12. Metabolic Emergencies

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Metabolic Emergencies

Edited by Mark Little

12.1 Acid–base disorders

David McCoubrie and Alan Gault

Introduction

Acid–base disorders are commonly encountered in the emergency department (ED) and their recognition is important for the diagnosis, assessment of severity and monitoring of many disease processes. Although these disorders are usually classified according to the major metabolic abnormality present (acidosis or alkalosis) and its origin (metabolic or respiratory), it is important to realize that acid–base disorders of a mixed type commonly occur and that the recognition and assessment of these are more complex.

CO2 produces acid when in solution and altering PaCO2 through changes in ventilation can produce or remove acid from the body. The terms respiratory acidosis/alkalosis refer to the pH shifts resulting from alterations in PaCO2 from changes in ventilation. Bicarbonate acts as a base in solution with bicarbonate accumulation resulting in a more alkaline state and its wasting or consumption indicating a more acidic state. The terms metabolic acidosis/alkalosis refer to pH shifts characterized by alterations in bicarbonate levels. By convention, the overall pH abnormality as defined by the blood gas assessment is termed alkalaemia (for pH>7.44) or acidaemia (pH<7.34).

Acid–base homeostasis

Acid–base status is one of the most tightly regulated systems in the body. The term compensation is used to describe the processes by which shifts in plasma pH are attenuated. These mechanisms include buffering, respiratory manipulation of CO2 and renal handling of bicarbonate. Buffering with plasma proteins, haemoglobin and the carbonic-acid–bicarbonate systems provide the most immediate mechanism. This is followed by respiratory compensation, which occurs within minutes and is achieved by alterations in alveolar ventilation. Renal compensation usually takes hours to days to take effect.

Acidaemia

Systemic acidaemia is defined as the presence of an increased concentration of H+ ions in the blood. An acidaemia can result from respiratory acidosis, metabolic acidosis or both. The physiological effects of acidaemia are a decrease in the affinity of haemoglobin for oxygen and an increase in serum K+ of approximately 0.4–0.6 mmol/L for each decrease in pH of 0.1 [1]. Although the presence of acidaemia is often associated with a poor prognosis, the presence of acidaemia per se usually has few clinically significant effects. It is the nature and severity of the underlying illness that principally determines outcome. A decrease in measured serum HCO3 of up to 5 mmol/L has also been reported as a result of underfilling of vacuum-type specimen tubes [2].

Metabolic acidosis

Metabolic acidosis is defined as an increase in the [H+] of the blood as a result of increased acid production or bicarbonate wasting from the gastrointestinal (GI) or renal tract. The cause is often multifactorial and can be further classified into ‘anion-gap’ and ‘non-anion-gap’ (or hyperchloraemic) metabolic acidosis.

Anion-gap metabolic acidosis (AGMA)

As electroneutrality must exist in all solutions, the anion gap represents the concentration of anions that are not commonly measured. The most commonly used formula for the calculation of the anion gap is:

< ?xml:namespace prefix = "mml" />Aniongap=[Na+]([Cl]+[HCO3])

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The normal value for the anion gap depends on the type of biochemical analyser used and, while the upper limit of normal has been commonly quoted as 14, the mean range with some modern analysers is only 5–12 [3]. In the normal resting state, the serum ionic proteins account for most of the anion gap, with a lesser contribution from other ‘unmeasured’ anions, such as PO4 and SO4. In pathological conditions where there is an increase in the concentration of unmeasured anions, an AGMA results. The anions responsible for the increase in the anion gap depend on the cause of the acidosis. Lactic acid is the predominant anion in hypoxia and shock, PO4 and SO4 in renal failure, ketoacids in diabetic and alcoholic ketoacidosis, oxalic acid in ethylene glycol poisoning and formic acid in methanol poisoning.

