Acidosis & Alkalosis physiology with signs and symptoms

Acidosis & Alkalosis Diagnosis And treatment–

acisosis alkalosis 1
Acidosis & Alkalosis

The arterial blood gases provide information about-

  • 1. Ventilation
  • 2. Oxygenation
  • 3.Acid base status This maintains the pH.
  • Arterial blood gases (ABG) help to diagnose respiratory failure.

Respiratory failure may be —

  • 1. Primary oxygenation failure
  • 2. Ventilatory failure

1–. Primary oxygenation failure is indicated by —

  • a. low PaCOz
  • b. low PaOz
  • c. alkaline pH

2– Ventilatory failure is indicated by —

  • a. high PaCOz
  • b. moderately low PaOz
  • c. acidic pH.

Arterial Blood Gas analysis tells about —

  • 1. Oxygenation
  • 2. Ventilatory status
  • 3. Acid base status.

The pH of blood is maintained by —

  • 1. Alveolar ventilation
  • 2. Oxygenation
  • 3. Acid base balance.

1– Alveolar ventilation

  • PaCOz is the best index for alveolar ventilation.
  • High PaCOz > 45 mmHg indicates alveolar hypo ventilation.
  • – Low PaCOz < 35 mmHg indicates alveolar hyperventilation.

2– Oxygenation:

  • Depends on —
  • arterial oxygen tension written as PaOz
  • – oxygen concentration of inspired air written
  • as FiOz
  • – Hb content.
  • PaOz is arterial oxygen tension.
  • SpOz is oxygen saturation by pulse oximetry. SaOz is oxygen saturation by ABG analysis. Hypoxemia: PaOz < 80 mmHg at sea level in an adult breathing room air is hypoxaemia.
  • PaOz <40 mmHg indicates severe hypoxemia. Hypoxia: Decreased oxygen saturation of tissues is known as hypoxia.
  • Normal Pa02 in adult breathing room air with FiOz of 0.2 is 80 – 100 mmHg.
  • Oxygenation ratio PaOz / FiOz is oxygenation ratio.
  • Normal PaOz / FiOz is 400 – 500 mmHg or 4 to 5. PaOz / FiOz ratio < 200 indicates shunt greater than 20%.

Acid Base balance —

  • – The pH is maintained through lungs, kidneys, and blood buffer.
  • HC03, PaCOz‘ and pH are interrelated.
  • HC03 is controlled by kidneys and blood buffers
  • The lungs control PCOz by volatile acids, carbonic acid in the blood.
  • Concentration of carbonic acid depends on dissolved COz
  • The blood buffer system is carbonic acid / bicarobonate base – HZC03 / HC03-.
  • Buffer system acts within a fraction of second to prevent change in pH.
  • Respiratory system takes few minutes only, but kidneys take minutes to days to adjust pH.
  • PaCOz is the best marker for respiratory acid base disturbance.
  • – Rise in PaCOzindicates increased carbonic acid
  • and decreased pH.
  • – Plasma bicarbonate indicates base status.
  • – Normal value of HC03 is 24 ± 2 meq/L.
  • – Standard bicarobonate (std HCO -) is plasma
  • bicarbonate at 37°C and PaCOz of 40 mmHg.
  • – Buffer base is sum of all buffer bases in a liter of blood.
  • – It is reported as base excess – negative or positive BB or BE.
  • – Normal value of BE is 0 ± 2 meq/L.

Plasma pH and potassium –

  • In acidemia, potassium moves out into the plasma leading to hyperkalemia and excess hydrogen ions are transported to intracellular space.
  • In alkalemia, potassium moves into cells leading to hypokalemia and hydrogen ions move to extracellular space.

Blood sampling —

  • · Arterial blood is taken.
  • · Always done in steady state.
  • · 0.05 ml heparin is mixed with 1 ml of blood.
  • · The sample is placed in ice and tested within 2 hours.

Normal values for ABG

  • · pH 7.35 to 7.45
  • · PaOz 80 – 100 mmHg
  • · PaCOz 35 – 45 mmHg
  • · HC03 24 ± 2 meq/L
  • · BE = 0 ± 2 meq/L.
  • Whenever there is ABG disturbance, there is compensatory response by respiratory system or kidneys.

RULES OF COMPENSATION IN ACID-BASE BALANCE  —

1. Metabolic acidosis: expected PaCOz decreases by 1 mmHg for every 1 meq/L decrease in HCO-3

  • PaCOz = last 2 digits of pH.

2. Metabolic alkalosis  :

  • PaCOz increases by 0.5 mmHg for every 1 meq/L increase in HCO-3 or PaCOz = last 2 digits of pH.

3. Respiratory acidosis  :

  • HC03 increases by 1 meq/ L for every 10 mmHg rise in PaCO in acuteacidosis. z
  • HCO-3 increases by 3.5 meq/L for each 10 mm increase in PaCOz in chronic acidosis.

4. Respiratory alkalosis :

  • In acute alkalosis HCO decreases by 2 meq/L for every 10 mmHg decrease in PaCOz
  • In chronic alkalosis HC03 decreases by 5 meq/L for every 10 mmHg decrease in PaCOz

Stepwise approach In acidosis & alkalosis —

  • 1. Look at pH.  — If pH <7.35 it is acidotic and if pH is >7.45, it is alkalotic.
  • 2. Look at PaCOz‘  – If >40 mmHg and pH <7.35, it is respiratory acidosis.
  • 3 • If more than 25 meq/L and pH more than 7.45, it is metabolic alkalosis.
  • 4. pH 7.35 to 7.40 is lower normal range (acidotic side).
  • 5. pH 7.40 to 7.45 is higher normal range (alkalotic side).
  • 6. If pH<7.35 and PaCOz>45, HC03 and BE normal, it is acute respiratory acidosis due to acute ventilatory failure.
  • 7. If pH<7.35, PaCOz>45 mmHg, HCO/BE increased, it is partially compensated respir~tory acidosis.
  • 8. If pH<7.35, PaCOz 35-45, HCO/BE decreased, it is acute metabolic acidosis (uncompensated).
  • 9. If pH is less than 7.35, PaCOz <35, HCO/BE decreased, it is partially compensated meta bOlic acidosis.
  • 10. If pH> 7.45, PaCOz>45, HCO/BE increased, it is partially compensated metabolic alkalosis.
  • 11. If pH is>7.45, PaCOz<35 mmHg, HCO/BE normal, it is acute respiratory alkalosis (A3cute alveolar ventilation).
  • 12. If pH is> 7.45, PaCOz<35, HC03 decreased, it is partially compensated respiratory alkalosis.
  • 13. If pH is 7.35 to 7.4, it is lower normal range acidosis.
  • 14. If pH is 7.35 to 7.4, PaCOz>45 mmHg, HCO /BE increased, it is compensated respiratory acid~sis.
  • 15. If pH 7.35 to 7.4, PaCOz<35, HCO /BE decreased, It IS compensated metabolic acidosis.
  • 16. If pH 7.4 to 7.45 it is higher normal range­alkalosis.
  • 17. If pH 7.4 to 7.45, PaCOz <35, HC03 decreased, it is compensataed respiratory alkalosis- chronic alveolar hyperventilation.
  • 18. If pH 7.4 to 7.45, PaCOz>45, HC03 increased, it IS compensated metabolic alkalosis.
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