Presentation on theme: "Acid Base Physiology By Heidi Allen, DVM, Dipl. ACVIM."— Presentation transcript:
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 CO 2 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 NaHCO 3 ? Dyspnic animals All though arterial blood gas is best we can make some assessments using venous samples.
Acid-Base Physiology pH = -log [H + ] H + pH Carbonic acid equation H + + HCO 3 H 2 CO 3 H CO 2
Acid-Base Physiology Metabolic acidosis Defined as an increase in H +
Metabolic Acidosis What effect does an increased H + have on the carbonic acid equation? H + + HCO 3 H 2 CO 3 H CO 2.
Metabolic Acidosis H + Causes a mild shift to the right H + + HCO 3 H 2 CO 3 H CO 2 Leading to HCO 3, & CO 2
Metabolic Acidosis Unfortunately this shift is not enough to overcome the increased hydrogen so H + is still very elevated. H, HCO 3, P CO 2, & pH
Metabolic Acidosis Compensatory mechanisms Extracellular buffering (decrease in HCO 3 ) 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)
Metabolic Acidosis Respiratory Compensation – Blow off CO 2 H + + HCO 3 H 2 CO 3 H CO 2 Allows equation to be pulled further to the right, decreasing H + and HCO 3
Compensated Metabolic Acidosis H + + HCO 3 H 2 CO 3 H CO 2 End result: Mildly increased H + Mildly decreased pH Decreased HCO 3 Decreased CO 2 (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
Metabolic Alkalosis What effect does decreased H+ have on the carbonic acid equation? H + + HCO 3 H 2 CO 3 H CO 2
Metabolic Alkalosis H + Causes a mild shift to the left H + + HCO 3 H 2 CO 3 H CO 2 Leading to HCO 3 & CO 2
Metabolic Alkalosis Unfortunately this shift is not enough to overcome the decreased hydrogen so H + is still very low. H + + HCO 3 H 2 CO 3 H CO 2 H, HCO 3, CO 2, & pH
Metabolic Alkalosis Respiratory Compensation – Retain CO 2 H + + HCO 3 H 2 CO 3 H CO 2 Allows equation to be pulled further to the left, increasing H + but also increasing HCO 3
Compensated Metabolic Alkalosis H + + HCO 3 H 2 CO 3 H CO 2 End result: Mildly decreased H + Mildly increased pH Increased HCO 3 Increased P CO 2 (compensation)
Acid-Base Physiology Respiratory alkalosis Defined as a CO 2
Respiratory Alkalosis Causes Hypoxemia In some cases of respiratory disease oxygen can not get into blood stream but CO 2 can get out. In these cases the body increases respiratory rate in response to hypoxemia which P CO 2
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 hrs
Respiratory Alkalosis What effect does CO 2 have on the body? H + + HCO 3 H 2 CO 3 H CO 2
Respiratory Alkalosis CO 2 Causes a shift to the right H + + HCO 3 H 2 CO 3 H CO 2 Leading to H + & HCO 3 & pH
Respiratory Alkalosis See both an acute and a chronic Metabolic compensatory response.
Respiratory Alkalosis Acute compensation Use nonbicarbonated buffers Cl/HCO 3 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 HCO 3 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 CO 2
Compensated Respiratory Alkalosis H + + HCO 3 H 2 CO 3 H CO 2 End result: Mildly decreased H + Mildly increased pH Decreased P CO 2 Decreased HCO 3 (Compensation)
Acid-Base Physiology Respiratory acidosis Defined as a CO 2
Respiratory Acidosis What effect does an increased CO 2 have on the carbonic acid equation? H + + HCO 3 H 2 CO 3 H CO 2
Respiratory Acidosis CO 2 Causes a shift to the left H + + HCO 3 H 2 CO 3 H CO 2 Leading to H + & HCO 3
Respiratory Acidosis See both an acute and a chronic Metabolic compensatory response.
Respiratory Acidosis Acute compensation Can not use HCO 3 buffers Use proteins such as hemoglobin H 2 CO 3 + Buf HBuf +HCO 3 Can cause an increase in HCO 3 1Meq/L per 10 mmHg P CO 2 Works poorly as a buffer
Respiratory Acidosis Chronic compensation Dogs Renal adaptation Increase H + excretion Increase HCO 3 retention Takes 2-5 days to reach steady state Cats may not be able to compensate
Respiratory Acidosis Metabolic Compensation – Increased HCO 3 H + + HCO 3 H 2 CO 3 H CO 2 -- Allows equation to be pulled back to the right, decreasing H +
Compensated Respiratory Acidosis Unfortunately this is not enough to override the CO 2
Compensated Respiratory Acidosis H + + HCO 3 H 2 CO 3 H CO 2 End result: Mildly increased H + Mildly decreased pH Increased P CO 2 Increased HCO 3 (Compensation)
Normal values Venous blood gases Canine Feline pH – 7.397pH – P CO 2 – 37.4 P CO 2 – 38.7 HCO 3 –22.5HCO 3 – 20.6 P O 2 – 52.1 (?)
Normal values Arterial blood gases Canine Feline pH – 7.407pH – P CO 2 – 36.8 P CO 2 – 31.0 HCO 3 –22.2HCO 3 – 18.0 P O 2 – 92.1P O 2 – 106.8
Acid Base Analysis 1. Is the patient acidic or alkalotic? 2. Does the P CO 2 or HCO 3 match the pH? 3. If P CO 2 matches then it is respiratory, if HCO 3 matches it is metabolic. 4. The other value measures compensation.
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 What is her acid/base status?
Maggie 10-year-old F/S Lab pH – 7.1 P CO HCO Classified as uncompensated metabolic acidosis
Maggie 10-year-old F/S Lab 20 minutes post initial presentation obtained arterial blood gas (On O 2 ) pH – 7.28 HCO PCO PO 2 – 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 HCO PCO PO 2 – 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 PCO PCO 2 – 10.1 HCO HCO 3 – 5.0 PO 2 – 189 Does she need oxygen?
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 – P CO HCO
Tom 9-year-old M/C DMH What is his acid/base status?
Tom 9-year-old M/C DMH pH – P CO HCO Classified as a metabolic alkalosis with respiratory compensation
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
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 HCO Classified as a compensated respiratory acidosis
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 NaHCO 3 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 P CO 2
Metabolic Acidosis Treatment NaHCO 3 supplement.3 x [Wt (Kg) ] x BE BE = Normal HCO 3 (24) – HCO 3 of patient If pH < 7.1 then bolus 25% Give 50% over 12 hrs.
Metabolic Acidosis Treatment NaHCO 3 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 HCO 3 and conserve H + Normal human patients given 1,000 mEq NaHCO 3 /day for 2 weeks excreted virtually all of the HCO 3 Metabolic alkalosis has significantly less HCO 3 load - Burton David Rose, 1994
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 Replenishes Cl - Acidic fluid With out Cl - you can not encourage H + retention and HCO 3 excretion K + supplementation Correct primary problem
Respiratory Acidosis and Alkalosis Treatment No specific treatment necessary Therapy directed towards the primary problem.