Normal serum osmolality is 270 to 290 mOsm/L. When there is a discrepancy of greater than 10 mOsm/L between the calculated and the directly measured values (osmolal, or osmolar gap), it can be assumed that additional circulating ions are present. Most often they are derived from an exogenous toxin or drug such as mannitol, but renal failure, ketonemia, or an increase of serum lactate may result in the accumulation of small molecules that contribute to the measured serum osmolality.
A point to be remembered is that the brain may be damaged, even to an irreparable degree, by a disturbance of blood chemistry (e.g., hypoglycemia, hypoxia) that has vanished by the time the patient is examined.
Here the basic disorder is a lack of oxygen and of blood flow to the brain, the result of failure of the heart and circulation or of the lungs and respiration. Often, both are responsible and one cannot say which predominates; hence the dually ambiguous allusions in medical records to “hypoxic-ischemic” encephalopathy. This combined encephalopathy in various forms and degrees of severity is one of the most frequent and disastrous cerebral disorders encountered in every general hospital.
Reduced to the simplest formulation, a deficient supply of oxygen to the brain is either the result of a failure of cerebral perfusion (ischemia) or of a reduced amount of circulating arterial oxygen, of diminished oxygen saturation, or of insufficiency of hemoglobin (hypoxia). Although they are often combined, the neurologic effects of ischemia and hypoxia are subtly different. The medical conditions that most often lead to it are as follows: 1. A global reduction in cerebral blood flow (myocardial infarction, ventricular arrhythmia, aortic dissection, external or internal blood loss, and septic or traumatic shock) 2. Hypoxia from suffocation (drowning, strangulation, or aspiration of vomitus, food, or blood; from compression of the trachea by a mass or hemorrhage; tracheal obstruction by a foreign body, general anesthesia accident, narcotic overdose, extreme asthma) 3. As a subset of the above, diseases that paralyze the respiratory muscles (Guillain-Barré syndrome, amyotrophic lateral sclerosis, myasthenia, and, in the past, poliomyelitis) or damages the medulla and leads to failure of breathing 4. The special case of carbon monoxide (CO) poisoning (nonischemic hypoxia)
The product of blood oxygen content and cardiac output is the ultimate determinant of the adequacy of oxygen supply to the organs. When blood flow is stable, the most important element in the delivery of oxygen is the oxygen content of the blood. This is the product of hemoglobin concentration and the percentage of oxygen saturation of the hemoglobin molecule. At normal temperature and pH, hemoglobin is 90 percent saturated at an oxygen partial pressure of 60 mm Hg and still 75 percent saturated at 40 mm Hg; that is, as is well known, the oxygen saturation curve is not linear.
Physiology of Hypoxic and Ischemic Damage