Evaluation of respiratory compensation in metabolic alkalosis

A mathematical model of tumour and blood pHe regulation: The HCO3-/CO2 buffering system

Malignant tumours are characterised by a low, acidic extracellular pH (pHe) which facilitates invasion and metastasis. Previous research has proposed the potential benefits of manipulating systemic pHe, and recent experiments have highlighted the potential for buffer therapy to raise tumour pHe, prevent metastases, and prolong survival in laboratory mice. To examine the physiological regulation of tumour buffering and investigate how perturbations of the buffering system (via metabolic/respiratory disorders or changes in parameters) can alter tumour and blood pHe, we develop a simple compartmentalised ordinary differential equation model of pHe regulation by the HCO3-/CO2 buffering system. An approximate analytical solution is constructed and used to carry out a sensitivity analysis, where we identify key parameters that regulate tumour pHe in both humans and mice. From this analysis, we suggest promising alternative and combination therapies, and identify specific patient groups which may show an enhanced response to buffer therapy. In addition, numerical simulations are performed, validating the model against well-known metabolic/respiratory disorders and predicting how these disorders could change tumour pHe.

Respiratory adjustment to chronic metabolic alkalosis in man

This study examined the ventilatory adjustment to chronic metabolic alkalosis induced under controlled conditions in normal human volunteers. Metabolic alkalosis induced by buffers (sodium bicarbonate, trishydroxymethylamine methane) or ethacrynic acid was associated with alveolar hypoventilation, as evidenced by a rise in arterial Pco(2), a fall in arterial Po(2), a reduced resting tidal volume, and a diminished ventilatory response to CO(2) inhalation. Alveolar hypoventilation did not occur when metabolic alkalosis was induced in the same subjects by thiazide diuretics or aldosterone despite comparable elevations of the arterial blood pH and bicarbonate concentration.The different ventilatory responses of the two groups could not be ascribed to differences among individuals comprising each group, pharmacological effects of the alkalinizing agents, differences in the composition of the lumber spinal fluid, changes in extracellular fluid volume, or sodium and chloride balance.The differences in ventilatory adjustments were associated with differences in the patterns of hydrogen and potassium ion balance during the induction of alkalosis. Alveolar hypoventilation occurred when hydrogen ions were buffered (sodium bicarbonate, trishydroxymethylamine methane) or when renal hydrogen ion excretion was increased (ethacrynic acid). Alveolar hypoventilation did not occur when induction of similar degrees of extracellular alkalosis was accompanied by marked potassium loss and no demonstrable increase in external hydrogen loss (thiazides and aldosterone).These observations suggest that respiratory depression does not necessarily accompany extracellular alkalosis but depends on the effect of the mode of induction of the alkalosis on the tissues involved in the control of ventilation.

Postoperative metabolic alkalosis following general surgery: its incidence and possible etiology

A prospective clinical study was performed on 293 patients, in order to elucidate the abnormalities in acid-base balance following general surgery. Six arterial blood gas and pH determinations were taken from each patient before surgery and on postoperative days zero, one, three, five and seven. A total of 1699 determinations were obtained. Although the majority of patients (87.5 per cent) had a normal acid-base balance before surgery, a postoperative metabolic alkalosis was seen in 50.5 per cent of the patients. However, there was an extremely low incidence of other postoperative acid-base abnormalities, apart from a transient increase in metabolic acidosis on the operative day. A significantly high mortality rate (32.3 per cent) was observed in 31 patients who had continuous metabolic alkalosis during the postoperative period. An excessive bicarbonate load resulting from the administration of fresh frozen plasma following surgery was strongly suggested as one of the major causes of postoperative metabolic alkalosis. Further investigation is required to elucidate the mechanism of the generation of metabolic alkalosis induced by the postoperative bicarbonate load in surgical patients.

Furosemide decreases ventilation in young rabbits

To test the hypothesis that furosemide would cause metabolic alkalosis and thus alveolar hypoventilation, normal rabbit pups were given either furosemide (4 mg/kg/day) or saline solution for the first 8 to 10 days of life. Pups given furosemide developed primary metabolic alkalosis and reduced ventilation, which resulted in secondary respiratory acidosis. Lung compliance was improved by furosemide, and the ventilatory response to CO2 was unaffected. KCl injection in alkalotic pups increased ventilation and decreased pH. The data show that conventional doses of furosemide can (1) cause metabolic alkalosis and reduce ventilation; (2) increase the PaCO2, which reflects changes in acid-base status and not changes in lung function; and (3) increase lung compliance, perhaps by decreasing lung water. When these effects occur in infants with chronic lung disease, the beneficial effect of furosemide may be obscured.

Compensatory hypoventilation in metabolic alkalosis

Although hyperventilation is a well-known compensatory mechanism in metabolic acidosis, compensatory hypoventilation has been inconsistent and controversial in metabolic alkalosis. Six healthy subjects were studied under baseline conditions and during steady-state metabolic acidosis (seven episodes) and alkalosis (14 episodes). Minute ventilation (VE) fell in metabolic alkalosis and rose in metabolic acidosis. These changes in ventilation were entirely due to reduction and elevation of tidal volume (VT) respectively, while respiratory frequency (f) remained unchanged. Alveolar ventilation fell during metabolic alkalosis and resulted in elevation of arterial PCO2 in all subjects. The ventilatory response to arterial PCO2 in all subjects. The ventilatory response to CO2 breathing was also diminished. There was a linear relationship between PaCO2 and plasma [HCO-3] in metabolic acidosis and alkalosis which was defined as PaCO2 (mm Hg = 0.7 [HCO-a] + 20 (+/- SEM), r = 0.95. Although arterial PO2 and plasma [K+] fell during metabolic alkalosis, minute ventilation did not change upon breathing oxygen and there was no correlation between changes in plasma [K+] and plasma H+ regulation.

Respiratory failure

Patients with respiratory failure should be approached in a systematic way, with emphasis both in diagnosis and treatment on arterial blood gases. The intelligent assessment of oxygenation, ventilation and acid-base balance, based on physiologic principles, can make the management of these patients very rewarding. The physiologic principles outlined here should be well understood by anyone entrusted with the care of these patients. They provide the cornerstone of diagnosis and management, and will remain valid long after current clinical dogma has been revised.

Case report: diuretics and non-respiratory alkalosis

A patient with chronic respiratory failure and cor pulmonale was admitted to hospital with an exacerbation of cardiac failure. In attempting to control cardiac failure, intensive diuretic therapy resulted in the development of dehydration, alkalosis, grand mal convulsions and signs of a left hemiparesis. Correction of the electrolyte and acid-base abnormalities resulted in complete neurological recovery.

Mechanisms of production by diuretics and the treatment of non-respiratory alkalosis is discussed.