Hypophosphatemia and respiratory failure: prolonged abnormal energy metabolism demonstrated by nuclear magnetic resonance spectroscopy

Hypophosphatemia in critically ill adults and children - A systematic review

BACKGROUND & AIMS

Phosphate is the main intracellular anion essential for numerous biological processes. Symptoms of hypophosphatemia are non-specific, yet potentially life-threatening. This systematic review process was initiated to gain a global insight into hypophosphatemia, associated morbidity and treatments.

METHODS

A systematic review was conducted (PROSPERO CRD42020163191). Nine clinically relevant questions were generated, seven for adult and two for pediatric critically ill patients, and prevalence of hypophosphatemia was assessed in both groups. We identified trials through systematic searches of Medline, EMBASE, Scopus, Cochrane Central Register of Controlled Trials, CINAHL, and Web of Science. Quality assessment was performed using the Cochrane risk of bias tool for randomized controlled trials and the Newcastle-Ottawa Scale for observational studies.

RESULTS

For all research questions, we identified 2727 titles in total, assessed 399 full texts, and retained 82 full texts for evidence synthesis, with 20 of them identified for several research questions. Only 3 randomized controlled trials were identified with two of them published only in abstract form, as well as 28 prospective and 31 retrospective studies, and 20 case reports. Relevant risk of bias regarding selection and comparability was identified for most of the studies. No meta-analysis could be performed. The prevalence of hypophosphatemia varied substantially in critically ill adults and children, but no study assessed consecutive admissions to intensive care. In both critically ill adults and children, several studies report that hypophosphatemia is associated with worse outcome (prolonged length of stay and the need for respiratory support, and higher mortality). However, there was insufficient evidence regarding the optimal threshold upon which hypophosphatemia becomes critical and requires treatment. We found no studies regarding the optimal frequency of phosphate measurements, and regarding the time window to correct hypophosphatemia. In adults, nutrient restriction on top of phosphate repletion in patients with refeeding syndrome may improve survival, although evidence is weak.

CONCLUSIONS

Evidence on the definition, outcome and treatment of clinically relevant hypophosphatemia in critically ill adults and children is scarce and does not allow answering clinically relevant questions. High quality clinical research is crucial for the development of respective guidelines.

X-linked hypophosphatemic rickets associated with respiratory failure

We report a 5-year-old girl who presented to our emergency room with respiratory arrest and limb deformities and was subsequently diagnosed with X-linked hypophosphatemic rickets. On normalization of the serum phosphorus concentration, her respiratory distress resolved, illustrating that untreated X-linked hypophosphatemic rickets can lead to life-threatening respiratory distress.

Neuromuscular dysfunction in critical illness: what are we dealing with?

Neuromuscular weakness is a very common and debilitating problem for survivors of critical illness. Neurophysiological abnormalities are almost ubiquitous in these patients, and often favour a diagnosis of axonal polyneuropathy, whereas muscle histology, where available, reveals a high incidence of atrophy and necrosis. The precise nature and aetiology of this complex disorder is not yet well understood.

Factors Affecting Exercise Capacity in Renal Transplantation Candidates on Continuous Ambulatory Peritoneal Dialysis Therapy

