MED IC A L PR OGR ES S
Medical Progress H YPOCAPNIA
JOHN G. LAFFEY, M.D.,
TABLE 1. CAUSES
BRIAN P. KAVANAGH, M.B.
RTERIAL carbon dioxide tension represents the balance between the production and elimination of carbon dioxide, and in healthy persons, it is maintained within narrow physiologic limits. Hypocapnia, even when marked, isnormally well tolerated, often with few apparent effects. Transient induction of hypocapnia can lead to lifesaving physiological changes in patients with severe intracranial hypertension or neonatal pulmonary-artery hypertension, but hypocapnia of longer duration in critically ill patients may adversely affect outcomes.1,2 Despite concern about adverse effects, the induction of hypocapnia has commonlybeen recommended for diverse disease states.3-5 Thus, hypocapnia, whether produced deliberately3-5 or accidentally,6,7 remains prevalent in clinical practice (Table 1). In addition, hypocapnia is a common component of many acute illnesses, although its importance is often underestimated.8-12 The prevalence of hypocapnia may be exacerbated by the belief held by some clinicians that hypocapnia isinherently safer than — or at least preferable to — hypercapnia.
DEVELOPMENT OF ARTERIAL HYPOCAPNIA
Hypoxemia High altitudes, pulmonary disease Pulmonary disorders Pneumonia, interstitial pneumonitis, fibrosis, edema, pulmonary embolism, vascular disease, bronchial asthma, pneumothorax Cardiovascular disorders Congestive heart failure, hypotension Metabolic disorders Acidosis (diabetic,renal, or lactic), hepatic failure Central nervous system disorders Psychogenic or anxiety-induced hyperventilation, central nervous system infection, central nervous system tumors Drugs Salicylates, methylxanthines, b-adrenergic agonists, progesterone Miscellaneous Fever, sepsis, pain, pregnancy
In its simplest form, the partial pressure of arterial carbon dioxide (PaCO2) reflects the balancebetween the production and elimination of carbon dioxide (CO2), as described by the following formula:
PaCO2 is proportional to CO2 production + inspired CO2 . CO2 elimination
The volume of inspired carbon dioxide is usually negligible, whereas reduced carbon dioxide producFrom the Department of Physiology, Conway Institute of Biomolecular and Biomedical Research, University College Dublin,and the Department of Anaesthesia and Intensive Care, St. Vincent’s University Hospital, Dublin, Ireland (J.G.L.); and the Department of Critical Care Medicine and the Lung Biology Program, the Research Institute, the Hospital for Sick Children, and the Program in Critical Care Medicine, University of Toronto, Toronto (B.P.K.). Address reprint requests to Dr. Kavanagh at the Department of CriticalCare Medicine, the Hospital for Sick Children, 555 University Ave., Toronto, ON M5G 1X8, Canada, or at email@example.com.
tion is an unusual, but possible, contributor to hypocapnia. Therefore, for practical purposes, a low partial pressure of arterial carbon dioxide reflects the rate of elimination of carbon dioxide. Thus, the principal physiologic causes of hypocapnia, including pregnancy, arerelated to hyperventilation (Table 1).13 Of course, hyperventilation can occur with mechanical ventilation, and artificial clearance of carbon dioxide with the use of extracorporeal techniques (e.g., cardiopulmonary bypass, extracorporeal membrane oxygenation, or devices for the removal of carbon dioxide) is extraordinarily efficient.6,14,15 One form of hypocapnic alkalosis that is rarelydiscussed occurs during critical reduction of pulmonary perfusion (for example, during cardiopulmonary resuscitation). In such cases, there is a dissociation between the condition of central venous blood, with a high partial pressure of arterial carbon dioxide and a low pH, and that of the systemic arterial blood, with a low carbon dioxide tension and an alkalemic pH; this dissociation is due to the...
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