Of the causes of an AGMA, lactic acidosis is the most commonly encountered in the ED and is defined as a serum lactate of>2.5 mmol/L (Table 12.1.1). The presence of lactic acidosis is determined by the balance between lactate production and metabolism. In the seriously ill patient, it is common for increased production and decreased metabolism to be present simultaneously.

It is important to realize that, in many conditions, a variety of factors may produce the acidosis and that multiple anions may be involved in the production of an anion-gap acidosis. In a patient with an AGMA, a non-anion-gap metabolic acidosis may also exist (see below).

Non-anion-gap metabolic acidosis

Non-anion-gap metabolic acidosis results from loss of HCO3 from the body, rather than increased acid production. To maintain electroneutrality, chloride is usually retained by the renal tubules when HCO3 is lost and the hallmark of non-anion-gap acidosis is an elevation of the serum chloride. The causes of non-anion-gap metabolic acidosis are further classified according to the site of HCO3 loss. Gastrointestinal losses can occur with lower gastrointestinal tract (GIT) fluid losses that are rich in HCO3 or with cholestyramine ingestion due to binding of HCO3 in the gut. Renal losses can occur with renal tubular acidosis, carbonic anhydrase inhibitor therapy or adrenocortical insufficiency. Occasionally, direct chloride excess drives the renal bicarbonate loss (again due to electroneutrality) – which can be observed with large volume chloride rich crystalloid administration (chiefly normal saline).

Renal tubular acidosis

Renal tubular acidosis (RTA) is a group of conditions where there is an impaired ability to secrete H+ in the distal convoluted tubule or absorb HCO3 in the proximal convoluted tubule. This may result in a chronic metabolic acidosis, with hypokalaemia, nephrocalcinosis, rickets or osteomalacia. There are many subtypes of RTA and many different causes. Most commonly, it is observed in patients with chronic renal impairment but it may also be drug induced with agents such as ibuprofen, toluene and carbonic anhydrase inhibitors most often implicated.

Treatment of metabolic acidosis

The treatment of acidosis should usually be directed primarily towards correction of the underlying cause. Intravenous HCO3 is of use in the presence of acidosis and severe hyperkalaemia, severe sodium channel (e.g. tricyclic antidepressant), salicylate and methanol poisoning. The use of HCO3 in patients with diabetic ketoacidosis and lactic acidosis associated with sepsis or severe cardiorespiratory disease does not appear to improve outcome [46]. The potential hazards of HCO3 therapy include a high solute load, hyperosmolarity, hypokalaemia, decreased ionized serum calcium and worsening of intracellular and cerebrospinal fluid acidosis (which may precipitate hepatic encephalopathy in susceptible patients).

Respiratory acidosis

Respiratory acidosis is defined as an elevation of the arterial partial pressure of carbon dioxide (PCO2) and is due to alveolar hypoventilation. This can result from central depression in respiriatory drive, neuromuscular weakness, mechanical factors, lung parenchymal disorders and ventilation/perfusion mismatch. With significant elevations in CO2, sweating, tachycardia, confusion and mydriasis occur. When the PCO2 is greater than 80 mmHg, the level of consciousness is usually depressed, known as CO2 narcosis.

Treatment

The treatment of respiratory acidosis is directed towards reversal of the causative factors while supporting and promoting ventilation. Indications for and methods of therapy are clinically determined.

Alkalaemia

Alkalaemia is defined as a decrease in [H+] in the blood. Extreme alkalaemia may cause altered mental status, tetany and seizures. These are predominantly related to a reduction in the concentration of ionized calcium, which is more commonly present in respiratory alkalosis due to anxiety, than from other causes. Alkalaemia in patients with chronic airways disease may exacerbate tissue hypoxia due to leftward shift of the oxygen-dissociation curve. Like acidaemia, there are metabolic and respiratory processes by which it occurs.

Metabolic alkalosis

Metabolic alkalosis most commonly results from loss of acid from the GIT, however, renal acid losses or accumulation of bicarbonate from exogenous sources can contribute. Diagnostically and therapeutically, metabolic alkalosis can be divided into two distinct aetiological groups – chloride-responsive and chloride-unresponsive metabolic alkalosis (Table 12.1.2).

