Basics of Acid Base Disorders

This lecture explains the basics of acid base disorders.

Outline of Lecture

Acid – Base Disorders
what is pH ?

[H⁺]

-log [H⁺]

log 1/[H⁺]

Blood pH

 

Sources of Hydrogen Ions

Most hydrogen ions originate from cellular metabolism

Breakdown of phosphorus-containing proteins releases phosphoric acid into the ECF

Anaerobic respiration of glucose produces lactic acid

Fat metabolism yields organic acids and ketone bodies

Transporting carbon dioxide as bicarbonate releases hydrogen ions

Acid-Base Homeostasis

 

DEFENCE AGAINST pH CHANGE BUFFERS

Proteins

Haemoglobin

Phosphate

Bicarbonate

Bone buffers

Protein Buffer System

Plasma and intracellular proteins are the body’s most plentiful and powerful buffers

Some amino acids of proteins have:

Free organic acid groups (weak acids)

Groups that act as weak bases (e.g., amino groups)

BICARBONATE BUFFER SYSTEM

Henderson-Hasselbalch Equation

CO2 + H2O         H2CO3        H + HCO3-

pH =          pK + log [Base] / [Acid]

7.4 =          6.1 + log [HCO₃^–] / [H₂ CO₃]

7.4 =          6.1 + log 1.3

=    6.1 + [20/1]

Reabsorption of Bicarbonate

Phosphate Buffer System

Nearly identical to the bicarbonate system

Its components are:

Sodium salts of dihydrogen phosphate (H₂PO₄¯), a weak acid

Monohydrogen phosphate (HPO₄²¯), a weak base

This system is an effective buffer in urine and intracellular fluid

Hydrogen Ion Excretion

Ammonium Ion Excretion

Pulmonary function

Acid-Base in the G-I Tract

NORMAL

Acid-Base Disorders

METABOLIC ACIDOSIS

Metabolic acidosis

Metabolic acidosis

Increased H⁺ formation

Acid ingestion

Reduced renal H⁺ excretion

Loss of bicarbonate

Increased H⁺ formation

ketoacidosis,  diabetic or alcoholic

lactic acidosis

poisoning

inherited organic acidoses

Reduced H⁺ excretion

Diabetic keto-acidosis

Lactic acidosis

METABOLIC ALKALOSIS

The primary lesion in metabolic alkalosis is increased [HCO₃^–] from 25®40 mmol/L

The compensatory response to elevated plasma bicarbonate is increased PCO₂ from 4050 mmHg

Observe pH  from 7.67.5

Metabolic Alkalosis

RESPIRATORY ACIDOSIS

Respiratory Acidosis

This is a disorder of ventilation.

Rate of CO2 elimination is lower than the production

Main categories:

CNS depression

Cardiac arrest

Air way obstruction

Pleural disease

Lung diseases such as COPD and ARDS

Musculoskeletal disorders

Compensatory mechanism for metabolic alkalosis

RESPIRATORY ALKALOSIS

Respiratory Alkalosis

CO₂ reduction due to excessive ventilation :

CNS : anxiety, hypoxia, head trauma, infection, fever

Pulmonary: PE , Pneumonia, RDS, high altitude, mild edema

Metabolic : Pregnancy ,Gram negative sepsis

MIXED ACIDOSIS

Laboratory investigations

Biochemical diagnosis

pH

HCO₃^–

PCO₂

Base excess

Etiological diagnosis

Chloride and anion gap

Electrolytes

Urea/creatinine

Glucose

Ketones

Lactate

Urinary chloride

Sequential Rules:

Rule #1

Must know the pH; pH determines whether the primary disorder is an acidosis or an alkalosis

Rule #2

Must know the PaCO₂ and serum HCO₃^–

Rule #3

Must be able to establish that the available data (pH, PaCO₂, and HCO₃^–) are consistent

 Acid-Base :Summary

Acid Base Disturbances

Be sure abnormalities not due to lab error

Henderson-Hasselbach equation

[H⁺] =24 x (pCO₂/HCO₃)

Convert [H⁺] to pH:

Subtract calculated [H⁺] from 80; this gives the last two digits of a pH beginning with 7

example: calculated [H⁺] of 24 converts to pH of (80-24)~7.56

example: calculated [H⁺] of 53 converts to pH of (80-53)~7.27

Refer to table 1 in handout for more precise conversion, or if calculated [H⁺] exceeds 80