb (suggestive of metabolic  Quality Assurance (QA) & Proficiency Testing (PT) Must cover: Pre-analytic, Analytic & Post-analytic phases of testing Preanalytic Considerations Proper patient identification, correct labeling of specimen & accurate info provided Experienced, knowledgeable collection personnel Proper collection & handling of blood gas specimens Transport time Analytic Considerations: QC Commercially available liquid control samples often used Sealed glass containers w/suitable matrix & gases of known concentration Split sample analysis on one device or running one sample on two or more instruments Post-Analytic Considerations: Result Interpretation Participating proficiency testing program is essential for quality of Quality Assurance (cont’d)  Common Sources of error Wrong collection device (usually syringe) Form & concentration of heparin used for anticoagulation Speed of syringe filling Not maintaining strict anaerobic environment Introduction of air bubbles Not mixing sample to ensure dissolution & distribution of heparin Long storage/waiting time between collection & analysis  Acid/Base Balance Disorders: Two primary concerns: Acidosis or Alkalosis Two subclasses each: Respiratory or Metabolic The following regulatory mechanisms help describe what is happening in body (clinically)  Respiratory Acidosis Acidosis: blood pH too low; can be caused by respiratory or metabolic condition Respiratory: decreased lung function Causes: AS A COW Airway Obstruction Sedatives Acute lung disease Chronic lung disease Opioids Weakness of respiratory muscles Body tries to compensate for Respiratory Acidosis via kidneys: Kidneys can increase rate of bicarbonate reabsorption to increase base Lung dysfunction is problem so kidneys must compensate  metabolic Acidosis (aka non-respiratory acidosis) Non-respiratory dysfunction, usually kidney (or GI), issue causing acidic pH Causes: MUDPILES (high AG) & HARDASS (normal AG) Methanol Hyperalimentation Uremia Addison’s Disease Diabetic ketoacidosis Renal Tubular Acidosis Propylene glycol Diarrhea Iron tablets & isoniazid Acetazolamide Lactic acid Spironolactone Ethylene glycol Saline Infusion Salicylates Body tries to compensate for Metabolic Acidosis via lungs Lungs expel more CO2 via hyperventilation Kidney dysfunction is problem so lungs must compensate  Respiratory Alkalosis Alkalosis (alkalemia): pH too high; can be caused by respiratory or metabolic condition Respiratory: hyperventilation; excessive elimination of CO2 Causes: PAST PH Panic Attacks Anxiety Attacks Salicylates Tumor Pulmonary embolism Hypoxemia Body tries to compensate for Respiratory Alkalosis via kidneys Kidneys can eliminate more bicarbonate (decreased reabsorption) to increase acid Lung dysfunction is problem so kidneys must compensate  metabolic Alkalosis (aka non-respiratory alkalosis) Metabolic dysfuntion: pH too hige; loss of hydrogen/gain in bicarbonate Causes: LAVA-UP Loop Diuretics Antacid Use Vomiting Aldosterone Up (increased) Body tries to compensate for Metabolic Alkalosis via lungs Lungs can slow respirations (hypoventilation) to increase acid via more CO2 in blood Kidney dysfunction so lungs must compensate  acid/base imbalance: 4 steps STEP 1: Is pH outside normal range? If yes, is it acidosis (7.45)? If pH normal, check pCO2 & HCO3 to determine normality isn’t compensatory mechanism STEP 2: Respiratory or Metabolic issue? *if pH, pCO2 & HC03 are all normal, there is no acid/base imbalance If pCO2 abnormal = suggests Respiratory If HCO3 abnormal = suggests Metabolic If Both Abnormal, use Step 3; if only 1 abnormal, skip to Step #4 STEP 3: only Use when both PCO2 & HCO3 are abnormal MOST abnormal parameter usually determines primary issue (respiratory