It is well known that reduced peak oxygen uptake (peak VO2) is a predictor for mortality in several chronic diseases and during the preoperative period. The aim of this study was to investigate the factors that influence peak VO2 in renal transplant candidates receiving continuous ambulatory peritoneal dialysis (CAPD) therapy. We included 22 chronic renal failure patients (12 men, 10 women; ages 29.64 +/- 8.29 years; CAPD duration, 37.35 +/- 7.15 months) in this study. Pulmonary function tests and symptom-limited cardiopulmonary exercise tests were administered to all patients. Cardiopulmonary exercise tests were performed on a cycle ergometry at the same time of day for all patients. We analyzed the exercise duration, maximum work rate, and peak VO2 level during cycle ergometry. Serum hemoglobin, hematocrit, total cholesterol, triglyceride, blood urea nitrogen, creatinine, albumin, prealbumin, C-reactive protein, sedimentation rate, ferritin, sodium, potassium, parathyroid hormone, calcium, and phosphorus levels were analyzed from samples. Mean values of exercise duration (6.86 +/- 1.56 minutes), peak VO2 (17.20 +/- 4.91 mL/min/kg), and maximum work rate (77.09 +/- 26.09 watts) were lower when we compared them with predicted values for a healthy population. Peak VO2 was well correlated with serum phosphorus levels (4.51 +/- 1.28 mg/dL, r = .592, P = .004). Test duration was correlated with peak VO2 (r = .489, P = .025) and serum phosphorus levels (r = .530, P = .024). There were no significant correlations with other factors. As a component of ATP, phosphorus is at the hub of the energy-related mechanisms operative in muscles of the respiratory and musculoskeletal systems. Therefore, we suggest that low exercise capacity might be related to low serum phosphorus levels, and that optimal control of serum phosphorus therapy would increase exercise capacity, exercise duration, and oxygen consumption resulting in a decrease of postoperative mortality in renal transplantation candidates.

The regulation and function of phosphate in the human body

Inorganic phosphate (Pi) is required for cellular function and skeletal mineralization. Serum Pi level is maintained within a narrow range through a complex interplay between intestinal absorption, exchange with intracellular and bone storage pools, and renal tubular reabsorption. Pi is abundant in the diet, and intestinal absorption of Pi is efficient and minimally regulated. The kidney is a major regulator of Pi homeostasis and can increase or decrease its Pi reabsorptive capacity to accommodate Pi need. The crucial regulated step in Pi homeostasis is the transport of Pi across the renal proximal tubule. Type II sodium-dependent phosphate (Na/Pi) cotransporter (NPT2) is the major molecule in the renal proximal tubule and is regulated by hormones and nonhormonal factors. Recent studies of inherited and acquired hypophosphatemia which exhibit similar biochemical and clinical features, have led to the identification of novel genes, phosphate regulating gene with homologies to endopeptidases on the X chromosome (PHEX) and fibroblast growth factor-23 (FGF-23), that play a role in the regulation of Pi homeostasis. The PHEX gene encodes an endopeptidase, predominantly expressed in bone and teeth but not in kidney. FGF-23 may be a substrate of this endopeptidase and inhibit renal Pi reabsorption. In a survey in the United States and in Japan, the amount of phosphorus from food is gradually increasing. It is thought that excess amounts of phosphorus intake for long periods are a strong factor in bone impairment and ageing. The restriction of phosphorus intake seems to be important under low calcium intake to keep QOL on high level.

Anions and the anaesthetist

Anions are the negative components of most chemical structures and play many important physiological and pharmacological roles that are of interest to the anaesthetist. Their relevance is reviewed with a particular emphasis on the inorganic anions (halides, bicarbonate, phosphate and sulphate) and the significance and limitations of the anion gap. Organic anions (albumin, lactate) are also discussed, albeit briefly. The suitability of anions for their role in neurotransmission and acid-base balance is outlined.

Phosphate supplementation for hypophosphataemia and parenteral nutrition

Routine detection and treatment of acute hypophosphataemia is important in intensive care unit and many other hospitalized patients, but metabolic bone disease and hypophosphataemia are still experienced as a result of parenteral nutrition. A significantly common problem that faces the compounding pharmacist when formulating parenteral nutrition regimens is the difficulty associated with the successful avoidance of calcium phosphate precipitation. Although incorporation of the normal calcium and phosphate requirements into regimens for metabolically stable adults is usually achievable, it can prove impossible in paediatric and neonatal mixtures when using the standard inorganic sources that are currently licensed for use in the UK and USA. In other countries, where organic compounds are routinely available, this problem does not exist.

