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Acid Base Imbalance ABG

Acid / Base Imbalance

  • An alteration in the acid/base balance  can result in diminished or increased  pH
  • An accumulation of hydrogen ions results in an acidosis
  • An alkalosis results from either a loss of hydrogen ions or an accumulation of a base.
  • These changes can be in response to a ventilation disorder (respiratory acidosis or alkalosis) or because of a general metabolic disturbance (metabolic acidosis or alkalosis).

METABOLIC ACIDOSIS

Cause

  • It occurs when there is an accumulation of acids, e.g. in anaerobic metabolism in metabolic disorders  or due to an infusion of acid when infusing  parental nutrition.
  • Septicemia is an important cause.
  • Hypovolemia is the most common cause.

Signs

  • Fall in  pH (due to rise in H+)
  • Negative base excess (as buffering  agents  are  being  used  up)
  • Normal  pco2
  • PO2 may be normal  or low

Treatment

  • Treating the cause of the acidosis is most important.
  • It may be necessary to increase the O2 to prevent anaerobic metabolism.
  • Infusion of amino acids may need to be stopped.
  • Sodium Bicarbonate should be given with extreme caution since it can lead to metabolic alkalosis and intra-ventricular hemorrhage.
  • Possible full septic screen, if unresponsive to Bicarb infusion.

METABOLIC ALKALOSIS

Cause

  • This is normally the result of inappropriate treatment with sodium bicarbonate.
  • However, where there is persistent vomiting, gastric hydrochloric acid may be lost and a metabolic alkalosis ensues.
  • Some babies with chronic lung disease have base excees to correct for a Resp Acidosis. Therefore they have a corrected Resp Acidosis.

Signs

  • Rise in pH is due to increased alkalinity of blood.
  • High St. HCO3
  • Normal PCO2 and PO2

Treatment

This involves correcting the underlying cause.

RESPIRATORY ACIDOSIS

Cause

  • It is due to poor gas exchange and is frequently seen in  respiratory distress syndrome.
  • It may also happen as a result of perinatal asphyxia.

 Signs

  • Fall in  pH
  • Rise in  pco2
  • PO2, BE and St HCO3 may be normal.

Treatment

Mechanical ventilation is usually necessary. Where it is already being used, the rate may need to be increased, with a shorter inspiratory time to allow the infant to breath out CO2. Pressure may also need to be increased.

RESPIRATORY ALKALOSIS

Cause

  • This condition is usually due to incorrect management of mechanical ventilation where excessive amounts of CO2 are eliminated.
  • It can be dangerous as a very low PCO2 leads to a fall in cardiac and cerebral blood flow.
  • However, It can be used therapeutically in cerebral edema.

Sign

  • pH is high
  • PCO2 is low

Treatment

  • The treatment is to reduce the rate and or pressures of mechanical ventilation.

MIXED ACIDOSIS

Cause

  • A mixed acidosis is usually the result of poor gas exchange which results in anaerobic metabolism.
  • This means that an excessive amount of hydrogen ions are produced.

Signs

  • pH is low due to general acidosis.
  • PO2 is low due to poor gas exchange.
  • PCO2 is high because of poor gas exchange.
  • St HCO3 is low as bicarbonate is used to buffer the carbonic acid produced.
  • Base Excess is low which indicates buffers are being used up.

Treatment

  • Improving the gaseous exchange will correct the respiratory acidosis and may help correct the metabolic acidosis .
  • If the metabolic acidosis is severe then plasma or Sodium Bicarbonate may be required.

COMPENSATION

Compensation is a physiological response to disturbance in acid/base balance. The blood gas may have a near normal appearance, but the primary cause of the disturbance remains. Treating the underlying cause of the disruption is necessary to restore the acid/base balance to normal.

Compensated Respiratory Acidosis

Compensated respiratory acidosis is characterized by the retention of bicarbonate as a result of adjustment in renal function. The primary disturbance is the accumulation of carbon dioxide, thus increasing carbonic acid concentration. The kidney responds to this disturbance by holding on to HCO3. This compensation by the holding on to HCO3. This compensation by the kidney can take several days if not corrected by ventilation therapy.

Sign

The PH may be near normal but the Paco2 and the St HCO3 will be low.

Compensated Metabolic Alkalosis

The underlying cause of the metabolic alkalosis is accumulation of bicarbonate so the body will try to retain CO2 to restore the balance .This can result in hypoventilation.

Signs

  • The pH will be normal but the Paco2 and the St HCO3 will be raised.
  • Interpreting  Blood  Gases
  • When interpreting a blood gas , you need to :
  • Evaluate the pH and decide if there is an acidosis or alkalosis.
  • Evaluate the PaCO2 value to determine if there is respiratory component.
  • Evaluate the HCO3 or base excess to evaluate if there is metabolic component.
  • Evaluate the PaO2 value and determine if there is hypoxia.

Correction of Acidosis – Alkalosis

A correction of acidosis or alkalosis can be achieved sooner if one manipulates ventilation settings or gives bicarbonate to achieve a desired value.

If you increase your pressure or rate on the ventilator, you will blow off CO2.

If you have an infant with severe metabolic acidosis, consider giving sodium bicarbonate 2 meq/kg slow IV push.

For acute correction of HCO3 base deficit:-

Take you deficit X (wt in kg) X (0.3). you can estimate base deficit by subtracting the infants bicarbonate value from the normal value of 24.

In giving HCO3 dilute (1:1) with sterile water give slowly (no faster than 1 mcq/kg/mt to avoid hyper osmolarity) and be sure you have adequate ventilation.

Acid Base Disturbances- How To Tell The Difference?

This table will PROVIDE a quick reference to help you distinguish between the different types of acid/base disturbances.

NORMAL VALUES

pH                  -7.35 -7.45

PaO2             – 60-80 mm Hg

PaCO2           -35-45 mm Hg

HCO3           – 18-24 mml/L

BE                  –   +4 – -4

pH PaCO2 HCO3
RESPIRATORY ACIDOSIS
Uncompensated            ↓              ↑         Normal
Partially compensated            ↓              ↑            ↑
Fully compensated      Normal              ↑            ↑
Respiratory alkalosis
Uncompensated             ↑              ↓         Normal
Partially  compensated             ↑              ↓         Normal
Fully compensated         Normal              ↓             ↓
Metabolic Acidosis
Uncompensated              ↓        Normal              ↓
Partially Compensated              ↓             ↓              ↓
Fuly Compensated         Normal             ↓              ↓
Metabollic Alkalosis
Uncompensated              ↑         Normal             ↑
Partially compensated              ↑              ↑             ↑
Fully compensated            Normal              ↑              ↑
Mixed Acidosis             ↓              ↑              ↓