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The acid base “balance”. Abelow, Understanding Acid-Base, Williams & Wilkins 1998. (Abelow B, 1998 “Understanding Acid-Base”). (Abelow B, 1998 “Understanding Acid-Base”). Neuromuscular chain defects -may alter alveolar ventilation. (Abelow B, 1998 “Understanding Acid-Base”).

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The acid base “balance”

Abelow, Understanding Acid-Base, Williams & Wilkins 1998




Neuromuscular chain defects

-may alter alveolar ventilation

(Abelow B, 1998 “Understanding Acid-Base”)


Pulmonary diseases

-may alter alveolar ventilation

(Abelow B, 1998 “Understanding Acid-Base”)


HENDERSON-HASSELBALCH EQUATION

pH = pK + log [HCO3-]/0.03PCO2

pH = 6.1 + log 24/(0.03 x 40)

pH = 6.1 + log 24/1.2

pH = 6.1 + log 20

pH = 6.1 + 1.3

pH = 7.4

(Abelow B, 1998 “Understanding Acid-Base”)


Renal bicarbonate reabsorption

Abelow, Understanding Acid-Base, Williams & Wilkins 1998


Distal nephron luminal H+/K+ exchanger

(Abelow B, 1998 “Understanding Acid-Base”)



RESPIRATORY ACIDOSIS

- Alveolar hypoventilation

: acute airway obstruction with underventilation

- Late acute asthma, acute COPD

: CNS - opiate overdose

- stroke

- neuropathy, myopathy


COMPENSATED RESPIRATORY ACIDOSIS

UNCOMPENSATED RESPIRATORY ACIDOSIS

Abelow, Understanding Acid-Base, Williams & Wilkins 1998


RESPIRATORY ALKALOSIS

- Alveolar hyperventilation

: Early acute asthma with over ventilation

: Pulmonary embolus, pneumonia, pulmonary oedema

: Anxiety


RESPIRATORY ALKALOSIS

Uncompensated

Compensated

Abelow, Understanding Acid-Base, Williams & Wilkins 1998


METABOLIC ACIDOSIS

- xs production of H+ ions

: Diabetic ketoacidosis

: Acute renal failure

: Circulatory shock (eg septic, cardiogenic, hypovolemic)


METABOLIC ACIDOSIS WITH RESPIRATORY COMPENSATION

UNCOMPENSATED METABOLIC ACIDOSIS

Abelow, Understanding Acid-Base, Williams & Wilkins 1998


METABLOIC ALKALOSIS

- xs HCO3- ions

: Loss of gastric fluid – vomiting

: Diuretics-K+ loss: xs renal HCO3reabsorption

:Post hypercapnic mechanical ventilation


METABOLIC ALKALOSIS

Abelow, Understanding Acid-Base, Williams & Wilkins 1998


When you see “respiratory”, think PCO2

and

When you see “metabolic”, think [HCO3-]

Abelow, Understanding Acid-Base, Williams & Wilkins 1998



EVALUATION OF BLOOD GASES

Abelow, Understanding Acid-Base, Williams & Wilkins 1998



RESPIRATORY FAILURE

Type 1: PaO2 PaCO2

- Alveolar hyperventilation

Type 2: PaO2 PaCO2

- Alveolar hypoventilation


ACUTE ASTHMA

Early: Alveolar hyperventilation -  respiratory drive PaO2 PaCO2

 give high concentration of O2 (60%)

Late: Alveolar hypoventilation - respiratorydrive

PaO2 PaCO2

: still relying on hypercapnic drive

 give high concentration of O2 (60%)

- may need mechanical ventilation


  • ACUTE EXACERBATION OF COAD

  • Chronic alveolar hypoventilation -  respiratory drive

  • - switch from hypercapnic to hypoxic drive

  •  Use low concentration of O2 (24%) to avoid suppressing hypoxic drive

  •  Can use central respiratory stimulation (doxapram) to permit higher concentration O2 (28-35%)


ARTERIAL BLOOD GASES IN ACUTE ASTHMA

Late Stage = Fatigue = Alveolar hypoventilation Early Stage = Alveolar hyperventilation

PaO2

1KPa = 7.5 mm Hg

PaCO2

  • If high PaCO2 (> 6KPa) and low PaO2 (< 8KPa) at presentation, or if rising PaCO2 and falling PaO2 despite treatment  mechanical ventilation (ie call anaesthetist)

  • Always use high flow O2 mask (> 60% inspired concentration) in acute asthma - even if high PaCO2 - as patient still relying on hypercapnic drive


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