Jeff Kaufhold, MD FACP 2013 Source: The ICU Book Chapter 36-38 Approach to Acid Base Jeff Kaufhold, MD FACP 2013 Source: The ICU Book Chapter 36-38

Acid Base Disorders Basic Physiology Approach to A-B disorders Equations Cases Electrolytes

Acid Base Physiology Balance based on Henderson-Hasselbach equation Hydrogen Ion measured in NanoEquivalents , i.e. very tightly regulated. Buffers for acid: Bicarbonate, Hemoglobin, Albumin, Bone Acid excretion: Lungs and Kidneys

Acid - Base Physiology Sources of Acid: 70 mEq daily as inorganic acids H2SO4, H3PO4 Excreted by kidney 16,000 mMole as CO2 Excreted by lungs 32,000 mEq organic acid which gets metabolised

Acid – Base Physiology Lung blows off acid as CO2 Kidney H2CO3  H2O + CO2 Kidney Proximal tubule has lumenal carbonic anhydrase to reclaim bicarb from urine Distal tubule has basolateral CA to pull H+ out of blood, complexed to NH3 to form NH4Cl (ammonium chloride) Can measure activity by urinary Anion Gap (Na+K – Cl)

ABG reporting Normals: Reported as pH / pCO2 / pO2 / HCO3 pCO2 36 – 44 mmHg pO2 over 60 HCO3 (bicarb) 22 – 26 Reported as pH / pCO2 / pO2 / HCO3 Example 7.40 / 40 / 80 / 23

Compensatory Mechanisms Disorder Resp Acidosis Resp Alkalosis Met Acidosis Met Alkalosis End point is a constant ratio of: Primary / Compens Pco2 up HCO3 up Pco2 down, HCO down HCO down, Pco2 down HCO up Pco2 up PCO2 / HCO3

Rules for interpretation 1. Primary Metabolic disorders: PCO2 and pH move in same direction Can be quickly identified if last 2 digits of pH = pCO2 i.e 7.20 / 20 / 70 7.51 / 50 / 120 If 7.20 / 40 / 70, not simple disorder Respiratory compensation not adequate

Rules for interpretation 3. Primary Respiratory disorders: PCO2 and pH move in opposite direction Resp acidosis: 7.20 / 58 / 68 Resp alkalosis: 7.50 / 25/ 80 7.25 / 48 / 52 Not a simple disorder Neither is 7.40 / 25 / 68

Rules for interpretation Rule 5 Mixed Disorders: Compensatory responses do not completely correct primary problem Can have BOTH metabolic acidosis and metabolic alkalosis simultaneously (sepsis plus vomiting is common) But can only hyperventilate (resp alk) OR hypoventilate (resp acidosis). Usually due to failure to adequately lower pCO2 (resp fatigue)

Expected Changes Rules of Thumb Met Acidosis Pco2 = 1.5 * bicarb + 8 +/- 2 7.20 / 20 / 68 / HCO3 = 8 Met Alkalosis Pco2 = 0.7 * HCO3 + 21 +/- 2 7.50 / 50 / 120 / HCO3 = 40

Expected Changes Acute Resp Acidosis Acute Resp Alkalosis Change in pH = 0.008 * change pCO2 7.30 / 50 / 68 / HCO3 = 22 Acute Resp Alkalosis 7.50 / 20 / 120 / HCO3 = 20 Shortcut: D pH = 0.08 for each 10 mmHg change in pCO2

Expected Changes Chronic Resp Acidosis Chronic Resp Alkalosis Change in pH = 0.003 * change pCO2 7.33 / 50 / 68 / HCO3 = 32 Chronic Resp Alkalosis Change in pH = 0.0017 * change pCO2 7.48 / 25 / 120 / HCO3 = 20

Expected Changes Note that the chronic respiratory conditions induce chronic changes in bicarbonate handling by the kidney, which takes time.

Common Causes of A-B Disorders Met Acidosis MUD PILES Met Alkalosis Vomiting, NG suction, IV infusion of bicarb or Acetate (TPN), Diuretics Resp Acidosis : respiratory failure, COPD exacerbation, Narcotics Resp Alkalosis: Hyperventilation/anxiety, Pregnancy, Early sepsis, early stage asthma, chronic alcoholism

Anion Gap Useful for evaluation of metabolic acidosis. Calculate AG every time you see electrolytes, esp on exams. AG = Anions – Cations AG = Na + K – (CL + HCO3) AG = 140 + 4 – (102 + 25) = 8 to 16 > 18 nephrologist gets excited

Anion Gap Low AG suggests extra plasma proteins, such as Myeloma Non Anion Gap acidosis suggests loss of bicarbonate through either: Diarrhea Renal Tubular Acidosis (RTA) Or Infusion of acid via NS, TPN High AG means accumulation of acid MUDD PILES or other nemonic

Anion Gap High AG means accumulation of acid Organic acid can be metabolised back to bicarbonate So AG over 16 or so can represent “potential Bicarb” or “delta gap”. Calculating the potential bicarb is useful for identifying mixed disorders, since both met acidosis and met alkalosis can be present at same time. Delta Gap = AG – 12 (extra anions) Potential bicarb = HCO3 + delta gap

Potential Bicarb HCO3 + Delta gap=Potential bicarb

Gapped Metabolic Acidosis Methanol (wood alcohol) Uremia/acute renal failure DKA D lactate (Metformin/blind loop in bowel) Paraldehyde Isopropyl alcohol Lactic acidosis (hypoxia/hypoperfusion) Ethanol Starvation Ketosis

Osmolal Gap Used to help identify presence of exogenous acids which may be causing gapped metabolic acidosis Measured Osm – calculated Osm Osm calc = 2(Na) + Glucose/18 + BUN/30 OG > 20 suggests Ethanol, Methanol

Clinical Problems in A-B 1. Calculate AG and potential bicarb 2. Is pH acidic or alkalotic 3. Is pCO2 alkalotic or acidotic < 40 > 40 4. Is the bicarb measured by the ABG machine same as that on lytes? If not the samples are not simultaneous 5. Apply the rules of thumb If values are consistent, Simple disorder If values are not consistent: Mixed.

