6 fluid and acid base balance m med07

8/17/2019 6 Fluid and Acid Base Balance M Med07

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Acid-Base disorders

DR. SHERIF EL DESOKY,MB.BCH, MSc, MRCP CH, MD

Prof. JAMEELA KARI, CABP, MD, CCST, FRCPCH, FRCP

PEDIATRICS DEPARTMENT

KING ABDULAZIZ UNIVERCITY HOSPITAL


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Learning objectives:

By the end of this lecture, you should be able to:

• Recognize normal acid base regulation

• Recognize the clinical acid-base relationship.

• Outline the causes of respiratory acidosis

• Outline the causes of respiratory alkalosis

• Outline the causes of metabolic acidosis

• Outline the causes of metabolic alkalosis

• Discuss the role of the kidney in handling the HCO3.


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Content of the lecture:

• Physiology of acid base balance.

• Case of respiratory acidosis (chest).

• Case of metabolic acidosis (Diabetic ketoacidosis , renal tubular acidosis)

• Example of metabolic alkalosis

•

Example of respiratory alkalosis


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"Potential of Hydrogen" (pH)• The acidity or alkalinity of a solution is measured as pH.

• The more acidic a solution, the lower the pH.

• The more alkaline a solution , the higher the pH.

• Water has a pH of 7 and is neutral.

• The pH of arterial blood is normally between 7.35 and

7.45


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Buffer Systems" Ability of weak acid and its corresponding base to

resist change in pH of a solution upon adding a strong acid or base"

• Regulate pH by binding or releasing Hydrogen

• Most important buffer system:

• Bicarbonate-Carbonic Acid Buffer System

• (Blood Buffer systems act instantaneously and thus constitute

the body’s first line of defense against acid-base imbalance)


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Clinically Significant Acid-Base Pairs

Acid

Carbonic acid (H2CO3)

Monobasic PO4 (H2PO4)

Ammonium (NH4+)

Lactic acid (H6C3O2)

Base

Bicarbonate (HCO3-)

Dibasic PO4 (HPO4-)

Ammonia (NH3)

Lactate (H5C3O2-)


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Respiratory Regulation

• Lungs

• help regulated acid-base balance by

eliminating or retaining carbon dioxide

• pH may be regulated by altering the rate and

depth of respirations• changes in pH are rapid,

• occurring within minutes

• normal CO2 level

• 35 to 45 mm Hg


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Renal Regulation

• Kidneys

• the long-term regulator of acid-base balance

• slower to respond

• may take hours or days to correct pH

• kidneys maintain balance by excreting or conserving

bicarbonate and hydrogen ions

• normal bicarbonate level

•

22 to 26 mEq/L.


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ABG interpretation


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Is it Respiratory or Metabolic?

1. Respiratory Acidosis

2. Respiratory Alkalosis

3. Metabolic Acidosis

4. Metabolic Alkalosis

• Increased pCO2 >50

• Decreased pCO2


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Compensated or Uncompensated—

what does this mean?

1. Evaluate pH—is it normal? Yes

2. Next evaluate pCO2 & HCO3

• pH normal + increased pCO2 + increased HCO3 =

compensated respiratory acidosis

• pH normal + decreased HCO3 + decreased pCO2 =

compensated metabolic acidosis


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Compensated vs. Uncompensated1. Is pH normal? No

2. Acidotic vs. Alkalotic3. Respiratory vs. Metabolic

• pH50 + normal HCO3 =uncompensated respiratory acidosis

• pH30 + normal pCO2 =uncompensated metabolic alkalosis


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Causes of Acidosis

•

Respiratory• Hypoventilation

• Impaired gas exchange

•

Metabolic• Ketoacidosis

• Diabetes

• Lactic Acidosis

• Decreased perfusion

• Severe hypoxemia

• Renal Failure

• Renal Tubular Acidosis

•

Severe Diarrheal illness


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Acidemia- Physiologic Effects

• Cardiovascular

• Mild acidemia--Tachycardia

• Severe acidemia -- Bradycardia

• Decreased fibrillation threshold

•

Decreased contractility• Neuromuscular

• Increase in CBF -- Headaches/Confusion

• Due to hypercarbia or pH ??


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Causes of Alkalosis

• Respiratory

• Hyperventilation due

to:

• Hypoxemia

• Metabolic acidosis

• Neurologic

CNS Lesions

CNS Trauma

Infection

• Metabolic

• Hypokalemia

• Gastric suction or

vomiting

• Hypochloremia


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Alkalemia- Physiology

•

CV• Mild--Slight increase contractility

• Oxyhgb curve shift left ( decrease O2 delivery to

tissue).