or metabolic) Based on distance away from mean of analyte normal range STEP 4: Is patient compensating (fully or partially) or not compensating? Body will attempt to compensate for whatever defect is causing acid/base imbalance  4 Step Process Used if any of the primary blood gas results (pH, pCO2, HCO3) are abnormal If all 3 primary results are normal, patient is not having an acid/base crisis STEP 1: Acidosis or Alkalosis? Is arterial blood pH outside of reference range? If pH is low (< 7.35) = ACIDOSIS If pH is high (> 7.45) = ALKALOSIS If Normal, are pCO2 and/or HCO3 abnormal? STEP 2: Respiratory or Metabolic? Look at pCO2 & HCO3 results If pCO2 is out of range = RESPIRATORY If HCO3 is out of range = METABOLIC Example #1 (Respiratory Acidosis) EXAMPLE #3 pH = 7.26 pH = 7.24 pCO2 = 56 pCO2 = 44 HCO3 = 24 HCO3 = 18 Example #2 (Respiratory Alkalosis) EXAMPLE #4 pH = 7.50 pH = 7.52 pCO = 30 pCO2 = 44  4 step process (cont’d) STEP 3: Used when both pCO2 & HCO3 are Abnormal If pCO & HCO are both abnormal: furthest away from 100% 2 3 is primary issue Verify which parameter is furthest from 100%: Divide pCO2 by 40 (reference mean) Divide HCO3 by 25.5 (reference mean) EXAMPLE #5: Metabolic Acidosis pH = 7.25 (acidic) pCO2 = 30 (abnormal) (check: 30/40 = 75.0%) HCO3 = 15 (abnormal) (check: 15/25.5 = 58.8% furthest away from 100%) EXAMPLE #6: Metabolic Alkalosis pH = 7.55 (Alkaline)  4 step process (cont’d) STEP 4: Is patient uncompensated, partially compensated or fully compensated Uncompensated: pH & 1 other parameter (pCO2 or HCO3) abnormal Partially Compensated: pH, PCO2 & HCO3 (all 3) are abnormal Compensation mechanism has kicked in but pH is still abnormal Fully Compensated: both PCO2 & HCO3 abnormal but pH has normalized Compensation mechanism has kicked in and brought pH back within limits Quick Reference Charts:  EXAMPLE #7 Patient Results: pH = 7.33 pCO2 = 60 HCO3 = 30 Step #1: is patient normal, acidic or alkaline? Step #2: is patient in metabolic or respiratory distress? If PCO2 = Respiratory HCO3 = Metabolic If Both = Go To Step 3 Step #3: If both PCO2 & HCO3 are abnormal, which is furthest from 100%? If PCO2 = Respiratory (do the math: 60/40 = 150.0%) HCO3 = Metabolic (do the math: 30/25.5 = 117.6%) Step #4: is patient uncompensated, partially compensated or fully compensated Are both PCO2 & HCO3 abnormal? If No = Uncompensated If Yes, has pH normalized yet? If No = Partially Compensated If Yes = Fully Compensated  EXAMPLE #8 Patient Results: pH = 7.52 pCO2 = 28 HCO3 = 21 Step #1: pH is alkaline Step #2: Both pCO2 & HCO3 are abnormal (low) Step #3: pCO2 is furthest from 100 % of mean reference range pCO2: 28/40 = 70.3% ; HCO3: 21/25.5 = 82.3% Step #4: pH has not normalized but HCO3 is abnormal due to attempted compensation  EXAMPLE #9 Patient Results: pH = 7.39 pCO2 = 25 HCO3 = 15 Answer: Fully Compensated Metabolic Acidosis Step #1: pH is normal Step #2: Both pCO2 & HCO3 are abnormal/critical low (pt in crisis) Step #3: HCO3 is furthest from 100% of it’s reference mean (metabolic) Step #4: pH has normalized (fully compensated) Difficult to distinguish between Fully Compensated Acidosis/Alkalosis Quick Rule: use whichever side of pH mean patient result lands In this example, pH is normal (7.39) but below pH mean (7.40); so slightly acidic Common sense check: since HCO3 is “more abnormal” lack of HCO3 is likely primary concern If not enough base, blood gets acidic; lungs compensate via hyperventilation to slowly lower PCO2  EXAMPLE #10 Patient Results: pH = 7.38 pCO2 = 42 HCO3 = 26 Step #1: pH is normal Step #2: pCO2 & HCO3 both normal Step #3: N/A Step #4: N/A Answer: Normal Patient Not in Acid/Base Crisis  Questions ??? 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