1 By Heidi Allen, DVM, Dipl. ACVIM Acid Base PhysiologyBy Heidi Allen, DVM, Dipl. ACVIM
2 Why is assessing acid-base status important? Assessing the tachypnic animalLow oxygenation vs. blowing off CO2Assessing vomiting animalsMetabolic alkalosis with high outflow obstruction vs. metabolic acidosisDiabetic animalsKetoacidosis vs. ketosis alone
3 Why is assessing acid-base status important? Renal failure animalsDo you need to supplement NaHCO3?Dyspnic animalsAll though arterial blood gas is best we can make some assessments using venous samples.
15 Acid-Base PhysiologyMetabolic alkalosisDefined as a ↓H+
16 Metabolic Alkalosis Causes High outflow GI obstruction Loss of hydrogen and chlorideDiuretic therapyIatrogenic
17 Metabolic Alkalosis Causes Hypokalemia Compensation for hypokalemia is to move K out of cells using H-K exchangeHydrogen goes into cells causing loss of H in blood stream and metabolic alkalosis
18 What effect does decreased H+ have on the carbonic acid equation? Metabolic AlkalosisWhat effect does decreased H+ have on the carbonic acid equation?H+ + HCO3 H2CO3 H20 + CO2
19 ↓H+ Causes a mild shift to the left Metabolic Alkalosis↓H+ Causes a mild shift to the left↓H+ + HCO3 H2CO3 H20 + CO2Leading to HCO3 & CO2
20 Metabolic AlkalosisUnfortunately this shift is not enough to overcome the decreased hydrogen so H+ is still very low.H+ + HCO3 H2CO3 H20 + CO2H, HCO3, CO2, & pH
21 Respiratory Compensation Metabolic AlkalosisRespiratory Compensation– Retain CO2H+ + HCO3 H2CO3 H20 + ↑CO2Allows equation to be pulled further to the left, increasing H+ but also increasing HCO3
23 Respiratory alkalosis Acid-Base PhysiologyRespiratory alkalosisDefined as a ↓CO2
24 Respiratory Alkalosis CausesHypoxemiaIn some cases of respiratory disease oxygen can not get into blood stream but CO2 can get out.In these cases the body increases respiratory rate in response to hypoxemia which PCO2
25 Respiratory Alkalosis CausesHypoxemia ExamplesMild to moderate pneumonia or CHFPTEInterstitial fibrosis
26 Respiratory Alkalosis CausesStimulation of respiratory centerIntracranial diseaseHepatic encephalopathyGram negative sepsisRapid correction of metabolic acidosisOver compensationLast hrs
27 Respiratory Alkalosis What effect does ↓CO2 have on the body?H+ + HCO3 H2CO3 H20 + CO2
28 Respiratory Alkalosis ↓CO2 Causes a shift to the rightH+ + HCO3 H2CO3 H20 + ↓CO2Leading to ↓H+ & ↓HCO3 & ↑pH
29 Respiratory Alkalosis See both an acuteand a chronicMetaboliccompensatoryresponse.
30 Respiratory Alkalosis Acute compensationUse nonbicarbonated buffersCl/HCO3 exchange in cell membranesSimilar for both dogs and catsOccurs with in 15 minutes
31 Respiratory Alkalosis Chronic compensationDogsRenal adaptationIncrease H+ retentionIncrease HCO3 excretionTakes 2-5 days to reach steady stateMaybe a similar mechanism in cats
32 Compensated Respiratory Alkalosis Unfortunately this is not enough to override the ↓CO2
46 Acid Base Analysis Is the patient acidic or alkalotic? Does the PCO2 or HCO3 match the pH?If PCO2 matches then it is respiratory, if HCO3 matches it is metabolic.The other value measures compensation.
55 Maggie 10-year-old F/S Lab How do her clinical signs match up?Tachycardia, pale gums, poor pulsesTachypneaVenous blood gas Arterial blood gaspH – pH – 7.28PCO PCO2 – 10.1HCO HCO3 – 5.0PO2 – 189Does she need oxygen?
74 Metabolic Acidosis Detrimental effects of Acidosis Decreased myocardial contractility when pH < 7.2Predispose heart to VPCsPeripheral insulin resistanceObtunded state or coma
75 Metabolic Acidosis Treatment IV fluids to address dehydration or hypovolemia –Use pH balanced fluids such as LRS or Norm R
76 Metabolic Acidosis Treatment Blood transfusions for anemia
77 Metabolic Acidosis Treatment NaHCO3 supplementUse only when dehydration, hypovolemia, and anemia have been addressedpH is < 7.2Do not use when body has not shown compensation with a low PCO2
78 Metabolic Acidosis Treatment NaHCO3 supplement.3 x [Wt(Kg)] x BEBE = Normal HCO3 (24) – HCO3 of patientIf pH < 7.1 then bolus 25%Give 50% over 12 hrs.
79 Metabolic Acidosis Treatment NaHCO3 supplementMonitor acid-base status q 6 hrsStop supplement when pH is 7.2Recheck acid-base status 6 hrs later to make sure further supplementation is not needed.
80 Metabolic Alkalosis Renal physiology Normally expect the kidney to excrete HCO3 and conserve H+Normal human patients given 1,000 mEq NaHCO3 /day for 2 weeks excreted virtually all of the HCO3Metabolic alkalosis has significantly less HCO3 load- Burton David Rose, 1994
83 Metabolic Alkalosis Renal physiology Hypokalemia Increased H+/K+ exchange leading to influx of H+ into cells. This leads to H+ secretion in renal tubules.Severe hypokalemia causes renal excretion of Cl-
84 Metabolic Alkalosis Treatment IV fluids - .9% NaCl K+ supplementation Replenishes Cl-Acidic fluidWith out Cl- you can not encourage H+ retention and HCO3 excretionK+ supplementationCorrect primary problem
85 Respiratory Acidosis and Alkalosis TreatmentNo specific treatment necessaryTherapy directed towards the primary problem.