By Heidi Allen, DVM, Dipl. ACVIM Acid Base Physiology By Heidi Allen, DVM, Dipl. ACVIM

Why is assessing acid-base status important? Assessing the tachypnic animal Low oxygenation vs. blowing off CO2 Assessing vomiting animals Metabolic alkalosis with high outflow obstruction vs. metabolic acidosis Diabetic animals Ketoacidosis vs. ketosis alone

Why is assessing acid-base status important? Renal failure animals Do you need to supplement NaHCO3? Dyspnic animals All though arterial blood gas is best we can make some assessments using venous samples.

Acid-Base Physiology pH = -log [H+] Carbonic acid equation ↑H+ ≈ ↓pH H+ + HCO3  H2CO3  H20 + CO2

Defined as an increase in H+ Acid-Base Physiology Metabolic acidosis Defined as an increase in H+

Metabolic Acidosis Causes Lactic acidosis Ketoacidosis Dehydration Hypovolemia Hypoxia Anemia Ketoacidosis

Metabolic Acidosis Causes Renal failure Gastrointestinal loss of HCO3 Decreased ability to excrete H+ Increased excretion of HCO3 Gastrointestinal loss of HCO3 Renal tubular acidosis

Metabolic Acidosis Causes Miscellaneous Aspirin overdose Methanol

What effect does an increased H+ have on the carbonic acid equation? Metabolic Acidosis What effect does an increased H+ have on the carbonic acid equation? H+ + HCO3  H2CO3  H20 + CO2 .

↑H+ Causes a mild shift to the right Metabolic Acidosis ↑H+ Causes a mild shift to the right ↑H+ + HCO3  H2CO3  H20 + CO2 Leading to HCO3, & CO2

Metabolic Acidosis Unfortunately this shift is not enough to overcome the increased hydrogen so H+ is still very elevated.  H, HCO3, PCO2, &  pH

Metabolic Acidosis Compensatory mechanisms Extracellular buffering (decrease in HCO3) Intracellular buffering Hydrogen enters cells in exchange for K, proteins, phosphate, and bone carbonate Respiratory compensation (takes 1-2 hrs) Renal hydrogen excretion (takes 2-5 days)

Respiratory Compensation Metabolic Acidosis Respiratory Compensation – Blow off CO2 H+ + HCO3 H2CO3  H20 + ↓CO2 Allows equation to be pulled further to the right, decreasing H+ and HCO3

Compensated Metabolic Acidosis H+ + HCO3 H2CO3  H20 + CO2 End result: Mildly increased H+ Mildly decreased pH Decreased HCO3 Decreased CO2 (compensation)

Acid-Base Physiology Metabolic alkalosis Defined as a ↓H+

Metabolic Alkalosis Causes High outflow GI obstruction Loss of hydrogen and chloride Diuretic therapy Iatrogenic

Metabolic Alkalosis Causes Hypokalemia Compensation for hypokalemia is to move K out of cells using H-K exchange Hydrogen goes into cells causing loss of H in blood stream and metabolic alkalosis

What effect does decreased H+ have on the carbonic acid equation? Metabolic Alkalosis What effect does decreased H+ have on the carbonic acid equation? H+ + HCO3  H2CO3  H20 + CO2

↓H+ Causes a mild shift to the left Metabolic Alkalosis ↓H+ Causes a mild shift to the left ↓H+ + HCO3  H2CO3  H20 + CO2 Leading to HCO3 & CO2

Metabolic Alkalosis Unfortunately this shift is not enough to overcome the decreased hydrogen so H+ is still very low. H+ + HCO3  H2CO3  H20 + CO2 H, HCO3, CO2, & pH

Respiratory Compensation Metabolic Alkalosis Respiratory Compensation – Retain CO2 H+ + HCO3  H2CO3  H20 + ↑CO2 Allows equation to be pulled further to the left, increasing H+ but also increasing HCO3

Compensated Metabolic Alkalosis H+ + HCO3  H2CO3  H20 + CO2 End result: Mildly decreased H+ Mildly increased pH Increased HCO3 Increased PCO2 (compensation)

Respiratory alkalosis Acid-Base Physiology Respiratory alkalosis Defined as a ↓CO2

Respiratory Alkalosis Causes Hypoxemia In 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

Respiratory Alkalosis Causes Hypoxemia Examples Mild to moderate pneumonia or CHF PTE Interstitial fibrosis

Respiratory Alkalosis Causes Stimulation of respiratory center Intracranial disease Hepatic encephalopathy Gram negative sepsis Rapid correction of metabolic acidosis Over compensation Last 24-48 hrs

Respiratory Alkalosis What effect does ↓CO2 have on the body? H+ + HCO3  H2CO3  H20 + CO2

Respiratory Alkalosis ↓CO2 Causes a shift to the right H+ + HCO3  H2CO3  H20 + ↓CO2 Leading to ↓H+ & ↓HCO3 & ↑pH

Respiratory Alkalosis See both an acute and a chronic Metabolic compensatory response.