[An unusual cause of acute respiratory distress]

A 54-year-old patient was admitted to our ICU for ketoacidosis with acute respiratory distress (ARD). The main and unusual cause of ARD was hypophosphataemia. Patient-related risk factors for chronic hypophosphataemia were malnutrition, chronic alcoholism, and diabetes mellitus. Correction of the metabolic acidosis by insulin therapy resulted in intracellular penetration of phosphate and potassium, causing severe hypophosphataemia and hypokaliaemia responsible for ARD. This case provides an opportunity for reviewing the main causes and consequences of hypophosphataemia, and for emphasising the value of monitoring serum phosphate levels and providing supplemental phosphate in ICU patients at risk for phosphate depletion.

Hypophosphataemia. Pathophysiology, effects and management on the intensive care unit

Routine detection and treatment of hypophosphataemia on the intensive care unit is commonplace. Hypophosphataemia has been associated with a multitude of clinical effects and there are many associations between correction of hypophosphataemia and improvement in symptoms. However, there is no evidence at present to support the routine correction of hypophosphataemia in the absence of clinical symptoms or signs.

Efficacy and safety of intravenous phosphate replacement in critically ill patients

OBJECTIVE

To evaluate the efficacy and safety of intravenous potassium phosphate administered in a fixed-dose regimen in critically ill patients.

DESIGN

Prospective, unblind study.

SETTING

Surgical-medical intensive care unit (ICU).

PARTICIPANTS

Patients who developed hypophosphatemia during their ICU admission.

INTERVENTIONS

Patients with a serum phosphate concentration between 1.27 and 2.48 mg/dL (group 1) and those with a concentration of 1.24 mg/dL or less (group 2) received 15 and 30 mmol, respectively, of phosphate as a potassium salt via a central line over 3 hours.

MAIN OUTCOME MEASURES

Normalization of serum phosphate within 6 hours of infusion, the development of arrhythmias during the infusion, and the development of hypocalcemia and hyperkalemia after the infusion were evaluated. Redevelopment of hypophosphatemia and the need for further therapy were also assessed.

RESULTS

Thirty-seven episodes of hypophosphatemia were entered in this study: 27 in group 1 (17 patients) and 10 in group 2 (10 patients). The mean serum phosphate concentration increased significantly from 2.02 to 2.82 mg/dL in group 1 and from 0.83 to 2.17 mg/dL in group 2, with no change in calcium or potassium. Normalization of serum phosphate with this initial dose occurred in 81.5% of the episodes in group 1 and 30% in group 2. However, over the following 2 days, 45% of the patients in group 1 and 60% in group 2 required further phosphate supplementation. No arrhythmias occurred during the 3-hour infusion that were related to the potassium phosphate. A significant drop in total serum calcium concentrations occurred in 2 patients who were slightly hypercalcemic prior to the infusion. Serum calcium concentrations remained above normal, but this was not associated with any adverse effects.

CONCLUSIONS

The administration of potassium phosphate 15 mmol to critically ill patients with mild-to-moderate hypophosphatemia over 3 hours is both effective and safe. The administration of potassium phosphate 30 mmol to severely hypophosphatemic patients was safe but achieved normalization of serum phosphate in a minority of patients. Either a higher dose or the subsequent administration of more potassium phosphate may be required to normalize serum phosphate concentrations. Once normalization has occurred, there is a high likelihood of redevelopment of hypophosphatemia over the following 2 days and supplementation should be given accordingly.

Intravenous phosphate repletion regimen for critically ill patients with moderate hypophosphatemia

OBJECTIVE

To document the safety and efficacy of an intravenous phosphate repletion regimen that is more aggressive than recommended by previously published guidelines, in intensive care unit (ICU) patients with hypophosphatemia.

DESIGN

Prospective evaluation of rapid, intravenous phosphate repletion in eligible patients.

SETTING

Surgical ICU in a teaching hospital.