Table 12.1.2

Causes of metabolic alkalosis

Low urine chloride variety (saline-responsive)

Gastric volume loss (vomiting, nasogastric suction, bulimia nervosa)

Diuretics

Licorice

Hypokalaemia

High urine chloride variety (not saline-responsive)

Primary and secondary hyperaldosteronism

Apparent mineralocorticoid excess

Liddle’s syndrome

Conn’s syndrome (aldosteronoma)

Cushing’s disease

Bartter’s syndrome

Gitelman’s syndrome

Excess bicarbonate administration – antacids, dialysis, milk-alkali syndrome

From Murray L, Daly F, Little M, Cadogan M. Toxicology Handbook, 2nd edn. Churchill Livingstone; 2011.

Chloride-responsive metabolic alkalosis arises from conditions that result in both chloride and volume loss. Reduction in extracellular volume leads to increased mineralocorticoid activity causing reabsorption of sodium and secretion of hydrogen. This, in turn, causes increased formation of bicarbonate which, ultimately, overwhelms the kidneys’ ability further to excrete it. In alkalosis, the urine is usually alkaline with higher concentrations of bicarbonate, there is minimal chloride excreted in order to maintain electroneutrality. Hence a urinary chloride<10 mmol/L is a common finding in these conditions. The commonest causes seen in the emergency department are upper GIT losses as a result of severe and prolonged vomiting and diuretic use.

Chloride-unresponsive metabolic alkalosis is typically due to disease states that either result in mineralocorticoid excess in the absence of hypovolaemia or chloride wasting, or congenital disorders with defects in the various ionic transport channels within the kidney. As extracellular volume is either normal or increased, urinary chloride is typically>10 mmol/L. These conditions are seen in the emergency department infrequently.

Treatment should be directed primarily towards correction of the underlying cause. In the presence of upper gastrointestinal fluid losses, intravenous fluids with high chloride content (such as 0.9% saline) should be used initially for rehydration and correction of hypokalaemia is also required.

Respiratory alkalosis

Respiratory alkalosis may be acute or chronic, of which the acute form is most commonly encountered in the emergency department.

Respiratory alkalosis may physiologically occur in the general population secondary to exercise, altitude-related hypoxia and stimulation of the medullary respiratory centre by progesterones during pregnancy. Disease states that give rise to respiratory alkalosis are more likely to be seen in the emergency department (Table 12.1.3). Treatment is again directed towards correction of the underlying cause.

Table 12.1.3

Causes of respiratory alkalosis

CNS-mediated hyperventilation Pulmonary
Increased intracranial pressure Congestive cardiac failure
Cerebrovascular accidents Mechanical hyperventilation
Psychogenic Pneumonia
  Pulmonary emboli
Hypoxia-mediated hyperventilation Sepsis
Altitude Toxin-induced hyperventilation
Anaemia Nicotine
V/Q mismatch Salicylate
Xanthines  

Systematic acid–base interpretation

A systematic stepwise approach to acid–base interpretation is beneficial in the evaluation of disturbances as they are often multiple. What follows is an example of a conventional methodology as outlined by Whittier and Rutecki [7]:

Step 1: what is the pH (primary acid–base disturbance)?

