Acid Base Physiology 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 animalLow oxygenation vs. blowing off CO2

Assessing vomiting animalsMetabolic alkalosis with high outflow

obstruction vs. metabolic acidosis

Diabetic animalsKetoacidosis vs. ketosis alone

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.

Acid-Base Physiology

pH = -log [H+] ↑H+ ≈ ↓pH ↓H+ ≈ ↑pH

Carbonic acid equation H+ + HCO3 H2CO3 H20 + CO2


Metabolic acidosis

Defined as an increase in H+

Metabolic Acidosis

CausesLactic acidosis

DehydrationHypovolemiaHypoxiaAnemia

Ketoacidosis

Metabolic Acidosis

Causes Renal failure

Decreased ability to excrete H+Increased excretion of HCO3

Gastrointestinal loss of HCO3

Renal tubular acidosis

Metabolic Acidosis

Causes Miscellaneous

Aspirin overdoseMethanolEthanol

Metabolic Acidosis

What effect does an increased H+ have on the carbonic acid equation?

H+ + HCO3 H2CO3 H20 + CO2

.

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 mechanismsExtracellular 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)

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


End result:Mildly increased H+

Mildly decreased pHDecreased HCO3

Decreased CO2 (compensation)


Metabolic alkalosis

Defined as a ↓H+

Metabolic Alkalosis

CausesHigh outflow GI obstruction

Loss of hydrogen and chlorideDiuretic therapyIatrogenic

Metabolic Alkalosis

CausesHypokalemia

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?


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, CO2, & pH

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


End result:Mildly decreased H+

Mildly increased pH Increased HCO3

Increased PCO2 (compensation)


Respiratory alkalosis

Defined as a ↓CO2

Respiratory Alkalosis

CausesHypoxemia

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


CausesHypoxemia Examples

Mild to moderate pneumonia or CHFPTEInterstitial fibrosis


CausesStimulation of respiratory center

Intracranial diseaseHepatic encephalopathyGram negative sepsisRapid correction of metabolic acidosis

Over compensationLast 24-48 hrs


What effect does ↓CO2 have on the body?



↓CO2 Causes a shift to the right

H+ + HCO3 H2CO3 H20 + ↓CO2

Leading to ↓H+ & ↓HCO3 & ↑pH


See both an acute

and a chronic

Metabolic

compensatory

response.


Acute compensationUse nonbicarbonated buffersCl/HCO3 exchange in cell membranesSimilar for both dogs and catsOccurs with in 15 minutes


Chronic compensationDogs

Renal adaptationIncrease H+ retentionIncrease HCO3 excretionTakes 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 pHDecreased PCO2

Decreased HCO3 (Compensation)


Respiratory acidosis

Defined as a ↑CO2

Respiratory Acidosis

Causes of Respiratory acidosisCongestive heart failurePrimary lower airway diseaseUpper airway diseaseOthers


What effect does an increased CO2 have on the carbonic acid equation?



↑CO2 Causes a shift to the left

H+ + HCO3 H2CO3 H20 + ↑CO2

Leading to ↑H+ & ↑HCO3


See both an acute

and a chronic

Metabolic

compensatory

response.


Acute compensationCan not use HCO3 buffersUse 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


Chronic compensationDogs

Renal adaptationIncrease H+ excretionIncrease HCO3 retentionTakes 2-5 days to reach steady state

Cats may not be able to compensate


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

1. Is the patient acidic or alkalotic?

2. Does the PCO2 or HCO3 match the pH?

3. If PCO2 matches then it is respiratory, if HCO3 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


PCV/TS – 36%/5.8

Venous blood gas-pH – 7.1PCO2- 35HCO3- 14

Blood glucose - 78


What is her acid/base status?


pH – 7.1PCO2- 35HCO3- 14

Classified as uncompensated metabolic acidosis


20 minutes post initial presentation obtained arterial blood gas (On O2)

pH – 7.28 HCO3- 5.0

PCO2- 10.1 PO2 – 189


Now what is her acid/base status?


pH – 7.28 HCO3- 5.0

PCO2- 10.1 PO2 – 189

Classified as compensated metabolic acidosis


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 – 5.0PO2 – 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


48 hrs post Lasix therapy Venous blood gas was performedpH – 7.553PCO2- 40.5HCO3- 35.7


What is his acid/base status?


pH – 7.553PCO2- 40.5HCO3- 35.7

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


Thoracic radiographs – cardiomegally, diffuse alveolar pattern

Venous blood gas- pH – 7.15PCO2 – 56HCO3 - 20


What is her acid/base status?


pH – 7.15PCO2 – 56HCO3 - 20

Classified as uncompensated respiratory acidosis

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


Physical examinationDepressed, pale pink mm, 5% dehydratedTachypnic and slightly dyspnicThoracic auscultation – fine crackles

bilaterally, harsh BV soundsThoracic radiographs, venous blood gas


Thoracic radiographs – diffuse bronchointerstitial pattern

Venous blood gas –pH 7.35PCO2 – 43HCO3 - 30


What is his acid/base status?


pH 7.35PCO2 – 43HCO3 - 30

Classified as a compensated respiratory acidosis

Questions

Therapy for Acid Base Disorders

Metabolic Acidosis

Detrimental effects of AcidosisDecreased myocardial contractility

when pH < 7.2Predispose heart to VPCsPeripheral 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


Blood transfusions for anemia


NaHCO3 supplementUse only when dehydration,

hypovolemia, and anemia have been addressed

pH is < 7.2Do not use when body has not shown

compensation with a low PCO2


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.


NaHCO3 supplementMonitor acid-base status q 6 hrs Stop supplement when pH is 7.2Recheck acid-base status 6 hrs later

to make sure further supplementation is not needed.

Metabolic Alkalosis

Renal physiologyNormally 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 alkalosisDecreased Cl- to

Macula Densa Increased Renin

excretion Increases distal

tubule H+ secretion

Metabolic Alkalosis

Renal physiologyHypochloremic

metabolic alkalosis

H+-ATPase pump in collecting tubule.

Metabolic Alkalosis

Renal physiologyHypokalemia

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

TreatmentIV fluids - .9% NaCl

Replenishes Cl-

Acidic fluidWith out Cl- you can not encourage H+

retention and HCO3 excretionK+ supplementationCorrect primary problem

Respiratory Acidosis and Alkalosis

TreatmentNo specific treatment necessaryTherapy directed towards the primary

problem.

Acid Base Physiology

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

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