Blood Gas Analysis Teguh Triyono Bagian Patologi Klinik FKUGM/ RSUP Dr Sardjito Yogyakarta

Normal acid-base balance Maintained through a number of buffer system but the most important is the bicarbonate system. The organs responsible are the kidneys and the lungs The body maintain pH or the hydrogen ion concentration (H+) of the blood 7.35-7.45

Assessment of acid-base balance is obtained by measuring the components of bicarbonate buffer system which can be expressed as the following modified Hendersen-Hasselbalch equation: pH= (HCO3)/ pCO2

Components of a Blood Gas Oxygenation: lungs/ECMO HCO3 Base: metabolic pCO2 Acid: lungs/ECMO pH The sum total of the acid/base balance, on a log scale (pH=-log[H+])

Normal Values pH pCO2 pO2 HCO3 BE Arterial 7.35-7.45 35-45 80-100 22-26 -2 to +2 Venous 7.30-7.40 43-50 ~45

Aims of Blood Gas Analysis Determine if pH is acidotic or alkalotic Determine cause: Respiratory Metabolic Mixed 3. Check oxygenation

Why Arterial Blood? Firmly establish the severity of an oxygenation abnormality To evaluate hyper- or hypoventilation Currently no convenient noninvasive way of evaluating pCO2 To determine acid-base status, particularly in patients with metabolic acidosis (e.g., diabetic ketoacidosis) To monitor the application of mechanical ventilation

The pH defines the (H+) of blood: increased pH = alkalaemia decreased pH = acidemia The (HCO3) defines metabolic component: increased (HCO3) = metabolic alkalosis decreased (HCO3) = metabolic acidosis The pCO2 defines respiratory component: increased pCO2 = respiratory acidosis decreased pCO2 = respiratory alkalosis

Acidaemia : pH < 7.35 metabolic acidosis : (HCO3) < 23 mmol/L respiratory acidosis : pCO2 > 45 mmHg Alkalaemia : pH > 7.45 metabolic alkalosis : (HCO3) > 33 mmol/L respiratory alkalosis : pCO2 < 35 mmHg

Simple metabolic acidosis Low blood bicarbonate The primary event is either an increased in hydrogen ion production or a decreased in its rate of excretion. Loss of bicarbonate via the gut (diarrhea) or the kidney is equivalent to increased H+ production Causes: renal failure, diarrhea, lactic acidosis, diabetic ketoacidosis

Simple metabolic alkalosis high blood bicarbonate The primary event is either an decreased in hydrogen ion production or a increased in its rate of excretion. Loss of hydrogen ions via the gut (vomiting) or the kidney (diuretic therapy) is equivalent to increased bicarbonate production Causes: diuretic therapy, vomiting, mineralocorticoid excess

Respiratory acidosis high blood pCO2 The primary event is retention of CO2 which can be due to lung disease or hypoventilation by a subject with a normal respiratory tract Causes: CNS depression(trauma, drugs), neuromuscular disorders(poliomyelitis, GBS, MG), thoracic disorder(hydrothorax, pneumothorax),lung disorders(bronchial obstruction, emphysema, oedema), mechanical ventilation

Respiratory alkalosis low blood pCO2 The primary event is hyperventilation resulting in a low pCO2 Causes: CNS disturbances (hypoxamia,trauma, infection), pulmonary disorders(embolus,oedema, asthma, pneumonia), mechanical ventilation

Acid-Base Regulation Three mechanisms to maintain pH Respiratory (CO2) Buffer (in the blood: carbonic acid/bicarbonate, phosphate buffers, Hgb) Renal (HCO3-)

Compensation of acid-base disorders Occurs in response to simple acid-base disturbances whereby homeostatic mechanism attempt to shift an abnormal pH back towards normal. A metabolic acidosis (low HCO3) is compensated by a respiratory alkalosis (low pCO2)

Practical-1 Every change in CO2 of 10 mEq/L causes pH to change by 0.08 (or Δ1 = 0.007) Increased CO2 causes a decreases in pH Decreased CO2 causes an increase in pH

Respiratory Acidosis Hypercarbia from hypoventilation Findings: pCO2 increased therefore… pH decreases Example: ABG : 7.32/50/ /25

Respiratory Alkalosis Hypocarbia from hyperventilation Findings: pCO2 decreased… therefore pH increases Example: ABG – 7.45/32/ /25

Metabolic Changes Remember normal HCO3- is 22-26

Practical-2 Every change in HCO3- of 10 mEq/L causes pH to change by 0.15 Increased HCO3- causes an increase in pH Decreased HCO3- causes a decrease in pH

Metabolic Acidosis Inability to excrete acid – e.g. renal tubular acidosis Loss of base – e.g. diarrhea Example: ABG – 7.25/40/ /15

Metabolic Alkalosis Loss of acid – e.g. vomiting (low Cl and kidney retains HCO3-) Gain of base – e.g. contraction alkalosis (lasix) Example: ABG – 7.55/40/ /35

Mixed pH depends on the type, severity, and acuity of each disorder Over-correction of the pH does not occur

Practical Application Check pH Check pCO2 Every change in CO2 of 10 mEq/L causes pH to change by 0.08 Every change in HCO3- of 10 mEq/L causes pH to change by 0.15

Example -1 ABG- 7.30/48/ /22 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change?

Example -1 ABG- 7.30/48/ /22 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change? Combined respiratory and metabolic acidosis

Example -2 ABG- 7.42/50/ /32 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change?

Example -2 ABG- 7.42/50/ /32 Acidotic or Alkalotic? pCO2 High or Low? pH change = pCO2 change? Metabolic alkalosis with respiratory compensation

For respiratory abnormalities, is the condition acute or chronic? Acute respiratory disturbances change pH 0.08 units for every 10 mmHg deviation from normal Therefore, in acute respiratory acidosis, the pH will fall by 0.08 x [(PCO2 - 40)/10] In acute respiratory alkalosis, the pH will rise by 0.08 x [(40-PCO2)/10]

For respiratory abnormalities, is the condition acute or chronic? Chronic respiratory disturbances only change pH 0.03 units for every 10 mmHg deviation from normal Therefore, in chronic respiratory acidosis, pH will fall by 0.03 x [(PCO2 - 40)/10] In chronic respiratory alkalosis, the pH will rise by 0.03 x [(40-PCO2)/10]

Limitations of ABGs ABGs measure gas partial pressures (tensions) Remember: PO2 is not the same as content! A severely anemic patient may have an oxygen content reduced by half while maintaining perfectly acceptable gas exchange and therefore maintaining pO2 Technical issues They hurt Sampling from a vein by mistake Finding an arterial pulse can be difficult in very hypotensive patients Complications such as arterial thrombosis are possible, but awfully rare

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