Acid Base Disorders

Hasan Al-Dorzi, MD

Pulmonary and Critical Care

Consultant, Intensive Care Department

AmjadSticky Note

Acid Base Balance

The body produces acids daily

15000 mmol of CO2 50-100 mEq of nonvolatile acids

The body prevents pH changes by three systems: Physiologic buffers

Respiratory system

Renal system

AmjadNote Correlation between pH and PaCO2. ( or PaCO2 by 10 mmHg pH by 0.08). Correlation between PaCO2 and HCO3. For equation purposes: Normal pH = 7.4 (7.35-7.45) Normal PaCO2 = 40 (40-45) Normal HCO3 = 24 (22-26).

Acid-Base Balance

Buffers bind or release hydrogen ion to limit the change in pH

Main body buffers: Bicarbonate

H+ + HCO3- H2CO3 H2O + CO2

Intracellular protein puffers: hemoglobin

Bone

Reservoir of bicarb and phosphate

Acid-Base Balance

Respiratory system

Changes in pH sensed by chemoreceptors

Peripherally (carotid bodies)

Centrally (medulla oblongata)

Drop in pH Increased minute ventilation

Lowers PaCO2

Increase in pH Decreased ventilatory effort

Increases PaCO2

Acid-Base Balance

Renal system

Plays no role in acute compensation

6-12hrs Acidosis

Active excretion of H+

Retention of HCO3-

>6hrs of Alkalosis

Active excretion of HCO3-

Retention of H+

pH pH

The negative logarithm of the hydrogen ion concentration

Henderson Hasselbalch equation:

pH = 6.1 + log [HCO3-]/ 0.03 PCO2

Normal pH is 7.35-7.45

Value 7.45 is alkalemia

Acid Base Disorders

Disturbances of acid base metabolism:

Acidosis process that increases [H+] by increasing PCO2 or by reducing [HCO3-]

Alkalosis process that reduces [H+] by reducing PCO2 or by increasing [HCO3-]

Hence, we can 4 acid-base disturbances

Acid Base Disorders

Hence, we can have 4 primary acid-base disturbances:

Respiratory acidosis

Respiratory alkalosis

Metabolic acidosis

Metabolic alkalosis

Compensatory processes try to correct pH

Mixed primary processes can exist

Respiratory Acidosis

CO2 Ventilation leading to ph Causes

CNS depression

Pleural disease

COPD/ARDS

Musculoskeletal disorders

Compensation for metabolic alkalosis

Respiratory Acidosis

Acute vs Chronic Acute - little kidney involvement.

pH by 0.08 for each 10 mmHg in CO2 HCO3 increase by ~1 for each 10 mmHg

in CO2

Chronic - Renal compensation via synthesis and retention of HCO3 (Cl to balance charges hypochloremia) pH by 0.03 for each 10 mmHg in CO2 HCO3 increase by 3 for each 10 mmHg in

CO2

Respiratory Alkalosis

CO2, Ventilation leading to pH

CO2 HCO3 (Cl to balance charges hyperchloremia)

Decreased HCO3 reabsorption and decreased ammonium excretion to normalize pH

Causes Intracerebral hemorrhage

Salicylate and Progesterone drug usage

Anxiety

Cirrhosis of the liver

Sepsis

Respiratory Alkalosis

Acute vs. Chronic Acute - HCO3 by 2 mEq/L for every 10

mmHg in PCO2

Chronic - HCO3 by 4 mEq/L of HCO3 for every 10 mmHg in PCO2

Metabolic acidosis

Two types:

Normal anion gap or non-anion gap

High anion gap or anion gap

Na+ Cl-

HCO3-

Anion Gap

Cations = Anions

Anion gap= Na+ - (Cl- + HCO3-)

Normal anion gap= 8-12

Na+ Cl-

Anion Gap

ie, Formate-

HCO3-

Addition of

exogenous acids

or Creation of

endogenous acids

Na+ Cl-

HCO3-

Anion Gap

Excessive loss of HCO3

-

or

Inability to excrete H+

Delta Gap

Checks for hidden metabolic process

Based on the 1:1 concept: AG = HCO3 (Normal HCO3 = 24)

Metabolic Acidosis

HCO3 leading to pH

12-24 hours for complete activation of respiratory compensation

The degree of compensation is assessed via the Winters Formula

PCO2 = 1.5(HCO3) +8 2

The Causes

Metabolic Gap Acidosis M - Methanol

U - Uremia

D - DKA

P - Paraldehyde

I - INH

L - Lactic Acidosis

E - Ehylene Glycol

S - Salicylate

Non Gap Metabolic

Acidosis

Acetazolamide

Renal tubular acidosis

Diarrhea

Pancreatic Fistula

Anion gap= Na+ (HCO3- + Cl-)

Metabolic alkalosis

Saline-responsive

Usually due to loss of hydrogen ions from the stomach or in the kidneys

Urinary chloride level

metabolic alkalosis

Saline-resistant Usually associated

with mineralcorticoid excess leading to Na+ reabsorption and secretion of K+ and H+

Urinary chloride >20mEq/L

Examples 1) Primary

aldosteronism 2) Exogenous steroids 3) Adenocarcinoma 4) Bartters Syndrome 5) Cushings disease 6) Ectopic

adrenocorticotropic hormone

Metabolic Alkalosis

HCO3 leads to pH

Causes

Vomiting

Diuretics

Hypokalemia

Hyperaldosteronism

Cushings syndrome

PCO2 by 0.7 for every 1mEq/L in HCO3

Mixed Acid-Base Disorders

Patients may have two or more acid-base disorders at one time

E.g. Metabolic acidosis and metabolic acidosis

E.g. Respiratory acidosis and metabolic acidosis

Arterial Blood Gas Interpretation

Check validity of laboratory measurements

H+ = 24 x PaCO2 HCO3

H+ = 80 last 2digits of pH

ie, pH=7.20 ------ H+ = 80-20=60

ie, pH=7.44 ------ H+ = 80-44=36

Arterial Blood Gas Interpretation

Step 1 Check pH Determine if acidosis or alkalosis If acidosis pH = decreased If alkalosis pH = increased Step 2 Check pCO2 If pCO2 is increased it is being retained If pCO2 is decreased it is being blown off

