ACID BASE DISORDER & COMPENSATORY

MECHANISM

Dr. Vijay Marakala, MBBS, MD.

Senior LecturerBIOCHEMISTRY

IMS, MSU.

DISTURBANCES IN ACID – BASE BALANCE

Ratio of HCO3-/H2CO3 is 20:1which is

constant at physiological pH

Derangements of hydrogen and bicarbonate concentrations in body

fluids are common in disease processes

WHEN TO ORDER ACID-BASE PARAMETERS

Any serious illness

Multi-organ failure

Respiratory failure

Cardiac failure

Uncontrolled diabetes mellitus

Poisoning (barbiturates, ethylene glycol)

CLASSIFICATION OF ACID-BASE DISTRURBANCES

ACIDOSISALKALOSIS

7.47.35 – 7.45 ACIDOSIS ALKALOSIS

CLASSIFICATION OF ACID-BASE DISTRURBANCES

Acidosis (fall in pH)

• Respiratory acidosis

• Metabolic acidosis

Alkalosis (Rise in pH)

• Respiratory alkalosis

• Metabolic alkalosis

METABOLIC ACIDOSIS

It is due to a primary deficit in the bicarbonate, resulting from an accumulation

of acid or depletion of bicarbonate

When there is excess acid production, the bicarbonate is used up for buffering. Depending on the cause, the anion gap is altered

[HCO-3] / [H2CO3] = <20:1

ANION GAP

• The sum of cations and anions in ECF is always equal

• Unmeasured anions constitute the anion gap.

• The anion gap is calculated as the difference between (Na+ + K+) and (HCO3

– + Cl–).

• Normally this is about 12 mmol/liter.

ANION GAP

• The sum of cations and anions in ECF is always equal

HIGH ANION GAP METABOLIC ACIDOSIS

Decreased acid excretion

• Renal failure

Increased acid production

• Lactic acidosis

• Ketoacidosis

Toxicity or overdose

• Salicylate Aspirin poisoning

• Methanol

NORMAL ANION GAP METABOLIC ACIDOSIS

HYPERCHLOREMIC OR NORMAL ANION GAP

Diarrhoea, Loss of bicarbonate and cations

RTA Defective acidification of urine.

METABOLIC ACIDOSIS

pH

Decreased

Primary deficit

Deficit of bicarbonate

Ratio of HCO3-/H2CO3

Less than 20

COMPENSATORY MECHANISM METABOLIC ACIDOSIS

Respiratory compensation

Hyperventilation

Renal compensationIncreased excretion

of acid and conservation of base

METABOLIC ACIDOSIS

In its most pronounced clinical manifestation,

the increase in ventilation is referred

to as Kussmaul Respiration

METABOLIC ACIDOSIS

METABOLIC ALKALOSIS

pH

Increased

Primary change

Excess of bicarbonate

Ratio of HCO3-/H2CO3

More than 20

[HCO-3] / [H2CO3] = >20:1

METABOLIC ALKALOSIS

Loss of hydrogen

• Severe vomiting• loop or thiazide

diuretics• Mineralocorticoid

excess

Exogenous Alkali

• Administration of NaHCO3, sodium citrate, gluconate, acetate, antacids

• Antacids

COMPENSATORY MECHANISM METABOLIC ALKALOSIS

Respiratory compensation

Hypoventilation

Renal compensationIncreased excretion

of HCO3- and

conservation of H+

METABOLIC ALKALOSIS

RESPIRATORY ACIDOSIS

pH

Decreased

Primary change

Excess of carbonic

acid

↑PCO2

Ratio of HCO3-/H2CO3

less than 20

[HCO-3] / [H2CO3] = <20:1

CAUSES OF RESPIRATORY ACIDOSIS

• Pneumonia • Emphysema, Asthma

Obstruction to respiration

• Respiratory depressant toxic drugs• Morphine

Depression of respiration

• Due to severe hypoxiaCardiac arrest

COMPENSATORY MECHANISM RESPIRATORY ACIDOSIS

Excess carbonic acid is buffered by haemoglobin

and protein buffer

Renal compensationIncreased excretion

of acid and conservation of base

RESPIRATORY ACIDOSIS

RESPIRATORY ALKALOSIS

pH

Increased

Primary change

Deficit of carbonic

acid

↓PCO2

Ratio of HCO3-/H2CO3

More than 20

[HCO-3] / [H2CO3] = >20:1

CAUSES FOR RESPIRAOTORY ALKALOSIS

High altitude

Hysteria

Septicaemia

Febrile conditions

COMPENSATORY MECHANISM RESPIRATORY ALKALOSIS

Rapid cell buffering

Decrease in net renal acid excretion.

Bicarbonate level is reduced by decreasing the reclamation of filtered

bicarbonate.

RESPIRATORY ALKALOSIS