Section 12 Metabolic
12.1 Acid–base disorders
Acidosis
Systemic acidosis is defined as the presence of an increased concentration of H+ ions in the blood. The physiological effects of acidosis 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, although this does not appear to occur during anaesthesia.1 It is commonly believed that acidosis decreases myocardial contractility, but this is usually of little clinical significance at a pH of more than 7.1. Although the presence of acidosis is often associated with a poor prognosis, the presence of acidosis per se usually has few clinically significant effects, and it is the nature and severity of the underlying illness that determines its outcome. There is also evidence that acidosis may have a protective effect in tissues by decreasing phospholipase activity and inhibiting the development of the mitochondrial permeability transition defect that leads to apoptosis. In certain situations, such as in pregnant women and the neonate, increased acid production may result in a greater change in serum pH (and possibly greater adverse physiological effects) than expected, owing to the decreased buffering capacity of the plasma. 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
Anion-gap metabolic acidosis
The normal value for the anion gap depends on the type of biochemical analyser used and whilst the upper limit of normal has been commonly quoted as 18, 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 anion-gap metabolic acidosis 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.
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, methanol or ethylene glycol toxicity. It should be used to attempt to normalize the pH before Factor VIIa therapy is administered. It may be of use in rhabdomyolysis and cardiac arrest in young children or pregnant women or cardiac arrest of more than 15 min duration. 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.4–6 The potential hazards of HCO3− therapy include a high solute load, hyperosmolarity, hypokalaemia, decreased ionized serum calcium, worsening of cerebrospinal fluid acidosis (which may precipitate hepatic encephalopathy in susceptible patients) and decreased metabolic degradation of citrate, lactate and ketone bodies in the liver. It also reduces oxygen off-loading by haemoglobin in the tissues and may inactivate calcium and adrenaline when administered through the same intravenous line.7
Respiratory acidosis
Respiratory acidosis is defined as an elevation of the arterial partial pressure of carbon dioxide (PCO2) and is due to alveolar hypoventilation. The effects of mild-to-moderate hypercarbia are usually confined to its effect on decreasing the alveolar partial pressure of oxygen (see alveolar gas equation). With more significant elevations, sweating, tachycardia, confusion and mydriasis occur. When the PCO2 is greater than 80 mmHg, the level of consciousness is usually depressed. There are many possible causes of alveolar hypoventilation. Central nervous system causes include severe hypotension, drugs with respiratory depressant effects (especially opioids and sedatives), cerebrovascular events, tumours, infections, neurotrauma and metabolic derangements. Ventilatory drive may also be reduced by high partial pressures of oxygen in patients with chronic obstructive airways disease (COAD) and chronic CO2 retention. Lesions of the spinal cord, such as tumours, infections, trauma or demyelination, may also result in alveolar hypoventilation if the lesion is above the level of C4. Lower in the afferent limb of respiratory muscle innervation, lesions of peripheral nerves such as Guillain–Barré syndrome or trauma to both phrenic nerves may also be causative. Neuromuscular junction dysfunction following postsynaptic destruction of acetylcholine receptors in myaesthenia gravis, inactivation of cholinesterase in organophosphate poisoning or the effects of spider or snake venoms and muscle relaxant drugs may also cause ventilatory failure. Aminoglycoside antibiotics may also precipitate ventilatory failure in susceptible patients. Muscular dystrophy, myopathies and severe electrolyte disorders may cause muscular weakness, and lesions of the chest wall, such as flail chest, severe kyphoscoliosis or arthritis, may also impair effective ventilation.
Alkalosis
Metabolic alkalosis
Alkali may be added to the body in the form of citrate by red cell transfusion, intravenous NaHCO3 administration or as urinary alkalinizers. The milk alkali syndrome may occur as a result of the chronic ingestion of more than 2 g of calcium salts each day (commonly in conjunction with vitamin D).8 The metabolic derangement known as post-hypercapnic alkalosis is caused by the decrease of a chronically elevated PCO2 to normal levels, when relative hyperventilation occurs. In such patients the HCO3− is usually elevated as a result of chronic hypercarbia, and when the PCO2 is acutely lowered to normal levels the appearance on blood gas analysis is that of a metabolic alkalosis, rather than that of a relative respiratory alkalosis. The most common example of this in the ED occurs in a patient who has chronic CO2 retention with an acute exacerbation of COAD. The ingestion of strong alkali is almost never a cause of systemic alkalosis.
