Acid/Base & ABG Analysis PDF

ACID / BASE & ABG ANALYSIS Crystal Fishman, MS, RRT Cecelia Szakolczay MAT, RRT updated 9/2024  2 Question of the Day #’s 8, 9 & 10: #8: What is a buffer; and what buffer is found in the RBC? #9: How does the bicarb...

ACID / BASE & ABG ANALYSIS Crystal Fishman, MS, RRT Cecelia Szakolczay MAT, RRT updated 9/2024  2 Question of the Day #’s 8, 9 & 10: #8: What is a buffer; and what buffer is found in the RBC? #9: How does the bicarbonate (HCO3-) buffer system get rid of fixed acid? 10: Interpret the following ABG’s: (on room air) a. pH 7.35; PaCO2: 48; PaO2: 79; HCO3-: 24; SaO2: 92% b. pH 7.50; PaCO2: 30; PaO2: 103; HCO3-: 24; SaO2: 99% c. pH 7.38; PaCO2:50; PaO2: 55; HCO3-: 30; SaO2: 88% d. pH 7.15; PaCO2: 23; PaO2: 105; HCO3-:19; SaO2: 99%  3 ABG SAMPLING: -Gas exchange between the lungs and the blood: -Arterial sample -Gas exchange between blood and tissues: -Mixed venous sample -Peripheral venous samples…NO VALUE! -Pulse oximetry reduces the need for ABG’s -But: -does not reflect CO2 -does not reflect Acid-base  4 The Physics of Blood Gases -Dalton’s Law of Partial Pressures: Pt P1  P2  P3 Pb PatmO2  Patm N 2  PatmOther Is it Groundhogs Day Pb 159  593  8 Again? Pb 760 mmHg -Gas we include, O2, N2, CO2 and other gases. -Gases are 100% saturated when we breathe in; the humidity exerts a partial pressure of 47mmHg  5 The Physics of Blood Gases2 -Note: Dalton’s Law states: the total pressure of a gaseous mixture is = to the sum of the partial pressures of its constituent gases. -Henry’s Law states: at a constant temp., the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the Partial Pressure of that gas in equilibrium with that liquid.  6 Gas Exchange by Pressure Gradient -O2: enters alveoli from atmosphere  PAO2 > the PO2 in pulmonary capillaries  O2 moves into the Pulmonary Capillaries (PcO2). -Newly oxygenated blood in Pulmonary Capillaries travels to the Pulmonary Veins  LA  LV  Aorta  systemic arteries  systemic arterioles  tissue capillaries of the body. -At the tissue level the PaO2 > PO2 in the tissues and O2 moves into the tissues for aerobic respiration (metabolism)  7 Gas Exchang e by Pressure Gradient  8 Gas Exchange by Pressure Gradient2 -At the tissue level the PCO2 is > than the PCO2 in the tissue capillaries  CO2 moves into tissue capillary blood (PvCO2) blood in the venules  veins  IVC & SVC  RA  RV  -Pulmonary Arteries  pulmonary arterioles  (in pulmonary capillaries - PcCO2) > alveoli (PACO2) CO2 moves into Alveoli (PACO2)  PACO2 > PCO2 of atmospheric air  CO2 is exhaled out into the atmosphere  9 Gas Exchange2  10 Oxygen - Hemoglobin Dissociation Curve Key Factors -pH -Temperature -2, 3 DPG (diphosphoglycerate)* -synthesized in the RBC -shifts the ODC to the right - ↓’in Hb’s affinity for O2 -makes loading O2 in the lungs harder, and unloading O2 at the tissues easier *without it the Hb will not let go of O2  11 Oxygen - Hemoglobin Dissociation Curve2 Key Factors (cont’d) -P50: the pressure at which the Hb is 50% saturated -Normal P50: 27 torr -The greater the partial pressure of O2, the higher the % Saturation of the blood -As the Oxyhemoglobin Dissociation Curve (ODC) moves, the P50 also moves -PaO2 of < 40 is quite dangerous  12 ↑’d affinity of Hb for O2, ↑’d ODC loading; ↓’d unloading Increased pH Fetal Hemoglobin Decreased CO2 Hypothermia Hyperventilation LEFT Decreased pH Increased CO2 RIGHT Fever Hypoventilation ↓’d affinity of Hb for O2, ↓’d loading; ↑’d unloading  13 Normal ABG/Co-oximeter Ranges Measured Calculated -pH  7.35-7.45 (7.40) - -PaCO2  35-45 mm Hg HCO3  22-26 mEq/L -PaO2  80-100 mm Hg BE  +/- 2 ______________________ -tHb  12-17 g/dL* SaO2  > 90% -O2Hb  94-98%* -COHb  0.5-1.5%* -DeoxyHb (reduced Hb)* -MetHb  0-1.5%* -*Co-oximeter values  Acid-Base and Ventilatory Classification Arterial Sample pH Normal 7.35-7.45 Acidemia < 7.35 Alkalemia > 7.45 PaCO2 Normal vent. 35-45 mm Hg Resp. Acidosis > 45 Resp. Alkalosis < 35 - Normal 22-26 mEq/L HCO3 Met. Acidosis < 22 Met. Alkalosis > 26 Base Excess Normal ±2 mEq/L Met. Acidosis (BE) < -2 Met. Alkalosis > +2 14  15 Acid/Base & Buffering: -Most H+ ions in the body come from: 1. Breakdown of phosphorous-containing proteins: - into phosphoric acid 2. Anaerobic metabolism of glucose: - into lactic acid 3. Metabolism of body fats: - into fatty and ketone acids 4. Transport of CO2 in the blood as HCO3- which releases H+ ions -Normally, both H+ and HCO3- ion concentrations in the blood are regulated by 3 major systems: 1. Chemical; 2. Respiratory: 3. Renal  16 Acid/Base & Buffering2 -Buffer: something that resists pH changes: very sensitive 1. Chemical Buffer System: body’s first line of defense -reacts fast a. Carbonic Acid-Bicarbonate Buffer system b. Phosphate Buffering System c. Protein Buffer System (e.g. Hb) -chemical buffers: -inactivate H+  releasing HCO3- in response to acidosis or -generates more H+ ions ↓’ing HCO3- ions in response to alkalosis  17 Acid/Base & Buffering3 2. Respiratory Buffering (via the lungs) -Reacts within 1 – 3 minutes ↑’s or ↓’s rate and depth of breathing to offset acidosis or alkalosis, respectively -Response to Metabolic Acidosis: -↑ in the rate and the depth of breathing -this ↓’s CO2 (“blowing off the CO2”); which in turn will increase pH; -and vice versa with a Metabolic alkalosis (↓ RR & depth  ↑’d CO2 and ↓’d pH)  18 Acid/Base & Buffering4 Renal System -When fluid becomes -Most effective acid-base acidic: monitor -renal system retains HCO3- ; -Takes time excretes H+ into the Requires a day or more to urine to ↑ the pH correct abnormal pH (24- 48 up to 72 hours) -When fluid becomes -Can expel fixed acids alkalotic: -renal system -Lactic retains H+ ; -Phosphoric excretes bases -Uric usually HCO3-, into the urine to ↓ the pH  19 Chemical Buffer System -Carbonic Acid-Bicarbonate Buffer System -plays extremely important role in maintaining pH homeostasis of the blood -inactivates H+ & liberates (releases) HCO3- -normally ratio of (Bicarbonate) HCO3- to (Carbonic Acid) H2CO3 is: 20:1 in the blood: -this relationship works to resist changes in pH response to ↑pH H2CO3 HCO 3- + H+ response to ↓pH  20 Chemical Buffer System2 -Carbonic Acid-Bicarbonate