Safe use of PEEP in patients with severe head injury.

Thirty-three patients with severe head trauma were studied to determine whether the use of positive end-expiratory pressure (PEEP) would cause an increase in intracranial pressure (ICP). Changes in ICP induced by PEEP were then correlated with a panel of physiological variables to try to explain these changes. Mean ICP increased from 13.2 +/- 7.7 mm Hg (+/- standard deviation) to 14.5 +/- 7.5 mm Hg (p less than 0.005) due to 10 cm H2O PEEP, but the eight patients with elevated baseline ICP experienced no significant increase. Cardiac output and venous admixture (Qs/Qt) declined significantly, while central venous pressure, peak inspiratory pressure, functional residual capacity, and arterial pCO2 increased significantly due to PEEP. Blood pressure and cerebral perfusion pressure were unchanged. The change in ICP due to PEEP correlated significantly with a combination of cardiac output, peak inspiratory pressure, Qs/Qt, and changes in blood pressure and arterial pCO2 due to PEEP, indicating that the effect of PEEP on ICP could be largely explained by its effect on hemodynamic and respiratory variables. No patient deteriorated clinically due to PEEP. It is concluded that 10 cm H2O PEEP increases ICP slightly via its effect on other physiological variables, but that this small increase in ICP is clinically inconsequential.

Detection of rubella-specific IgM in subclinical rubella reinfection in pregnancy.

Seven cases of asymptomatic rubella reinfection in early pregnancy are described. In each, there was a history of exposure to a rubelliform illness and low levels of rubella-specific IgM subsequently appeared in the serum. Four of the women had been immunised, after having been shown to be susceptible to rubella, one had been immunised at school without previous antibody screening, and two were uncertain about immunisation. One pregnancy was terminated and rubella virus was not isolated from the products of conception. Six pregnancies went to term and the infants showed no evidence of intrauterine infection. In a further case it was impossible to discriminate between reinfection and primary infection, and termination of pregnancy was offered.

Reduced functional residual capacity and abnormal oxygenation in patients with severe head injury.

Severe head injury often results in hypoxemia, but the pathophysiology of this phenomenon is unclear. We studied 24 patients hospitalized after severe head injury to determine whether the abnormality of oxygen transfer as measured by venous admixture (Qs/Qt) was associated with a reduction in functional residual capacity (FRC) and also what changes in these variables could be induced by positive end-expiratory pressure (PEEP). Mean FRC was 68 percent of the value predicted for the upright position, and mean Qs/Qt was 0.196. The FRC and Qs/Qt were significantly related so that patients with the lowest FRC had the highest Qs/Qt (p less than 0.001). The FRC was small enough to expect closure of small airways in many of these patients. The addition of 10 cm H2O of PEEP resulted in an increase in FRC of 28 percent (+/- 15 percent [SD] of the value predicted for upright posture; Qs/Qt declined by 0.05 (+/- 0.05 [SD] ). Most of these patients had no spontaneous breathing due either to the severity of the brain injury or to the therapeutic hyperventilation and muscular paralysis employed to control intracranial pressure. We conclude that FRC is often reduced in patients hospitalized after severe head injury and that associated abnormalities of ventilation-perfusion matching often lead to an elevated Qs/Qt. Therapy with PEEP in the range of 10 to 15 cm H2O was well tolerated by our patients and can be used safely to reduce Qs/Qt.

Accurate measurement of functional residual capacity and oxygen consumption of patients on mechanical ventilation.

We developed an apparatus and technique for the simultaneous measurement of functional residual capacity and oxygen uptake (VO2) for use in intensive care unit (ICU) patients. The accuracy of the functional residual capacity measurement was proven using an in vitro lung model and the reproducibility of this measurement was established by use in ICU patients. We tested the accuracy of the VO2 measurement in comparison with two other methods in common use among ICU patients and our method proved accurate. We conclude that this technique for measurement of functional residual capacity and VO2 is highly accurate and easily applied to patients on any mode of mechanical ventilation.

Kinetic studies on the effects of ouabain on Na+ fluxes in frog skin.

Among 48 pieces of paired frog skins of Rana pipiens in Ringer's solution, 10 pieces showed a strictly monotone decrease in the short circuit current (SCC) following ouabain treatment (10(-4) M). In 9 cases a transient attenuation, and in 27 cases a distinct wave in the ebb of the SCC, was seen. In 2 instances, two waves were seen. Associated with the not-monotone events was a transient rise in electrical skin conductance. The reasons for these mixed skin responses are unknown. One possible reason is considered here: Early during the ouabain action, some of the Na+ entering from the mucosal side is trapped in the skin by electroneutral processes, in keeping with the already known fact that ultimately cellular KCl is partly replaced by NaCl. Computer assisted model studies show how monotone, and not-monotone "transepithelial" net Na+ flux curves can be generated. Essential conditions for the generation of not-monotone Na+ flux curves are: 1. Presence of two distinct "cellular", active Na+ pools in the model. 2. Presence of a loop pathway in which a principal "transepithelial Na+ transport compartment", and a constituent "Na+/K+ maintenance compartment", are connected to each other and to the "extracellular" compartment. The model, then, predicts under which kinetic conditions monotone and not-monotone transepithelial Na+ flux curves will be seen.