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Acid –Base Imbalance

Dr. Eman EL Eter

Acid-Base Imbalances

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pH< 7.35 acidosispH > 7.45 alkalosisPCO2= 35-45 mmHgHCO3- = 22-26 mEq/LThe body response to acid-base imbalance is

called compensationMay be complete if brought back within

normal limitsPartial compensation if range is still outside

norms.

Compensation

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If underlying problem is metabolic, hyperventilation or hypoventilation can help : respiratory compensation +buffer system.

If problem is respiratory, renal mechanisms can bring about metabolic compensation.

Acidosis

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Principal effect of acidosis is depression of the CNS through ↓ of synaptic transmission.

Generalized weaknessDeranged CNS function the greatest threatSevere acidosis causes

Disorientationcoma death

Causes of acidosis

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A. Respiratory causes:- CNS depression (anaesthesia).- Resp. muscle paralysis/ diaphragm paralysis,- Rib fractures, etc..

- Obstructive lung diseases e.g. Emphysema

Pulmonary edema.

A. Metabolic causes:- Diabetic ketoacidosis.-Severe diarrehea.-Hypoaldosteronism-Acute renal failure

Alkalosis

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Alkalosis causes over excitability of the central and peripheral nervous systems.

NumbnessLightheadednessIt can cause :

Nervousnessmuscle spasms or tetany Convulsions Loss of consciousnessDeath

Causes of alkalosis

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A- Respiratory:Hyperventilation:- High altitude.- Hysterical.

- B. Metabolic:- -Severe vomiting.- Excess antacids.- Hyperaldosteronism.

Respiratory Acidosis

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Carbonic acid excess caused by blood levels of CO2 above 45 mm Hg.

Compensation for Respiratory Acidosis

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Kidneys eliminate hydrogen ion and retain bicarbonate ion.

Kidney also generates new bicarbonate.

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Respiratory Alkalosis

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Carbonic acid deficitpCO2 less than 35 mm Hg (hypocapnea)Most common acid-base imbalancePrimary cause is hyperventilation

Respiratory Alkalosis

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Conditions that stimulate respiratory center and wash out CO2 (Hyperventilation):

Oxygen deficiency at high altitudes.Anorexia nervosa.Early salicylate intoxication

Compensation of Respiratory Alkalosis

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Kidneys conserve hydrogen ionExcrete bicarbonate ion

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Metabolic Acidosis

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Bicarbonate deficit - blood concentrations of HCO3- drops below 22mEq/L

Causes:Loss of bicarbonate through diarrhea or renal

dysfunctionAccumulation of acids (lactic acid or ketones) Failure of kidneys to excrete H+

Compensation for Metabolic Acidosis

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Increased ventilationRenal excretion of hydrogen ions if

possibleK+ exchanges with excess H+ in ECF( H+ into cells, K+ out of cells)

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Metabolic Alkalosis

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Bicarbonate excess - concentration in blood is greater than 26 mEq/L

Causes:Excess vomiting = loss of stomach acidExcessive use of alkaline drugsCertain diureticsEndocrine disorders: Hyperaldosteronism.Heavy ingestion of antacidsSevere dehydration

Compensation for Metabolic Alkalosis

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Kidney excretes alkaline urine and retain H+

Respiratory compensation difficult – hypoventilation limited by hypoxia.

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Diagnosis of Acid-Base Imbalances

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1. Note whether the pH is low (acidosis) or high (alkalosis)

2. Decide which value, pCO2 or HCO3- , is

outside the normal range and could be the cause of the problem. If the cause is a change in pCO2, the problem is respiratory. If the cause is HCO3

- the problem is metabolic.

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Compensated or uncompensated?If pH is normal (between 7.35-7.45)

….CompenstaedIf pH is abnormal (<7.35 or >7.45)

….uncompenstated.Respiratory or metabolic?If PCO2>45 = Respiratory acidosisIf PCO2<35= Respiratory alkalosisIf HCO3-< 22= Metabolic acidosis.If HCO3-> 26 = metabolic alkalosis.

Example:

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A patient is in intensive care because he suffered a severe myocardial infarction 3 days ago. The lab reports the following values from an arterial blood sample:

pH =7.21, PCO2= 42, HCO3- = 12:List the condition: acidosis or alkalosis,

metabolic or respiratory, compensated or uncompensated?

Answer:Metabolic acidosis, uncompensated

Practice

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pH PaCO2 PaHCO3

Com/un Resp/metab

Acid/alkalosis

7.21 32 14 uncompensated

metabolic acidosis

7.5 26 21 uncompensated

respiratory

alkalosis

7.36 54 32 compensated

respiratory

acidosis

7.38 38 25 Normal

7.44 30 20 compensated

respiratory

alkalosis

7.52 36 34 uncompensated

metabolic alkalosis

7.37 52 30 compensated

respiratory

acidosis

7.18 68 29 uncompensated

Respiratory

acidosis