Buffer System(cont’d) -ratio of HCO3- to H2CO3 is: 20:1 in the blood: -one way to look at it: -for every 1 ion of carbonic acid (H2CO3) loss in the blood plasma, 20 HCO3- ions must be eliminated to maintain the pH -another way to look at it: a H2CO3 ion is 20x more powerful - than a HCO3 ion in changing the pH -this relationship works to resist changes in pH  21 pH -as H+ increases: -pH ↓’s -becomes more acidic -as Hydroxide (OH-) increases: -pH ↑’s -becomes more basic (alkalotic) -Scale runs 1 – 14  22 Henderson-Hasselbalch Equation [HCO3-] (base) -Good for pH = pK + log _______ [H2CO3] (acid) estimating the pH of a [HCO3-] pH = pK + log ___________ buffer [PCO2 x 0.03] solution 24 mEq/L -For more = 6. 1 + log __________ (40 x 0.03) information review 24 mEq/L = 6. 1 + log __________ p. 312 – (1.2 mEq/L) 313 in Des = 6. 1 + log 20 (20:1 ratio) Jardins 1 = 6. 1 + 1.3 = 7.4 23 pH of Buffer System: Henderson-Hasselbalch Equation (H- H) Describes [H+] as ratio of [H2CO3] / [HCO3–] -pH = 6.1 + log [HCO3–]/(PaCO2 × 0.03) -pH is logarithmic expression of [H+]. -6.1 is the log of the H2CO3 equilibrium constant -(PaCO2 × 0.03) is in equilibrium with, & directly proportional to, blood [H2CO3] -pH = -log [H+] -Blood gas analyzers measure: -pH & PaCO2 -then use H-H equation to calculate HCO3– 24 Bicarbonate Buffer System -HCO3– buffer system can continue to buffer fixed acid H+ as long as ventilation is adequate to exhale volatile acid CO2 Ventilation  H+ + HCO3–↔H2CO3 ↔ H2O + CO2  Fixed acid -the process of buffering fixed acid produces CO2 -this CO2 is removed by ventilation *def. of a fixed acid: ‘acid produced in the body that is non-volatile, i.e. it cannot be expelled through the lungs’. 25 Bicarbonate Buffer System2 -HCO3– system cannot buffer H2CO3 (volatile acid) during hypoventilation -i.e. in hypoventilation: -CO2 is not ventilated out as fast as it is produced -when this happens: -the H2CO3 accumulates in the blood -the CO2 drives the equation to the right - CO2 + H2O ↔ H2CO3 ↔HCO3 + H+ ↑ Aerobic Metabolism See: Fig. 7-2 p. 318 in Des Jardin  26 Phosphate Buffer System 1. Almost identical to Carbonic Acid-Bicarbonate buffer system 2. Primary components = sodium salts of HPO4 and H2PO4: -NaH2PO4 -NaHPO4 3. H+ ions released by strong acid  phosphate buffer system kicks in and inactivates the acidic effects of the H+: HCL + Na2HPO4  NaH2PO4 + NaCL strong acid weak base weak acid salt NaOH + NaH2PO4  Na2HPO4 + H2O  27 Protein Buffer System -Protein Buffer System is the body’s most abundant and influential supply of buffers - ≈ 75% of buffering power of the body’s fluids are found in: -proteins in plasma and cells -these proteins can: 1. release H+ ions ↓’ing the pH 2. bind H+ ions ↑’ing the pH -a prime example of a protein that works as an intracellular buffer is: Hb 28 Acid Excretion -Buffer systems are the immediate defense against the accumulation of H+ -If acids were not excreted, a life-threatening acidosis would follow -The Lungs: -excrete CO2, which is in equilibrium with H2CO3 -Extremely important!: -body produces huge amounts of CO2 during aerobic