PATIENTS

Patients with a serum phosphorus concentration of < 2 mg/dL (< 0.65 mmol/L) while in the ICU.

INTERVENTIONS

Enrolled patients received 15 mmol of sodium phosphate in 100 mL of 0.9% sodium chloride, infused intravenously over a period of 2 hrs. Patients with a serum potassium concentration of < 3.5 mmol/L received potassium phosphate, if no other potassium supplementation was ordered. The same dose could be repeated to a maximum of 45 mmol in a 24-hr period if either the 6-hr or follow-up (18- to 24-hr) postinfusion serum phosphorus remained < 2 mg/dL (< 0.65 mmol). Serum electrolytes, renal function, vital signs, and reflexes were closely monitored.

MEASUREMENTS AND MAIN RESULTS

Eleven patients enrolled had baseline serum phosphorus values of 1.6 to 1.9 mg/dL (0.51 to 0.61 mmol/L). The serum phosphorus value immediately postinfusion was 2.3 to 5.3 mg/dL (0.74 to 1.7 mmol/L). Only one patient had a 6-hr postinfusion serum phosphorus of < 2 mg/dL (< 0.65 mmol/L), requiring two additional doses. Two other patients each required a second dose. Serum phosphorus was corrected in other patients with a single dose. No significant changes were noted in serum calcium, magnesium, or potassium concentrations, urine output, vital signs, or reflexes throughout the repletion period.

CONCLUSIONS

All patients were successfully repleted using the described protocol without any significant adverse effects. This repletion regimen may have widespread applicability in the ICU setting.

Hypophosphatemia and phosphorus depletion in respiratory and peripheral muscles of patients with respiratory failure due to COPD

In 22 patients (19 men, 3 women; mean [+/- SD] age, 63 +/- 6 years) with chronic obstructive pulmonary disease (COPD), phosphorus content was measured by spectrophotometric methods on muscle fragments of both peripheral (quadriceps femoris needle biopsy in 22 patients) and respiratory muscles (external intercostal muscle surgical biopsy in 14 patients). Thirty age- and sex-matched subjects were used as controls (19 for quadriceps femoris muscle biopsy and 11 for intercostal muscle biopsy). Serum phosphorus levels, as well as the main determinants of overall phosphorus metabolism (dietary intake of phosphorus and renal phosphate handling), were also obtained in all patients and control subjects. Muscle phosphorus content of both respiratory and peripheral muscles was significantly reduced in the COPD patient group, no matter what reference index was used (fat-free dry muscle weight or muscle fragment DNA content); muscle phosphorus depletion was present in about 50 percent of patients with COPD. In the same patient group, a significant relationship between muscle and serum phosphorus levels was demonstrable in the case of peripheral muscles only. No relationship was found between phosphorus content of both types of skeletal muscles and dietary phosphorus intake levels or with nutritional status, even though patients with COPD had significantly reduced anthropometric, biochemical, and immunologic indices as compared with controls. Renal phosphorus handling indices of the COPD patient group were compatible with a condition of inadequacy of the renal compensatory mechanism to hypophosphatemia and phosphorus depletion (low percent tubular reabsorption of phosphorus, low renal threshold concentration values). Our study suggests that phosphorus depletion occurs frequently in COPD, but in this clinical condition serum phosphorus levels are not representative of cellular phosphorus levels. Phosphorus depletion, which is equally severe in respiratory and peripheral muscles, could depend, at least in part, on malnutrition and a condition of renal phosphorus wasting possibly linked to some drugs commonly used in patients with COPD (xanthine derivatives, diuretics, etc).

