blood gases interpretation elkhatib

By M.elkhatib CIN

Oxygenation. Ventilation. Acid base balance. Electrolytes. Haemoglobin. LACT

More accurate. SaO2 has limitations as does pO2 but both

together provide useful information.

Normal range (for what it is worth!) = 80-95 mm Hg.

Different in children with cardiac lesions (usually lower)

Ventilation is assessed by looking at pCO2.

Normal range 35-45 mm Hg.

Hyperventilation will reduce pCO2.

Hypoventilation will increase pCO2.

How do we manipulate these parameters?

Acidaemia – pH < 7,35 Alkalaemia – pH > 7,45 Acidosis – Process causing acid to accumulate

(abnormal pH not necessary) Alkalosis – Process causing alkali accumulation

(abnormal pH not necessary) pH - negative log of [H+]

Normal pH is 7.35 – 7.45. Normal pH range is essential for cellular function Normal pH is maintained by the respiratory and

renal systems working together. Cells produce CO2 as a result of cellular

respiration. CO2 causes increased acidity.

Renal system produces sodium bicarbonate.Normal range = 22 -26.

Base excess (or deficit) The amount of acid that must be added to a litre

of blood to return the pH to 7.4 at a pCO2 of 39 mm HgNormal value +/- 1

The body’s attempt to return the acid/base status to normal (i.e. pH closer to 7.4) by over or under producing bicarbonate or CO2.

How long does it take to compensate? Buffers ◦ immediate

Lungs◦ 10-15 min

Kidneys ◦ 12-24 h

Is the PH normal?

Is it acidotic or alkalotic?

Look at the oxygenation separately.

Is it primarily respiratory or metabolic?

Is it high LACT ?

pH < 7.35Acidosis (metabolic and/or

respiratory)

pH > 7.45Alkalosis (metabolic and/or

respiratory) paCO2 > 6.0 kPaRespiratory acidosis

(alveolar hypoventilation)

paCO2 < 4.0 kPaRespiratory alkalosis

(alveolar hyperventilation)

HCO3 < 22 mEq/LMetabolic acidosis

HCO3 > 26 mEq/LMetabolic alkalosis

paCO2 > 45, pH < 7.35 - respiratory acidosis paCO2 < 35, pH > 7.45 - respiratory alkalosis

Primary respiratory problem - pH & paCO 2 move in opposite direction

HCO3 < 22, pH < 7.35 - metabolic acidosis HCO3 > 26, pH > 7.45 - metabolic alkalosis

Primary metabolic problem - pH & HCO 3 are in same direction, and paCO 2 is also in same direction

paCO2 elevated & pH acidotic ↓pH accounted for entirely by ↑paCO2

HCO3 & BE - in Normal range ◦ No adequate time for kidneys to establish effective compensatory

mechanisms Causes◦ Respiratory pathophysiology

airway obstruction severe pneumonia chest trauma pneumothorax◦ Acute drug intoxication (narcotics, sedatives)◦ Residual neuromuscular blockade◦ CNS disease – decreased level of consciousness

paCO2 low & pH alkalotic ↑pH accounted for entirely by ↓paCO2

HCO3 & BE - Normal range ◦ No adequate time for kidneys to establish effective

compensatory mechanisms

Causes◦ Pain◦ Anxiety◦ Restrictive lung disease◦ Severe congestive heart

failure◦ Pulmonary emboli

◦ Sepsis◦ Fever◦ Overaggressive

mechanical ventilation

Causes

◦ Ketoacidosis - diabetic, alcoholic, starvation◦ Lactic acidosis - hypoxia, shock, sepsis, seizures◦ Toxic ingestion – salicylates, methanol, ethylene glycol,

ethanol, isopropyl alcohol, paraldehyde, toluene◦ Renal failure - uremia

12/06/14ABG Interpretation 18

◦ Renal tubular acidosis◦ Post respiratory

alkalosis◦ Hypoaldosteronism◦ Potassium sparing

diuretics◦ Pancreatic loss of

bicarbonate

◦ Diarrhea◦ Carbonic anhydrase

inhibitors◦ Acid administration (HCl,

NH4Cl, arginine HCl)◦ Cholestyramine◦ Ureteral diversions◦ NaCl excess

Causes

◦ Gastric acid loss due to vomiting or nasogastric suction◦ Gitelman syndrome◦ Diuretics◦ Hypokalemia◦ Renal failure

Further evaluation of blood gas - assessment of effectiveness of blood oxygenation

Hypoxemia – ↓O2 content of blood ◦ paO2 < 8 kPa & SO2 < 90%

Hypoxia – inadequate amount of O2 available to or used by tissues for metabolic needs

1. Does the patient have acidosis or alkalosis ?◦ Look at the pH

1. What is the primary problem – metabolic or respiratory ?◦ Look at the pCO2

pCO2 change in opposite direction of pH change - primary problem is respiratory