Step 2: determine whether the primary process is respiratory, metabolic or both

Step 3: calculate the anion gap

Aniongap=[Na+]([Cl]+[HCO3])

image
Causes of anion-gap acidosis (mnemonic ‘CAT MUDPILES’)

Causes of a low anion gap (<6)

Causes of non-anion-gap metabolic acidosis

Causes of respiratory acidosis

Acute Chronic
Airway obstruction Lung diseases, e.g. COPD, pulmonary fibrosis
Aspiration Neuromuscular disorders, e.g. muscular atrophy
Bronchospasm Obesity
Drug-induced CNS depression Severe kyphoscoliosis
Hypoventilation of CNS or muscular origin
Hypoventilation of PNS origin, e.g. GBS, OP poisoning
Pulmonary disease

Image

Step 4: check for the degree of compensation

Step 5: determine if there is a 1:1 relationship between the anions in the blood (presence of a delta gap)

In an anion-gap metabolic acidosis, this step determines whether there is a non-anion-gap (hyperchloraemic) component as a contributing explanantion of the bicarbonate fall. There should be a 1:1 relationship between the rise in the anion gap over normal and the decrease in the bicarbonate. If the bicarbonate is higher than predicted then a metabolic alkosis is also present. If the bicarbonate is lower than predicted then a non-anion-gap acidosis is also present.

References

1. Natalini G, Seramondi V, Fassini P, et al. Acute respiratory acidosis does not increase plasma potassium in normokalaemic anaesthetized patients A controlled randomized trial. Eur J Anaesth. 2001;18:394–400.

2. Herr RD, Swanson T. Pseudometabolic acidosis caused by underfill of vacutainer tubes. Ann Emerg Med. 1992;21:177–180.

3. Paulson WD, Roberts WL, Lurie AA, et al. Wide variation in serum anion gap measurements by chemistry analyzers. Am J Clin Pathol. 1998;110:735–742.

4. Cooper DJ. Bicarbonate does not improve haemodynamics in critically ill patients who have lactic acidosis: a prospective controlled clinical study. Ann Intern Med. 1990;112:492–498.

5. Mathieu D, Neviere R, Billard V, et al. Effects of bicarbonate therapy on hemodynamics and tissue oxygenation in patients with lactic acidosis: a prospective, controlled clinical study. Crit Care Med. 1991;19:1352–1356.

6. Okuda Y, Adrogue HJ, Field JB, et al. Counterproductive effects of sodium bicarbonate in diabetic ketoacidosis. J Clin Endocrinol Metab. 1996;81:314–320.

7. Whittier WL, Rutecki GW. Primer on clinical acid-base problem solving. Dis Monitor. 2004;50:117–162.

12.2 Electrolyte disturbances

John Pasco

Essentials

Sodium disorders are relatively common in hospitalized patients and elderly people.

The brain is most at risk from acute hyponatraemia because the osmotically expanded intracellular volume may induce increased intracranial pressure (hyponatraemic encephalopathy).

Treatment of hyponatraemia needs to be carefully individualized because of the risk of osmotic myelinolysis.

Hypernatraemia has a high in-hospital mortality rate, which often reflects severe associated medical conditions.

Although usually benign, hypokalaemia may cause cardiac arrhythmias and rhabdomyolysis. Oral replacement is usually sufficient, except where there is severe myopathy or cardiac arrhythmias.

Electrocardiogram changes in the presence of hyperkalaemia require urgent potassium-lowering measures and myocardial protection with calcium.

Management of severe hypercalcaemia includes enhancement of renal excretion of calcium, inhibition of osteoclast activity and treatment of the underlying condition.

Acute symptomatic hypocalcaemia should be treated with IV calcium.

Hypomagnesaemia is difficult to diagnose because its symptoms are non-specific and the serum level often does not reflect the true magnesium status of the patient. It usually exists as a ‘deficiency triad’ with hypokalaemia and hypocalcaemia.

10 Hypermagnesaemia is often iatrogenic, particularly in elderly patients or patients with renal impairment and/or chronic bowel conditions receiving magnesium therapy.

Hyponatraemia

Introduction

Hyponatraemia, defined as serum sodium concentration of less than 130 mmol/L, is a common condition. The prevalence is estimated at 2.5% in hospitalized patients, of which two-thirds develop the condition while in hospital.

Pathophysiology

Hyponatraemia is almost always associated with extracellular hypotonicity, with an excess of total body water relative to sodium (hypotonic hyponatraemia). The exceptions are:

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