Respiratory alkalosis
Common causes of respiratory alkalosis in the general population include exercise, altitude-elated hypoxia and stimulation of the medullary respiratory centre by progesterones during pregnancy. In the ED setting, causes such as hypoxia, early sepsis, cerebral oedema, hepatic cirrhosis, mechanical ventilation, anxiety and salicylate, theophylline and carbon monoxide toxicity should be considered.9 Treatment is directed towards correction of the underlying cause and the treatment of hypokalaemia or hypocalcaemia as required. Hydrochloric acid can be administered to correct the metabolic abnormality, but it is rarely required. A dose of 1–3 mmol/kg of hydrochloric acid can be given through a central venous line at a rate of no faster than 1 mEq/min.10
Whilst it was previously thought that, due to levels decreasing rapidly following sampling, serum lactate needed to be measured by immediate assay of arterial blood, more recent evidence has challenged this assumption with one study failing to demonstrate any significant difference in lactate levels measured at 15 min after sampling.11
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. European Journal of Anaesthesiology. 2001;18(6):394-400.
2 Herr RD, Swanson T. Pseudometabolic acidosis caused by underfill of vacutainer tubes. Annals of Emergency Medicine. 1992;21(2):177-180.
3 Paulson WD, Roberts WL, Lurie AA, et al. Wide variation in serum anion gap measurements by chemistry analyzers. American Journal of Clinical Pathology. 1998;110(6):735-742.
4 Cooper DJ. Bicarbonate does not improve haemodynamics in critically ill patients who have lactic acidosis: a prospective controlled clinical study. Annals of International Medicine. 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. Critical Care Medicine. 1991;19(11):1352-1356.
6 Okuda Y, Adrogue HJ, Field JB, et al. Counterproductive effects of sodium bicarbonate in diabetic ketoacidosis. Journal of Clinical Endocrinology and Metabolism. 1996;81(1):314-320.
7 Australian Resuscitation Council, Medications in cardiac arrest, February 2006.
8 Whiting SJ, Kim K, Wood R. Calcium supplementation. Journal of the American Academy of Nurse Practitioners. 1997;9(4):187-192.
9 Laffey JG, Kavanagh BP. Hypocapnia. New England Journal of Medicine. 2002;347(1):43-53.
10 Adrogue HJ, Madias NE. Management of life-threatening acid base disorders. New England Journal of Medicine. 1998;338(2):107-111.
11 Jones AE, Leonard MM, Hernandez-Nino J, et al. Determination of the effect of in vitro time, temperature, and tourniquet use on whole blood venous point-of-care lactate concentrations. Academic Emergency Medicine. 2007;14:587-591.
12.2 Electrolyte disturbances
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 whilst in hospital.1
Pathophysiology
Hyponatraemia causes cellular swelling as water moves down an osmotic gradient into the intracellular fluid. Most of the symptomatology of hyponatraemia is produced in the central nervous system (CNS) by the swelling of brain cells within the rigid calvarium, causing raised intracranial pressure (hyponatraemic encephalopathy). As intracranial pressure rises, adaptive responses come into play. Initially there is a reduction of the cerebral blood and cerebrospinal fluid (CSF) pools. Later, neuronal intracellular osmolality is reduced by extrusion of potassium, followed within hours to days by organic solutes such as amino acids, phosphocreatine and myoinositol. These processes return brain volume towards normal and restore cellular function.
Aetiology and classification
Hypovolaemic hyponatraemia
These patients have deficits in both total body sodium and total body water, but the sodium deficit exceeds the water deficit. Causes include renal and extra-renal fluid losses, and are listed in Table 12.2.1. Determination of the urinary sodium concentration can differentiate these two groups. Extrarenal losses are associated with low urinary sodium concentrations (<20 mmol/L) and hyperosmolar urine. The exception is with severe vomiting and metabolic alkalosis, where bicarbonaturia obligates renal sodium loss and urinary sodium is high (>20 mmol/L), despite volume depletion. However, urinary chloride, a better indicator of extracellular fluid (ECF) volume, is low.
TURP, transurethral resection of prostate; ADH, antidiuretic hormone; SSRI, selective serotonin reuptake inhibitor; TCA, tricyclic antidepressant; MAOI, monoamic oxidase inhibitor; SIAOH, syndrome of inappropriate antidiuretic hormone secretion.
Clinical features
In addition to the features of the underlying medical condition and alteration in extracellular volume, clinical manifestations of hyponatraemia per se usually develop when serum sodium is less than 130 mmol/L. The severity of symptoms depends partly on the absolute serum sodium concentration and partly on its rate of fall. At sodium concentrations from 125 to 130 mmol/L the symptoms are principally gastrointestinal, whereas at concentrations below 125 mmol/L the symptoms are predominantly neuropsychiatric. The principal signs and symptoms of hyponatraemia are listed in Table 12.2.3.
Anorexia |
Nausea |
Vomiting |
Lethargy |
Muscle cramps |
Muscle weakness |
Headache |
Confusion/agitation |
Altered conscious state |
Seizures |