ACID–BASE IMBALANCE -

Introduction:

The term used to define the acidity or alkalinity of solutions or blood, pH, is the negative logarithm of the hydrogen ion concentration ([H+]).
Normal pH = -log [H+] = 7.40 (7.35-7.45)
Acid = proton donor (H+)
Base = proton acceptor
Solution acidity reflects [H+]
Pa02: Partial pressure of oxygen in arterial blood (mmHg). Normal is 40
PaCo2: Partial pressure of carbon dioxide in arterial blood (mmHg). Normal 75-100 mm Hg

Buffers

A buffer is solution containing a weak acid and its conjugate base or a weak base and its conjugate acid
It minimizes changes in pH by accepting or giving up H+
Examples: Bicarbonate, hemoglobin, proteins, phosphates, amnonia

Physiologic Responses to pH Changes

Pulmonary Compensation

Mediated by chemoreceptors in brain stem
VO2 1-4 L/min for every 1mm Hg PaCO2
Response to metabolic acidosis is hyperventilation, response to metabolic alkalosis is hypoventilation (but less predictable)

PaCO2 usually does not rise above 55 mmHg in response to metabolic alkalosis because hypoxemia produced by hypoventilation drives ventilation and limits compensation
PaCO2 decreases 1-1.5 mmHg for every 1 mEq/L decrease in HCO3-
PaCO2 increases 0.25-1 mmHg for every 1 mEq/L increase in HCO3- up to 55mmHg

Renal Compensation

Increased secretion of HCO3-
Generally occurs only with concomitant Na+ deficit or with mineralocorticoid excess

Metabolic Acidosis

Definition:

Metabolic acidosis, usually characterized by an acidemic pH (<7.35) and hypobicarbonatemia

Etiology:

Treatment

Treatment is aimed at the underlying disorder, such as insulin and fluid therapy for diabetes and appropriate volume resuscitation to restore tissue perfusion.

Metabolic Alkalosis

Definition

Etiology:

Factors that generate metabolic alkalosis include nasogastric suction and diuretic administration
Abnormalities that generate HCO3- within the body are called “initiation factors” of metabolic alkalosis, whereas abnormalities that promote renal conservation of HCO3- are called “maintenance factors.”

Treatment

Mild alkalosis is generally well tolerated.
Severe or symptomatic alkalosis (pH > 7.60) requires urgent treatment.
Depending on the degree of hypovolemia, adequate amounts of 0.9% NaCl and KCl should be administered.
Discontinuation of diuretics and administration of H2-blockers in patients whose alkalosis is due to nasogastric suction can be useful.

Respiratory Acidosis

Definition:

Respiratory acidosis, usually characterized by a low pH (<7.35) and always characterized by hypercarbia, occurs when minute ventilation is insufficient to eliminate CO2 production without an increased capillary–alveolar CO2 gradient.
Occurs when an acute or chronic rise in arterial CO2

Etiology:

Respiratory acidosis results from decreased alveolar ventilation and subsequent hypercapnia

Symptoms and Signs

With acute onset, there is somnolence and confusion, and myoclonus with asterixis may be seen. Coma from CO2 narcosis ensues.
Severe hypercapnia increases cerebral blood flow and cerebrospinal fluid pressure.
Signs of increased intracranial pressure (papilledema, pseudotumor cerebri) may be seen.

Treatment

Since drug overdose is an important reversible cause of acute respiratory acidosis, administration of naloxone (opiate antagonist), 0.04-2 mg intravenously is given to all such patients if no obvious cause for respiratory depression is present
In all forms of respiratory acidosis, treatment is directed at the underlying disorder to improve ventilation.

Respiratory Alkalosis

Definition & etiology: