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AUTHOR AND EDITOR INFORMATION
Section 1 of 11  
Author: Mary C Mancini, MD, PhD, Director of Cardiothoracic Transplantation, Professor, Department of Surgery, Louisiana State University Health Sciences Center
Mary C Mancini is a member of the following medical societies: American Heart Association, American Medical Association, American Thoracic Society, Association for Academic Surgery, Association for Surgical Education, International College of Surgeons, International Society for Heart and Lung Transplantation, New York Academy of Sciences, Phi Beta Kappa, and Southern Thoracic Surgical Association
Coauthor(s):
Girish G Deshpande, MD, MBBS, FAAP, Assistant Professor, Department of Pediatrics, Division of Critical Care Medicine, Children's Hospital of Illinois at OSF St Francis Medical Center
Editors: G Patricia Cantwell, MD, Associate Clinical Professor, Department of Pediatrics, University of Miami; Director of Pediatric Critical Care Medicine, Miller School of Medicine, Jackson Children's Hospital; Mary L Windle, PharmD, Adjunct Assistant Professor, University of Nebraska Medical Center College of Pharmacy, Pharmacy Editor, eMedicine.com, Inc; Barry J Evans, MD, Assistant Professor of Pediatrics, Temple University Medical School; Director of Pediatric Critical Care and Pulmonology, Associate Chair for Pediatric Education, Temple University Children's Medical Center; Mary E Cataletto, MD, Associate Director, Division of Pediatric Pulmonology, Winthrop University Hospital; Associate Professor, Department of Clinical Pediatrics, State University of New York at Stony Brook; Michael R Bye, MD, Attending Physician, Pediatric Pulmonary Medicine, Columbia University Medical Center; Professor of Clinical Pediatrics, Division of Pulmonary Medicine, Columbia University College of Physicians and Surgeons
Author and Editor Disclosure
Synonyms and related keywords:
respiratory alkalosis, alveolar hyperventilation, hyperventilation, acid-base disorders, increased blood pH, alkalemia, hypocapnia, urinary bicarbonate, plasma bicarbonate, acute alkalosis, chronic alkalosis, tetany, symptomatic hypocalcemia, dizziness, mental confusion, seizures, hypocarbia, excessive elimination of carbon dioxide, respiratory abnormality
Background
Respiratory alkalosis is one of many acid-base disorders found among critically ill patients. It is detected by arterial blood gas and electrolyte levels. To diagnose respiratory alkalosis or assess the severity of the condition, the physician must understand clinical acid-base balance. Alkalosis, by definition, is a pathologic state that causes or tends to cause an increase in blood pH. Hence, one can have an alkalosis with normal pH if compensation has occurred; alkalemia is defined as a blood pH above 7.44. The term respiratory in respiratory alkalosis refers to the primary respiratory mechanism responsible for the change.
Pathophysiology
Hypocapnia (low PCO2) develops whenever CO2 elimination by the lungs exceeds tissue production. One or more of 3 basic mechanisms usually underlie respiratory alkalosis (see Image 2).
- Hypoxia
- Metabolic acidosis
- Direct CNS stimulation of respiration.
Compensation In respiratory acid-base disturbances, changes in ventilation, and hence PCO2, represent the primary disturbance, and compensation occurs by alterations in plasma bicarbonate. In chronic respiratory alkalosis, increased urinary bicarbonate excretion resists the pH change caused by hypocapnia. This renal compensation begins within several hours and takes several days for the maximal response. In acute respiratory alkalosis, an initial small decrease may occur in plasma bicarbonate concentration because of chemical mass action. Hypocapnia leads to increased formation of carbonic acid, to lowered plasma hydrogen ion concentration (alkalemia), and to concomitant reduced plasma bicarbonate concentration. This is quantitatively less profound than renal compensation and is not related to change in bicarbonate excretion. Formulas for estimating appropriate compensation in simple respiratory alkalosis (limit of compensation is [HCO3-] of approximately 15) include the following:
- Acute alkalosis - Change in pH = (change in PCO2) X 0.08
- Chronic alkalosis - Change in pH = (change in PCO2) X 0.003
History
Patients primarily have clinical manifestations of the disorder causing the respiratory alkalosis; the effects of respiratory alkalosis per se are fewer.
Acute respiratory alkalosis has more intense features than chronic respiratory alkalosis because later renal compensation and cellular adaptation minimize the pH change.
Alkalosis, by promoting the binding of calcium to albumin, can reduce the fraction of ionized calcium in blood, causing tetany. Symptomatic hypocalcemia is more common with respiratory alkalosis than with metabolic alkalosis.
- Patients have symptoms of underlying disorders.
- Rapid decrease in PCO2 can result in dizziness, mental confusion, and (rarely) seizures, even with a PO2 that is within the reference range. This is probably due to the cerebral vasoconstriction caused by the hypocarbia.
- Patients may have tetany due to reduced ionized calcium in blood.
Physical
- Vital signs
- Patients have fever if respiratory alkalosis is the result of an infectious disorder.
- Hyperthermia of any origin may in turn result in respiratory alkalosis.
- Acute respiratory alkalosis may cause mild tachycardia.
- Respiratory rate is usually high. In some cases, the hyperventilation is primarily a manifestation of increased tidal volume and the respiratory rate may not be markedly elevated. This is often observed in the respiratory alkalosis compensating diabetic ketoacidosis.
- Blood pressure is usually maintained, except when respiratory alkalosis is caused by massive pulmonary embolism or sepsis.
- Central nervous system
- Effects are secondary to the reduction in cerebral blood flow (CBF) caused by reduction in PCO2. CBF may decrease by 1-2 mL/100 g/min for each 1 mm Hg fall in PCO2, with maximum reduction in CBF of 40-50% achieved with a PCO2 of 20-25 mm Hg.
- Reduced CBF may cause altered mentation, dizziness, and sometimes seizures.
- Cardiovascular system
- Cardiovascular effects of acute hypocapnia are minimal in patients who are awake. Tachycardia may be the only observable manifestation.
- Electrolyte imbalance resulting from respiratory alkalosis very rarely may induce dysrhythmias, although only in patients with underlying heart disease.
Causes
- Hypoxia and hypoxemia: Any condition associated with a fall in the PaO2 below 55 mm Hg or with decreased oxygen delivery to the tissues increases minute ventilation, causing respiratory alkalosis. Causes include the following:
- Altitude/low fraction of inspired oxygen (FIO2)
- Anemia
- Hypotension
- Lung disease
- Pulmonary disorders: Interstitial, airway, and parenchymal pulmonary diseases affect PO2 more prominently than PCO2, and hyperventilation usually results in hypocapnia. Inflammation of the irritant receptors in the airways and parenchyma also causes hyperventilation, resulting in respiratory alkalosis. Causes include the following:
- Edema (hydrostatic or permeability)
- Embolism
- Airway obstruction/inflammation
- Pneumonia: A classic presentation of Pneumocystis pneumonia is hypoxemia with respiratory alkalosis.
- Interstitial lung disease
- Mechanical ventilation: Respiratory alkalosis could result from a ventilatory rate or tidal volume that is too high or from the patient triggering excessive additional breaths.
- Extrapulmonary disorders: In these cases, the child has normal lung function with an overriding ventilatory stimulus. These disorders usually result in the most severe respiratory alkalosis. Causes include the following:
- Anxiety, stress
- Neurologic disease (eg, stroke, infection, trauma, tumor)
- Hormones/drugs (eg, catecholamines, progesterone, methylxanthines, salicylates/doxapram, nicotine)
- Pregnancy
- Hyperthermia
- Liver failure, especially with hepatic encephalopathy
- Sepsis
- Recovery from metabolic acidosis
Acidosis, Metabolic
Alkalosis, Metabolic
Anemia, Acute
Anemia, Chronic
Anxiety Disorder: Panic Disorder
Asthma
Dehydration
Diabetic Ketoacidosis
Hyperthyroidism
Hypocalcemia
Meningitis, Aseptic
Mood Disorder: Depression
Pneumococcal Bacteremia
Pneumonia
Pulmonary Infarction
Respiratory Distress Syndrome
Shock
Status Asthmaticus
Thyroid Storm
Toxicity, Salicylate
Toxicity, Theophylline
Other Problems to be Considered
Mood disorder, anxiety
Lab Studies
- A simple step-wise approach proves useful for further workup.
