Acidosis & Alkalosis Diagnosis And treatment–

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 PaCO_z
• b. low PaO_z
• c. alkaline pH

2– Ventilatory failure is indicated by —

• a. high PaCO_z
• b. moderately low PaO_z
• 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

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

2– Oxygenation:

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

Acid Base balance —

• – The pH is maintained through lungs, kidneys, and blood buffer.
• HC0₃, PaCO_z‘ and pH are interrelated.
• HC0₃ is controlled by kidneys and blood buffers
• The lungs control PCO_z 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 – H_ZC0₃ / 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.
• PaCO_z 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 PaCO_z 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
• · PaO_z 80 – 100 mmHg
• · PaCO_z 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 PaCO_z decreases by 1 mmHg for every 1 meq/L decrease in HCO-₃•

• PaCO_z = last 2 digits of pH.

2. Metabolic alkalosis  :

• PaCO_z increases by 0.5 mmHg for every 1 meq/L increase in HCO-₃ or PaCO_z = 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 PaCO_z in chronic acidosis.

4. Respiratory alkalosis :

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

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 PaCO_z‘  – 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 PaCO_z>45, HC0₃ and BE normal, it is acute respiratory acidosis due to acute ventilatory failure.
• 7. If pH<7.35, PaCO_z>45 mmHg, HCO/BE increased, it is partially compensated respir~tory acidosis.
• 8. If pH<7.35, PaCO_z 35-45, HCO/BE decreased, it is acute metabolic acidosis (uncompensated).
• 9. If pH is less than 7.35, PaCO_z <35, HCO/BE decreased, it is partially compensated meta bOlic acidosis.
• 10. If pH> 7.45, PaCO_z>45, HCO/BE increased, it is partially compensated metabolic alkalosis.
• 11. If pH is>7.45, PaCO_z<35 mmHg, HCO/BE normal, it is acute respiratory alkalosis (A³cute alveolar ventilation).
• 12. If pH is> 7.45, PaCO_z<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, PaCO_z>45 mmHg, HCO /BE increased, it is compensated respiratory acid~sis.
• 15. If pH 7.35 to 7.4, PaCO_z<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, PaCO_z <35, HC0₃ decreased, it is compensataed respiratory alkalosis- chronic alveolar hyperventilation.
• 18. If pH 7.4 to 7.45, PaCO_z>45, HC0₃ increased, it IS compensated metabolic alkalosis.

• These are the main points to remember in ACIDOSIS AND ALKALOSIS .