• Regional vasoconstriction


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Assessing Oxygenation

• Normal value for arterial blood gas 80-

100mmHg

• Normal value for venous blood gas 40mmHg

• Normal SaO2

• Arterial: 97%

• Venous: 75%

I t t i t f i ti


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Important points for assessing tissue

oxygenation

•This is the O2 that’s really available at the tissuelevel.

• Is the Hb normal?

• Low Hb means the ability of the blood to carry the O2

to the tissues is decreased

• Is perfusion normal?

• Low perfusion means the blood isn’t even getting to

the tissues


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Respiratory Acidosis

• Alveolar hypoventilation

• Retained Co2 drives release of free H+

• If acute, pH by .08 for pCo210 mm

• Correct any underlying cause


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Respiratory Alkalosis

•

min. Vent. pCo2 & pH• Most common causes

• Response to hypoxemia

• Response to acute metabolic acidosis

• CNS malfunction

• Correct underlying cause

• Rarely life-threatening


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Metabolic Acidosis

• Causes

• High anion gap = Na - (Cl + HC03)(eg.LA)

• Normal-anion gap (Hyperchloremic)

• Treatment• Correction of underlying cause

• Administer bicarbonate for life-threatening

acidosis


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Metabolic Alkalosis

• Usually results from excess acid losses

• Causes• Loss of gastric juices

• Diuretic therapy

• Adrenal cortical hormone excess

• Hepatic coma(hyper ammonemia )

• Administration of exogenous base

• Almost always accompanied by low K+

• Treatment


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Highly acidic, pH =1.0

Secretes HCO3

-

pH varies from

4.0 to 8.0

Vomiting:Loss of H+

leading to

alkalosis

Diarrhea:Loss of HCO3

-

leading to

acidosis

Gastrointestinal losses can create acid-base disturbances


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Approach to ABG

• Check serum pH –

• Acidemia or Alkalemia ?

• Check PCO2 –

• Is disturbance respiratory or metabolic ?

• Is respiratory disturbance acute ?? Change inpH= -0.08 x (d Pco2/10)

• Is respiratory disturbance non acute ?? Bicarb

change = 1-5 x (d Pco2/10)


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Approach to ABG

•

Check PaO2 ?? -Good guide to patient course..Not important with regard to Oxygen Delivery..

• SaO2 - 90 % acceptable for Oxygen delivery.


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• Metabolic acidosis

• Increased Acid Generation ??

• Metabolic alkalosis

• Urine chloride > 20 meq/L(Chloride Unresponsive - Eg.Adrenal excess)

• Urine chloride < 10 meq/L

(Chloride Responsive -dehydration)

• Primary problem is never exceeded by

compensation...


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Let’s Practice


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Case 1;

12 year old diabetic presents with Kussmaul breathing

• pH : 7.05

• pCO2: 12 mmHg

• pO2: 108 mmHg

• HCO3: 5 mEq/L

• BE: -30 mEq/L

• Severe partly compensated metabolic acidosis

without hypoxemia due to ketoacidosis

Case 2;


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Case 2;9 year old w/hx of asthma, audibly wheezing x 1 week, has

not slept in 2 nights; presents sitting up and using accessory

muscles to breath w/audible wheezes• pH: 7.51

• pCO2: 25 mmHg

• pO2 55 mmHg

• HCO3: 22 mEq/L

• BE: -2 mEq/L

• Uncompensated respiratory alkalosis with severe

hypoxia due to asthma exacerbation

If the previous child was untreated or came


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If the previous child was untreated or came

later:

• pH: 7.28

• pCO2: 55 mmHg

• pO2 35 mmHg

• HCO3: 28 mEq/L• BE: +6 mEq/L

• Partially compensated respiratory acidosis with

severe hypoxia due to asthma exacerbation

Case 3;


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Case 3;

7 year old post op presenting with chills, fever and

hypotension

•pH: 7.25

• pCO2: 32 mmHg

• pO2: 55 mmHg

•

HCO3: 10 mEq/L• BE: -15 mEq/L

• Uncompensated metabolic acidosis due to low

perfusion state and hypoxia causing increased

lactic acid

Case 4


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Case 4A 6 year old girl with severe gastroenteritis is admitted to thehospital for fluid rehydration, and is noted to have a high [HCO3

-]

on hospital day #2. An ABG is ordered:

ABG: pH 7.47 Chem : Na+ 130

PCO246 K+ 3.2

HCO3- 32 Cl- 86PO2 96 HCO3

- 33

Urine pH: 5.8

G S O G


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GE, SEVERE VOMITING

• Hypokalemic hypocholiremic metabolic alkalosis

Thank you


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Thank you

6 fluid and acid base balance m med07

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