- Step 1: Prove the presence of respiratory alkalosis by an arterial blood gas. A PCO2 less than 35 indicates alveolar hyperventilation. A pH greater than 7.40 makes it likely to be alkalosis. When both are found, this is probably respiratory alkalosis.
- Step 2: Assess the chronicity of hyperventilation. Reference range HCO3- with a pH greater than 7.45 suggests acute hyperventilation, whereas low HCO3- with a pH of 7.40-7.45 suggests a chronic partially compensated process.
- Step 3: An arterial-alveolar oxygen gradient within the reference range and a pH greater than 7.40 is consistent with hyperventilation secondary to direct CNS stimulation, with normal lung function.
- Step 4: Arterial pH less than 7.40 is usually observed with alveolar hyperventilation as compensation for metabolic acidosis (overcompensation for metabolic acidosis is very rare).
- Step 5: Respiratory alkalosis is likely with hypoxemia with alveolar hyperventilation. However, determining if the alkalosis is caused by the hypoxia or if the hypoxia and the alkalosis are caused by the underlying pulmonary disease is difficult.
- Measurement of arterial pH, HCO3-, and PCO2 are crucial. Transcutaneous or end-tidal PCO2 may be used in place of arterial PCO2; however, transcutaneous PCO2 requires normal skin perfusion, and end-tidal pCO2 is useful only in the presence of normal lung function and when no other acid-base disturbance is suspected. Furthermore, the noninvasive tests do not measure the pH.
- A detailed history and careful physical examination should indicate an underlying disorder.
- Standard nomograms (see Image 1) help diagnose simple acid-base disorders, despite the following limitations.
- They describe acid-base status in children with a steady-state condition. Hence, nomograms are not helpful for patients with rapidly changing status.
- Nomograms lose precision at extremes.
- Values falling in respiratory alkalosis may overlap with other mixed disorders and ultimately require clinical judgment.
- Hyperventilation syndrome is often considered a diagnosis of exclusion. Physicians must consider other causes before making the diagnosis. However, in the typical patient with a normal alveolar-arterial oxygen gradient with an acute stress, the diagnosis can be made with confidence.
Imaging Studies
- Chest radiography
- Ventilation/perfusion imaging, helical chest CT imaging, or CT angiography may be performed if pulmonary embolism is suspected.
- CT imaging or MRI of the brain may be indicated if CNS pathology is suspected.
Other Tests
- Drug screening may be helpful.
Medical Care
- Care primarily is directed to the underlying etiology.
- Respiratory alkalosis rarely is life threatening. Direct measures to correct it are usually unnecessary and often may not work unless the underlying cause is treated.
- Respiratory alkalosis encountered during mechanical ventilation may be corrected by reducing the rate or the tidal volume or using sedation and paralysis (if it is caused by patient-triggered ventilator breaths).
- Patients with hyperventilation syndrome may be immediately relieved by rebreathing into a small-volume paper bag; then, the physician should address treating the patient's psychological stress.
Drug therapy for respiratory alkalosis is directed toward alleviation of the underlying causative disorder.
Complications
- The major complication of the alkalosis per se is the concomitant hypocalcemia and potential for tetany.
Prognosis
- Prognosis relates to the underlying pathology.
Medical/Legal Pitfalls
- Identification of any acid-base disturbance requires a good knowledge of physiology and blood gas interpretation.
- Patients with respiratory alkalosis can present with a subtle change in blood bicarbonate. Hence, always interpret the electrolytes and blood gas abnormalities in association with one another and put everything in clinical context upon arriving at the diagnosis.
- Failure to diagnose respiratory alkalosis can lead to underdiagnosis of some potentially lethal diseases such as pulmonary embolism or severe pneumonia.
- An example of medicolegal importance is overzealous ventilator management. This could cause respiratory alkalosis causing cerebral vasoconstriction and hypocalcemia, both of which could potentiate a seizure by reducing seizure threshold. Such seizures could increase the morbidity and mortality of the original disease. This can be easily avoided by routinely monitoring arterial blood gases of patients on ventilator management.
| Media file 1:
Acid-base nomogram shows confidence bands for simple acid-base disturbances. Conversion factor is 1 torr = 0.13 kPa. |
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Media type: Graph
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| Media file 2:
Schematic presentation of pathophysiology of hyperventilation. |
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Media type: Image
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Alkalosis, Respiratory excerpt Article Last Updated: Jul 11, 2006
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