Respiratory Alkalosis Acute compensation Use nonbicarbonated buffers Cl/HCO3 exchange in cell membranes Similar for both dogs and cats Occurs with in 15 minutes

Respiratory Alkalosis Chronic compensation Dogs Renal adaptation Increase H+ retention Increase HCO3 excretion Takes 2-5 days to reach steady state Maybe a similar mechanism in cats

Compensated Respiratory Alkalosis Unfortunately this is not enough to override the ↓CO2

Compensated Respiratory Alkalosis ↓H+ +  HCO3  H2CO3  H20 +  CO2 End result: Mildly decreased H+ Mildly increased pH Decreased PCO2 Decreased HCO3 (Compensation)

Acid-Base Physiology Respiratory acidosis Defined as a ↑CO2

Respiratory Acidosis Causes of Respiratory acidosis Congestive heart failure Primary lower airway disease Upper airway disease Others

What effect does an increased CO2 have on the carbonic acid equation? Respiratory Acidosis What effect does an increased CO2 have on the carbonic acid equation? H+ + HCO3  H2CO3  H20 + CO2

↑CO2 Causes a shift to the left Respiratory Acidosis ↑CO2 Causes a shift to the left H+ + HCO3  H2CO3  H20 + ↑CO2 Leading to ↑H+ & ↑HCO3

Respiratory Acidosis See both an acute and a chronic Metabolic compensatory response.

Respiratory Acidosis Acute compensation Can not use HCO3 buffers Use proteins such as hemoglobin H2CO3 + Buf HBuf +HCO3 Can cause an increase in HCO3 1Meq/L per 10 mmHg PCO2 Works poorly as a buffer

Respiratory Acidosis Chronic compensation Dogs Renal adaptation Increase H+ excretion Increase HCO3 retention Takes 2-5 days to reach steady state Cats may not be able to compensate

Respiratory Acidosis Metabolic Compensation – Increased HCO3 ↓H+ + ↑HCO3  H2CO3  H20 + CO2 -- Allows equation to be pulled back to the right, decreasing H+

Compensated Respiratory Acidosis Unfortunately this is not enough to override the ↑CO2

Compensated Respiratory Acidosis ↑H+ +  HCO3  H2CO3  H20 +  CO2 End result: Mildly increased H+ Mildly decreased pH Increased PCO2 Increased HCO3 (Compensation)

Normal values Venous blood gases Canine Feline pH – 7.397 pH – 7.343 PCO2 – 37.4 PCO2 – 38.7 HCO3 – 22.5 HCO3 – 20.6 PO2 – 52.1 (?)

Normal values Arterial blood gases Canine Feline pH – 7.407 pH – 7.386 PCO2 – 36.8 PCO2 – 31.0 HCO3 – 22.2 HCO3 – 18.0 PO2 – 92.1 PO2 – 106.8

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.

Maggie 10-year-old F/S Lab

Maggie 10-year-old F/S Lab History of acute collapse 1 hr ago Presents with pale gums, tachypnea, tachycardia, poor pulses PCV/TS/ Venous Blood gas/Glucose

Maggie 10-year-old F/S Lab PCV/TS – 36%/5.8 Venous blood gas- pH – 7.1 PCO2- 35 HCO3- 14 Blood glucose - 78

Maggie 10-year-old F/S Lab What is her acid/base status?

Maggie 10-year-old F/S Lab pH – 7.1 PCO2- 35 HCO3- 14 Classified as uncompensated metabolic acidosis

Maggie 10-year-old F/S Lab 20 minutes post initial presentation obtained arterial blood gas (On O2) pH – 7.28 HCO3- 5.0 PCO2- 10.1 PO2 – 189

Maggie 10-year-old F/S Lab Now what is her acid/base status?