Arterial Blood Gas Interpretation

Step 3

Check HCO3

If increased acid is being excreted

If decreased acid is being added

Step 4

Determine primary process

Anion GAP

Calculation of AG is useful approach to

analyze metabolic acidosis

AG = (Na+ (Cl- + HCO3-)

Step 5: Determine Compensation

Metabolic Acidosis

PaCO2 decreases 1.2 mmHg per 1 meq/L bicarbonate fall

Winters formula: PCO2= 1.5 HCO3 + 8 2

Metabolic Alkalosis

PaCO2 increases 6-7 mmHg per 10 meq/L bicarbonate rise

Step 5: Determine Compensation

Acute Respiratory Acidosis

Bicarbonate increases 1 meq/L per 10 mmHg PaCO2 rise

Chronic Respiratory Acidosis

Bicarbonate increases 4 meq/L per 10 mmHg PaCO2 rise

Step 5: Determine Compensation

Acute Respiratory Alkalosis

Bicarbonate decreases 2 meq/L per 10 mmHg PaCO2 fall

Chronic Respiratory Alkalosis

Bicarbonate decreases 4 meq/L per 10 mmHg PaCO2 fall

Other Components of ABGs

Bicarbonate (HCO3-) Calculated from the CO2 and pH using the

Henderson-Hasselbach equation Allows assessment of the metabolic component

of acid-base balance

Base excess (or deficit)

A measure of the amount of acid or alkali that must be added to a sample under standard conditions to return the pH to 7.4

Calculated from the pH and PaCO2

Case # 1

A 65 year old man who is a heavy smoker presents to the Emergency department because of shortness of breath and sputum production of one day duration.

ABGs showed pH=7.30, PO2: 54 mm Hg, PCO2=52 mm Hg, HCO3=25, BE=+1, O2 saturation=85% on room air.

Case # 1

This patient has:

a) type I respiratory failure

b) type II respiratory failure

This patient has:

a) Respiratory acidosis

b) Metabolic acidosis

Analysis of case 1

Check for internal consistency: 50= 24 x50/25 OK

pH is 7.30= acidemia

PCO2 is high = respiratory acidosis

HCO3: slightly elevated = metabolic alkalosis

Primary process= respiratory acidosis

Acute vs chronic: history suggests it is acute

Compensation: 10 increase in PCO2, 1 increase in HCO3: OK

Diagnosis= primary acute respiratory acidosis

Case # 2

A 65 yo man presents after motor vehicle collision. He has facial injuries and requires to be intubated. He is now on mechanical ventilation.

ABGs: 7.26 / 40 / 65 / 12 on FiO2 =1.0 Serum chemistry: Na= 140 / K= 3.5 / Cl= 104 /

HCO3= 12

Case # 2

This patient has

a) type I respiratory failure

b) type II respiratory failure

c) None of the above

This patient has

a) Acidemia

b) Acidosis

c) alkalosis

Analysis of case 2

Check for internal consistency

Patient has academia

Patient has metabolic acidosis

Anion gap is high = 140 -116= 24

Drop in HCO3 = increase in anion gap

Winters formula: expected PCO2= (1.5 x 12) +8 +/- 2:= 24-28

But on ABGs, PCO2 = 40: higher than expected

Diagnosis: primary metabolic acidosis (high AG) and primary respiratory acidosis

Case # 2

His acid base disorder can be explained by:

Shock state

Ethanol intoxication

Severe vomiting before presentation to the hospital

Severe dehydration and diarrhea

Case # 3

A 65 yo man presents with nausea and

vomiting.

ABGs: 7.4 / 41 / 85 / 22

Na- 137 / K- 3.8 / Cl- 90 / HCO3- 22

Does this patient have any acid-base disorder?

Case # 3

Anion Gap = 137 - (90 + 22) = 25

anion gap metabolic acidosis

Winters Formula = 1.5(22) + 8 2

= 39 2

compensated

Delta Gap = 25 - 10 = 15

15 + 22 = 37

metabolic alkalosis

This patient is alcoholic!!!

Case # 4

22 year old female presents for attempted overdose. She has history of chronic headache.

On exam she is experiencing respiratory distress.

Case # 4

ABG - 7.47 / 19 / 123 / 14

Na- 145 / K- 3.6 / Cl- 109 / HCO3- 17

Case # 4

Anion Gap = 145 - (109 + 17) = 19

anion gap metabolic acidosis

Winters Formula = 1.5 (17) + 8 2

= 34 2

uncompensated: primary respiratory alkalosis

Delta Gap = 19 - 10 = 9

9 + 17 = 26

no metabolic alkalosis

Case # 5

47 year old male experienced crush injury at construction site.

ABG - 7.3 / 32 / 96 / 15

Na- 135 / K-5 / Cl- 98 / HCO3- 15 / BUN- 38 / Cr- 1.7

CK- 42,346

Case # 5

Anion Gap = 135 - (98 + 15) = 22

anion gap metabolic acidosis

Winters Formula = 1.5 (15) + 8 2

= 30 2

compensated

Respiratory acidosis

pH PaCo2 HC03

normal

Respiratory Alkalosis

normal

Metabolic Acidosis

normal

Metabolic Alkalosis

normal

Summary

Thank You