Case 1 38 y.o. male with chronic GN. Admitted with weakness, fatigue. BUN 100 cr 6 Labs: Na 134, K 5.6, Cl 100, CO2 14. ABG: 7.26 / 27 / 72 / 14 What is A-B disturbance? AG = Delta gap/Potential bicarb = Acidosis or alkalosis ? Direction of CO2 ? MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

Case 1 Labs: Na 134, K 5.6, Cl 100, CO2 14. ABG: 7.26 / 27 / 72 / 14 What is A-B disturbance? AG = 20 Delta gap/Potential bicarb = 8 / 22 Acidosis or alkalosis ? acidosis Direction of CO2 ? alkalosis MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 works So simple metabolic acidosis with appropriate respiratory compensation

Case 2 23 y.o. garage mechanic presents with acute confusion after drinking antifreeze. Lab: Na 137 K 5.4 Cl 105 HCO 5 ABG: 6.95 / 15 / 80 / 4 What is A-B disturbance? AG = Delta gap/Potential bicarb = Acidosis or alkalosis ? Direction of CO2 ? MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

Case 2 Lab: Na 137 K 5.4 Cl 105 HCO 5 ABG: 6.95 / 15 / 80 / 4 What is A-B disturbance? AG = 27 Delta gap/Potential bicarb = 15 / 20 Acidosis or alkalosis ? acidosis Direction of CO2 ? alkalosis MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 works so simple met acidosis with appropriate resp compensation.

Case 3 65 y.o. recently discharged after pneumonia. Presents with C dif Diarrhea. Lab: Na 132 K 2.4 Cl 105 HCO 15 ABG: 7.30 / 30 / 80 / 14 What is A-B disturbance? AG = Delta gap/Potential bicarb = Acidosis or alkalosis ? Direction of CO2 ? MA rule: pCO2 = 1.5*HCO3 + 8 +/-2

Case 3 Lab: Na 132 K 2.4 Cl 105 HCO 15 ABG: 7.30 / 30 / 80 / 14 What is A-B disturbance? AG = 12 Delta gap/Potential bicarb = 0 / 15 Acidosis Direction of CO2 ? alkalotic MA rule: pCO2 = 1.5*HCO3 + 8 +/-2 Works so Non Gap Metabolic Acidosis

Case 4 40 y.o. with status asthmaticus. Lab: Na 135 K 3.4 Cl 100 HCO 21 ABG: 7.50 / 30 / 80 / 20 What is A-B disturbance? AG = Delta gap/Potential bicarb = Acidosis or alkalosis ? Direction of CO2 ? Acute RA rule: DpH = 0.008 * DpCO2

Case 4 40 y.o. with status asthmaticus. Lab: Na 135 K 3.4 Cl 100 HCO 21 ABG: 7.50 / 30 / 80 / 20 What is A-B disturbance? AG = 14 Delta gap/Potential bicarb = 23 alkalosis Direction of CO2 ? alkalosis Acute RA rule: DpH = 0.008 * DpCO2 works

Case 5 32 y.o. chronic pyelo, CRF, pleural effusion. Admitted for thoracentesis Lab: Na 130 K 5.0 Cl 94 HCO 15 ABG: 7.32 / 32 / 55 / 15 Attempted thoracentesis unsuccessful, became dyspneic and coded as blood drawn: Na 131/k 7.8/cl 92/CO 9/ 6.9/50/40

Case 5 CRF with pneumothorax Admission AG = 20 Appropriate resp compensation Uremic metabolic acidosis After thoracentesis: Resp acidosis, plus met acidosis Met acidosis worse, gap up to 30. Due to uremia plus lactic acidosis.

Case 6 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. Lab: Na 142 K 3.4 Cl 98 HCO 20 ABG: 7.28 / 41 / 58 / 20 What is A-B disturbance? AG = Delta gap/Potential bicarb = Acidosis or alkalosis ? Direction of CO2 ?

Case 6 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. Lab: Na 142 K 3.4 Cl 98 HCO 20 ABG: 7.28 / 41 / 58 / 20 What is A-B disturbance? AG = 24 Delta gap/Potential bicarb = 12 / 32 Acidosis or alkalosis ? Acidosis Direction of CO2 ? Not appropriate What is going on?

Case 6 44 y.o. alcoholic admitted with abd pain and N/V/Fever/chills. SOB and CXR shows pneumonia with effusion. Lab: Na 142 K 3.4 Cl 98 HCO 20 ABG: 7.28 / 41 / 58 / 20 Pneumonia is interfering with resp compensation. Also see this with mechanical problems of ventilation such as flail chest, paralyzed hemidiaphragm, abdominal compartment syndrome.