Maggie 10-year-old F/S Lab pH – 7.28 HCO3- 5.0 PCO2- 10.1 PO2 – 189 Classified as compensated metabolic acidosis

Maggie 10-year-old F/S Lab How do her clinical signs match up? Tachycardia, pale gums, poor pulses Tachypnea Venous blood gas- Arterial blood gas pH – 7.1 pH – 7.28 PCO2- 35 PCO2 – 10.1 HCO3- 14 HCO3 – 5.0 PO2 – 189 Does she need oxygen?

Tom 9-year-old M/C DMH

Tom 9-year-old M/C DMH Presented for acute GI signs Developed CHF post abdominal exploratory Treated with Lasix

Tom 9-year-old M/C DMH 48 hrs post Lasix therapy Venous blood gas was performed pH – 7.553 PCO2- 40.5 HCO3- 35.7

What is his acid/base status? Tom 9-year-old M/C DMH What is his acid/base status?

Classified as a metabolic alkalosis with respiratory compensation Tom 9-year-old M/C DMH pH – 7.553 PCO2- 40.5 HCO3- 35.7 Classified as a metabolic alkalosis with respiratory compensation

Missy – 12-year-old F/S Bichon

Missy – 12-year-old F/S Bichon History of acute onset respiratory distress Presents with grade IV/VI heart murmur, crackles bilaterally, cyanosis Thoracic radiographs, venous blood gas

Missy – 12-year-old F/S Bichon Thoracic radiographs – cardiomegally, diffuse alveolar pattern Venous blood gas- pH – 7.15 PCO2 – 56 HCO3 - 20

Missy – 12-year-old F/S Bichon What is her acid/base status?

Missy – 12-year-old F/S Bichon pH – 7.15 PCO2 – 56 HCO3 - 20 Classified as uncompensated respiratory acidosis

Toby 12-year-old M/C Westie

Toby 12-year-old M/C Westie History of chronic cough x 2 years Recent worsening of cough Decreased appetite, lethargy x 1 week Increased respiratory rate, unwilling to walk this evening

Toby 12-year-old M/C Westie Physical examination Depressed, pale pink mm, 5% dehydrated Tachypnic and slightly dyspnic Thoracic auscultation – fine crackles bilaterally, harsh BV sounds Thoracic radiographs, venous blood gas

Toby 12-year-old M/C Westie Thoracic radiographs – diffuse bronchointerstitial pattern Venous blood gas – pH 7.35 PCO2 – 43 HCO3 - 30

Toby 12-year-old M/C Westie What is his acid/base status?

Toby 12-year-old M/C Westie pH 7.35 PCO2 – 43 HCO3 - 30 Classified as a compensated respiratory acidosis

Questions

Therapy for Acid Base Disorders

Metabolic Acidosis Detrimental effects of Acidosis Decreased myocardial contractility when pH < 7.2 Predispose heart to VPCs Peripheral insulin resistance Obtunded state or coma

Metabolic Acidosis Treatment IV fluids to address dehydration or hypovolemia – Use pH balanced fluids such as LRS or Norm R

Metabolic Acidosis Treatment Blood transfusions for anemia

Metabolic Acidosis Treatment NaHCO3 supplement Use only when dehydration, hypovolemia, and anemia have been addressed pH is < 7.2 Do not use when body has not shown compensation with a low PCO2

Metabolic Acidosis Treatment NaHCO3 supplement .3 x [Wt(Kg)] x BE BE = Normal HCO3 (24) – HCO3 of patient If pH < 7.1 then bolus 25% Give 50% over 12 hrs.

Metabolic Acidosis Treatment NaHCO3 supplement Monitor acid-base status q 6 hrs Stop supplement when pH is 7.2 Recheck acid-base status 6 hrs later to make sure further supplementation is not needed.

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 HCO3 Metabolic alkalosis has significantly less HCO3 load - Burton David Rose, 1994

Metabolic Alkalosis Renal physiology Hypochloremic metabolic alkalosis Decreased Cl- to Macula Densa Increased Renin excretion Increases distal tubule H+ secretion

Metabolic Alkalosis Renal physiology Hypochloremic metabolic alkalosis H+-ATPase pump in collecting tubule.

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-

Metabolic Alkalosis Treatment IV fluids - .9% NaCl K+ supplementation Replenishes Cl- Acidic fluid With out Cl- you can not encourage H+ retention and HCO3 excretion K+ supplementation Correct primary problem

Respiratory Acidosis and Alkalosis Treatment No specific treatment necessary Therapy directed towards the primary problem.

Acid Base Physiology