respiration (metabolism): by- product is CO2 + H2O 29 Acid Excretion2 -Lungs2: -through HCO3– , fixed acids can be eliminated indirectly as by-products CO2 & H2O -remove ≈ 24,000 mmol/L CO2 daily Ventilation  H+ + HCO3–↔H2CO3 ↔ H2O + CO2  Fixed acid  Acid Excretion3 -Kidneys 1. physically remove H+ from body -excrete < 100 mEq fixed acid per day 2. also control excretion or retention of HCO3– -If blood is acidic: -more H+ are released in the urine & HCO3– is retained (and vice versa ) -Note: The lungs can alter [CO2] in seconds, the kidneys require hours/days to change HCO3– & affect the pH 30  Acid-Base Disturbances -Normal acid-base balance -Kidneys maintain HCO3– of 22-26 mEq/L -Lungs maintain CO2 of 35-45 mm Hg -These produce pH of 7.35-7.45 (H-H equation) -pH = 6.1 + log (24/(40 × 0.03) → pH = 7.40 -Note pH determined by ratio of HCO3– to dissolved CO2 -Ratio of 20:1 will provide normal pH (7.40) -Increased ratio results in alkalemia -Decreased ratio results in acidemia 31 32 Clinical Acid-Base States Respiratory Acidosis -High PaCO2, with a low pH -Causes: -Loss of drive to breathe (e.g. narcotics / neurologic damage) Respiratory Alkalosis -Low PaCO2, with a high pH -Causes: -Hyperventilation (anxiety / pain) 33 Clinical Acid-Base States2 Metabolic acidosis -Low HCO3–, with a low pH -Causes: -Increased fixed acid accumulation -Lactic acidosis in anaerobic metabolism -Excessive loss of HCO3– -Diarrhea -Anion gap can help identify cause of metabolic acidosis (see notes on Anion Gap in folder in Blackboard for future reference) 34 Uncompensated Clinical Acid- Base States3 1. ABG in Alveolar hyperventilation (RA): -pH 7.50, PaCO2 30 mm Hg, PaO2: 105 mmHg, HCO3– : 30 mEq/L; SaO2: 99% 2. ABG in Alveolar Hypoventilation (RA): -pH: 7.30; PaCO2: 50 mmHg; PaO2: 76; HCO3-: 24 SaO2: 91% 3. ABG in Metabolic Acidosis: -pH 7.25, PaCO2 40 mm Hg, HCO3– : 19 mEq/L 4. ABG in Metabolic Alkalosis: -pH: 7.51; PaCO2: 40 mmHg; HCO3-: 29 mEq/L 35 Compensated Clinical Acid- Base State4 3. ABG in Chronic Ventilatory Failure: -pH 7.38, PaCO2 52 mm Hg, PaO2: 50 mmHg, HCO3– : 30 mEq/L -Looks like it can be a: a. Compensated Respiratory Acidosis or b. Compensated Metabolic Acidosis -Only medical history & knowledge of situation allow correct interpretation of this ABG! -Cecelia’s Trick: look at which side of 7.40 the pH is coming from: -if coming from acidotic side, it’s a Comp. Resp. Acidosis -if coming from the alkalotic side, it’s a Comp. Met. Alkalosis 36 Clinical Acid-Base State3 4. Alveolar hyperventilation superimposed on compensated respiratory acidosis (chronic ventilatory failure) Referred to as: “acute onchronic” ‘Decompensation’ of Compensated Respiratory Acidosis -E.g.: normal ABG: -pH 7.38, PaCO2 52 mmHg, PaO2: 50 mmHg, HCO3– : 30 mEq/L -pH: 7.29, PaCO2: 63 mmHg, PaO2: 46 mmHg HCO3-: 30  37 Compensatory Response Acid-base Disorder Primary Defect Compensatory Response  HCO3  HCO3 Respiratory Acidosis [ ]  pH [ ]  pH  PaCO2  PaCO2  HCO3  HCO3 Respiratory Alkalosis [ ]  pH [ ]  pH  PaCO2  PaCO2  HCO3  HCO3 Metabolic Acidosis [ ]  pH [ ]  pH  PaCO2  PaCO2 Metabolic Alkalosis  HCO3  HCO3 [ ]  pH [ ]  pH  PaCO2  PaCO2 From Beachey W: Respiratory care anatomy and physiology: foundations for clinical practice. St. Louis, 1998, Mosby.  - No change, - increased,  -decreased  38 Oxygen Cascade Gas moves across system by simple diffusion (with a pressure gradient) Oxygen cascade moves from PO2 of 159 mm Hg in atmosphere to intracellular PO2 of ≈ 5 mm Hg  39 Venous vs. Arterial Sample Venous Arterial pH: 7.30-7.40 pH  7.35-7.45 (7.40) PvCO2: 40-46 mmHg PaCO2  35-45 mm Hg PvO2: 35-45 mmHg PaO2  80-100 mm Hg SvO2: 65-75 % SaO2  > 90%  40 Never Gone! Assessment of Oxygenation Alveolar-air equation: P A O2 .21(760mmHg – 47) - 1.25 x PaCO2.21 ( 713) x 50 PAO2 = 100 If FIO2 > 0.6 do not use R (0.8) use above better! A-a Difference: P( A  a) P O A 2  The (A‐a) Difference at RA (FIO2 of 21%) should be 5‐10 mm Hg The (A‐a) Difference at FIO2 of 100% should be 25‐65 mm Hg On FIO2 of 1.0 for every 100 mmHg of P(A‐a)O2 there is a ≈ 5% of shunting  41 Assessment of Oxygenation 1. Assessment of Oxygenation on Room Air (0.21) -when a patient is on room air (RA) the oxygenation is quantified as : -PaO2: 60-79 mmHg = Mild hypoxemia - PaO2: 40-59 mmHg = Moderate hypoxemia -PaO2: < 40 mmHg = Severe hypoxemia PaO2 or 2. PaO2 / PAO2 ratio (a/A ratio): a -Normal ‘a/A ratio’: > 0.75 PAO2 -a/A can’t ever exceed 1 (If it does A there are analytical or sampling errors)  42 Assessment of Oxygenation 3. PaO2 / FIO2*: (P/F ratio): PaO2 -Normal values: 350-450 FIO2* -Failure: < 200 *In decimal form -When on supplemental O2: -follow the Berlin criteria for P/F ratio**: < 300 = ‘Hypoxemia present’ due to: -V/Q mismatch -Shunt **Ref. Berlin Criteria for P/F (part of Berlin ARDS criteria): -Mild: P/F ratio of 201-300 -Moderate: P/F ratio of 101-200 -Severe: P/F ratio of < 100  43 Causes of Hypoxemia  44 Hypoxemia due to: Hypoventilation 1. a/A ratio: normal 2. abnormally high PaCO2 and acidemia -Example: -a/A = 0.85 -pH = 7.30 -PaCO2 = 49 mm Hg -PaO2 = 55 mm Hg  45 Hypoxemia due to: V/Q Mismatch 1. Normal PaCO2 2. a/A ratio between 0.15 - 0.75 3. PaO2 is > 50 on an FIO2 < 0.5 (‘50/50’ rule) or PaO2 is > 60 on an FIO2 < 60 (‘60/60’ rule)  46 Hypoxemia due to: Physiologic Shunt 1. PaCO2: normal or abnormal 2. a/A ratio: < 0.15 3. PaO2 is < 50 on an FIO2 > 0.5 or PaO2 is < 60 on an FIO2 > 60  47 pH 7.40 1 7 14 Acid Base Normal High pH = Low H+ Low pH = High H+ 48  49 Common Causes of Resp. Acidosis Normal Lungs: Abnormal Lungs: CNS Depression: Anesthesia COPD Sedative drugs Acute Airway Narcotic analgesics obstruction (late Neuromuscular phase) Disease: Poliomyelitis Lung contusion Myasthenia gravis Guillain-Barre syndrome Trauma: Spinal cord injury Brain injury Chest wall injury  50 Common Causes of Respiratory Alkalosis Normal Lungs: Abnormal Lungs: Anxiety Hypoxemia-causing conditions Fever Acute asthma Stimulant drugs Pneumonia CNS Lesions Vagal stimulation Pain Pulmonary edema Sepsis Pulmonary vascular disease Either Normal or Abnormal Lungs: -Iatrogenic hyperventilation  51 Respiratory Alkalosis Causes: Hyperventilation (anxiety, fear) CNS problem (fever, brainstem tumors, infection) Drugs (ASA, theophylline, catecholamines) Hypoxemia, pulmonary disease, PE, Pulm. Edema Sepsis, liver disease S & S: lightheadedness, tingling of fingers and toes, syncope, seizure, dysrhythmias  52 Causes of Metabolic Alkalosis Loss of H+ : Gastrointestinal: Vomiting Nasogastric drainage Renal: Diuretics (Loss of Cl-, K+, fluid volume) Hypochloremia (⇈ H+ secretion and HCO3- reabsorption) Hypokalemia (⇈ H+ secretion and HCO3- reabsorption) Hypovolemia (increased H+) Retention of Bicarbonate Ions – Increased HCO3- NaHCO3 infusion or ingestion  53 Metabolic Alkalosis Causes: Excess of bicarbonate Ingestion of alkali (GI upset meds) NGT drainage Vomiting & HCL losses Kidneys may not be able to excrete excess bicarbonate Hypokalemia, chloride depletion, excess mineralocorticoid or glucocorticoid  54 Causes of Metabolic Acidosis Increase in Body Acid and H+: Lactic acidosis Diabetic Ketoacidosis Chronic Renal Failure Renal: Kidneys are not removing enough acid from the body. Loses Bicarbonate Ions – Decreased HCO3-  55 Metabolic Acidosis Causes: Loss of base (HCO3-) DKA Lactic acidosis Renal failure Alcoholic ketoacidosis ASA intoxication Diarrhea Shock  56 ABG’s NICU  57 Venous Blood Gases in the NICU  58 Acceptable Ranges  59 Interpretation of the ABG -Acid/Base Compensation mechanisms: -Primary compensation = lungs, blowing off or retaining of CO2 (acid) -Secondary compensation = kidneys, retaining or losing of HCO3- (base) -Compensation can be partial or full. -In an attempt to regain homeostasis the body will compensate for changes in acid/base status bringing the pH back toward normal.  60 Interpret The ABG2 Partial Compensation: -the body is trying to compensate but has not moved the pH back to normal. Partial Compensation Sample ABG’s: pH: 7.33; PaCO2: 55; PaO2: 89; HCO3-: 28; SaO2: 96% on Room Air. pH: 7.47; PaCO2: 31; PaO2: 98; HCO3-: 19; SaO2: 99% on Room Air.  61 Interpret The ABG4 Sample ABG’s: pH: 7.30; PaCO2: 40; HCO3-:20 pH: 7.50; PaCO2: 40; HCO3-:29  62 Interpret The ABG5 Sample ABG’s: pH: 7.50; PaCO2: 30; PaO2: 105; HCO3-: 24; SaO2: 99% on Room Air. pH: 7.30; PaCO2: 50; PaO2: 59; HCO3-:24; SaO2: 89%  63 Interpret The ABG6 pH: 7.30; PaCO2: 40; PaO2:95; HCO3-:20; SaO2: 97% Interpretation: pH: 7.50; PaCO2: 40; PaO2:97; HCO3-:29; SaO2: 99% Interpretation:  64 Sample ABG’s to Interpret7 (all ABG’s taken on Room Air) 1. pH: 7.26; PaCO2: 56; PaO2: 55; HCO3-: 24 2. pH: 7.36; PaCO2: 83; PaO2: 50; HCO3-: 48 3. pH: 7.24; PaCO2: 30; PaO2:102; HCO3-:14 4. pH: 7.55; PaCO2: 52; PaO2: 81; HCO3-: 44 5. pH: 7.42; PaCO2: 28; PaO2: 65; HCO3-: 18  65 Sample ABG’s to Interpret8 (all ABG’s taken on Room Air) 6. pH: 7.48; PaCO2: 54; PaO2: 40; HCO3-: 37 7. pH: 7.15; PaCO2: 65; PaO2: 39; HCO3-: 17 8. pH: 7.55; PaCO2: 20; PaO2: 62; HCO3-: 18 9. pH: 7.24; PaCO2: 38; PaO2: 98; HCO3-: 15 10. pH: 7.27; PaCO2: 53; PaO2: 50; HCO3-: 24  66 References: 1. Radaeopedia.org 2. Wikipedia 3. Fundamental Critical Care Support (FCCS) – 5th ed. Society of Critical Care Medicine – 2012 4. Kettering Registered Respiratory Therapy Review - 2007