1.5 Disorders of acid–base balance Metabolic acidosis A metabolic acidosis is any process, other than a rise in PaCO2, that acts to lower blood pH. It may occur through accumulation of metabolic acids (excess ingestion, increased production or reduced renal excretion) or through excessive loss of base (HCO3). Calculating the anion gap (see over) may help to establish the cause of a metabolic acidosis. Metabolic acidosis is recognised on an ABG by low HCO3 (and negative base excess (BE)). There is normally a compensatory increase in alveolar ventilation to lower PaCO2. If respiratory compensation is overwhelmed, an acidaemia will result. Severity must be judged according to both the underlying process and the resulting acidaemia. An HCO3 < 15 mmol/L (or BE < 10) indicates a severe acidotic process whereas a pH < 7.25 (H+ > 55) constitutes serious acidaemia. The dominant symptom in metabolic acidosis is often hyperventilation (Kussmaul’s respiration) owing to the respiratory compensation. Other signs are fairly non-specific or related to the underlying cause. Profound acidaemia (pH < 7.15; H+ > 70) may lead to circulatory shock, organ dysfunction, and, ultimately death. Specific causes of metabolic acidosis are discussed in greater detail in the relevant cases in Part 2. The anion gap In blood, positively charged ions (cations) must be balanced by negatively charged ions (anions) to maintain electroneutrality. But when the main cations (Na+ + K+) are compared with the main anions (Cl− + HCO3−), there appears to be a shortage of anions or an anion gap. The gap is made up of unmeasured anions such as phosphate and sulphate and negatively charged proteins (these are difficult to measure). A raised anion gap (>18 mmol/L) therefore indicates the presence of increased unmeasured anions, e.g. lactate, salicylate. A note on … lactic acidosis This common cause of metabolic acidosis occurs when tissues receive an inadequate supply of O2 due to either hypoxaemia or impaired perfusion. Normal aerobic metabolism (which relies on O2) is then replaced by anaerobic metabolism (which generates lactic acid). Lactic acidosis is therefore a marker of tissue hypoxia and a useful indicator of severity in a variety of conditions, including severe hypoxaemia or shock from any cause. Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Common ABG values When and why is an ABG required? Making ABG interpretation easy Acid–base balance: the basics Stay updated, free articles. Join our Telegram channel Join Tags: Arterial Blood Gases Made Easy Dec 18, 2016 | Posted by admin in CARDIOLOGY | Comments Off on Disorders of acid–base balance Full access? Get Clinical Tree
1.5 Disorders of acid–base balance Metabolic acidosis A metabolic acidosis is any process, other than a rise in PaCO2, that acts to lower blood pH. It may occur through accumulation of metabolic acids (excess ingestion, increased production or reduced renal excretion) or through excessive loss of base (HCO3). Calculating the anion gap (see over) may help to establish the cause of a metabolic acidosis. Metabolic acidosis is recognised on an ABG by low HCO3 (and negative base excess (BE)). There is normally a compensatory increase in alveolar ventilation to lower PaCO2. If respiratory compensation is overwhelmed, an acidaemia will result. Severity must be judged according to both the underlying process and the resulting acidaemia. An HCO3 < 15 mmol/L (or BE < 10) indicates a severe acidotic process whereas a pH < 7.25 (H+ > 55) constitutes serious acidaemia. The dominant symptom in metabolic acidosis is often hyperventilation (Kussmaul’s respiration) owing to the respiratory compensation. Other signs are fairly non-specific or related to the underlying cause. Profound acidaemia (pH < 7.15; H+ > 70) may lead to circulatory shock, organ dysfunction, and, ultimately death. Specific causes of metabolic acidosis are discussed in greater detail in the relevant cases in Part 2. The anion gap In blood, positively charged ions (cations) must be balanced by negatively charged ions (anions) to maintain electroneutrality. But when the main cations (Na+ + K+) are compared with the main anions (Cl− + HCO3−), there appears to be a shortage of anions or an anion gap. The gap is made up of unmeasured anions such as phosphate and sulphate and negatively charged proteins (these are difficult to measure). A raised anion gap (>18 mmol/L) therefore indicates the presence of increased unmeasured anions, e.g. lactate, salicylate. A note on … lactic acidosis This common cause of metabolic acidosis occurs when tissues receive an inadequate supply of O2 due to either hypoxaemia or impaired perfusion. Normal aerobic metabolism (which relies on O2) is then replaced by anaerobic metabolism (which generates lactic acid). Lactic acidosis is therefore a marker of tissue hypoxia and a useful indicator of severity in a variety of conditions, including severe hypoxaemia or shock from any cause. Only gold members can continue reading. Log In or Register to continue Share this:Click to share on Twitter (Opens in new window)Click to share on Facebook (Opens in new window) Related Related posts: Common ABG values When and why is an ABG required? Making ABG interpretation easy Acid–base balance: the basics Stay updated, free articles. Join our Telegram channel Join Tags: Arterial Blood Gases Made Easy Dec 18, 2016 | Posted by admin in CARDIOLOGY | Comments Off on Disorders of acid–base balance Full access? Get Clinical Tree