Pharmacologic elevation of blood inorganic phosphate in hypoxemic patients with COPD

We have shown that in patients with COPD, myocardial efficiency during exercise is enhanced following acute elevations of plasma phosphate (Pi). A decrease in Hb-O2 affinity (increase in P50) was not responsible for the improvement. We postulated that the physiologic benefit was due to the acute reversal of a subclinical myocardial Pi depletion. To further test this hypothesis in a chronic state, we studied nine stable hypoxemic (PaO2 = 64 +/- 2 mm Hg [+/- SEM]) patients with COPD over five weeks: two weeks at normal plasma Pi; and three weeks at elevated plasma Pi, induced by etidronate disodium (Didronel; 750 mg orally daily). Administration of etidronate disodium increased (p less than 0.05) plasma level of Pi (4.4 +/- 0.2 to 5.8 +/- 0.1 mg/dl), RBC level of Pi (3.1 +/- 0.2 to 4.1 +/- 0.2 mg/dl), RBC level of 2,3-DPG (16.2 +/- 1.1 to 21.3 g+/- 1.3 mumol/g of Hb) and P50 (23.7 +/- 0.5 to 26.0 +/- 0.8 mm Hg). At the end of the treatment, the widening of the C(a-v)O2 with exercise (7.1 +/- 0.8 to 8.9 +/- 0.6 ml/dl) was less pronounced than under control conditions (6.9 +/- 0.4 to 10.1 +/- 0.6 ml/dl; p less than 0.02); concomitantly, the crossover point (COP; the PaO2 below which a rightward-shifted Hb-O2 curve causes the C(a-v)O2 to become narrower rather than wider) increased (37 +/- 2 to 49 +/- 1 mm Hg). Indicators of myocardial work efficiency were not affected by etidronate disodium at rest or during exercise. We postulate that during exercise the potential beneficial effect of the rightward shift of the Hb-O2 curve upon cardiac function was negated by the fall of PaO2 to or below the COP level, a situation which would limit increases in tissue O2 extraction.

Potential role of acute hypophosphatemia during hypokalemic periodic paralysis attack

A paralysis attack was induced by glucose load in a patient with hypokalemic periodic paralysis. A profound drop in serum phosphorus was observed (from 3.0 to 0.8 mg/dl) in parallel to the serum potassium decrease. The potential role of phosphorus metabolism in the pathophysiology of muscle weakness in this disease is discussed.

Hypophosphatemia in course of chronic obstructive pulmonary disease. Prevalence, mechanisms, and relationships with skeletal muscle phosphorus content

Serum phosphorus levels (Ps), dietary intake of phosphorus, and renal phosphate handling indexes were evaluated in 158 patients with chronic obstructive pulmonary disease (COPD) of varying degrees of severity; moreover, skeletal muscle phosphorus content (Pm) was measured in muscle samples obtained by quadriceps femoris needle biopsy in 14 of the same patients. Hypophosphatemia (Ps less than or equal to 2.5 mg/dl) was found in 34 (21.5 percent) of 158 patients without differences between groups of COPD patients presenting increasing severity of respiratory illness. No relationship was found between serum levels and dietary intake of phosphorus; hypophosphatemia was associated with low renal phosphate threshold (TmPO4/GFR) values in 31 (91 percent) of 34 patients. The prevalence of hypophosphatemia was significantly higher among COPD patients taking one or more drugs commonly used in COPD and known as negatively influencing renal phosphate handling: xanthine derivatives, corticosteroids, loop diuretics, and beta 2-adrenergic bronchodilators. Short-term administration of therapeutic doses of these drugs in COPD patients previously not taking any drug reduced TmPO4/GFR values; phosphaturic effect of short-term theophylline administration on renal phosphate handling was additive to that of long-term assumption of the drug. Muscle phosphorus content was both reduced in COPD patients as compared with control subjects and significantly correlated to serum phosphorus levels and to TmPO4/GFR values. The present investigation revealed a high prevalence of hypophosphatemia among COPD patients as well as a defect in renal phosphate reabsorption secondary, at least in part, to pharmacologic therapy. Moreover, it also suggests that in COPD patients muscle phosphorus content is likely to be reduced in presence of hypophosphatemia.