pressure (P) [presh´ur]
the pressure exerted by the atmosphere, usually considered as the downward pressure of air onto a unit of area of the earth's surface; the unit of pressure at sea level is one atmosphere
. Pressure decreases with increasing altitude.
2. pressure of blood on walls of any blood vessel.
capillary pressure the blood pressure in the capillaries.
cerebral perfusion pressure the mean arterial pressure minus the intracranial pressure; a measure of the adequacy of cerebral blood flow.
cerebrospinal pressure the pressure of the cerebrospinal fluid, normally 100 to 150 mm Hg.
high blood pressure hypertension.
intraocular pressure the pressure exerted against the outer coats by the contents of the eyeball.
intrinsic positive end-expiratory pressure
elevated positive end-expiratory pressure
and dynamic pulmonary hyperinflation
caused by insufficient expiratory time or a limitation on expiratory flow. It cannot be routinely measured by a ventilator's pressure monitoring system but is measurable only using an expiratory hold maneuver done by the clinician. Its presence increases the work needed to trigger the ventilator, causes errors in the calculation of pulmonary compliance, may cause hemodynamic compromise, and complicates interpretation of hemodynamic measurements. Called also auto-PEEP
and intrinsic PEEP
maximal inspiratory pressure
the pressure during inhalation against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning
from mechanical ventilation. A maximum inspiratory pressure above −25 cm H2
O is associated with successful weaning.
maximum expiratory pressure
(MEP) a measure of the strength of respiratory muscles, obtained by having the patient exhale as strongly as possible against a mouthpiece; the maximum value is near total lung capacity
maximum inspiratory pressure
(MIP) the inspiratory pressure generated against a completely occluded airway; used to evaluate inspiratory respiratory muscle strength and readiness for weaning from mechanical ventilation. A maximum inspiratory pressure above −25 cm H2
O is associated with successful weaning
mean airway pressure the average pressure generated during the respiratory cycle.
mean circulatory filling pressure a measure of the average (arterial and venous) pressure necessary to cause filling of the circulation with blood; it varies with blood volume and is directly proportional to the rate of venous return and thus to cardiac output.
negative pressure pressure less than that of the atmosphere.
oncotic pressure the osmotic pressure of a colloid in solution.
osmotic pressure the pressure required to stop osmosis through a semipermeable membrane between a solution and pure solvent; it is proportional to the osmolality of the solution. Symbol π.
partial pressure the pressure exerted by each of the constituents of a mixture of gases.
peak pressure in mechanical ventilation, the highest pressure that occurs during inhalation.
plateau pressure in mechanical ventilation, the pressure measured at the proximal airway during an end-inspiratory pause; a reflection of alveolar pressure.
positive pressure pressure greater than that of the atmosphere.
positive end-expiratory pressure
(PEEP) a method of control mode ventilation in which positive pressure is maintained during expiration to increase the volume of gas remaining in the lungs at the end of expiration, thus reducing the shunting of blood through the lungs and improving gas exchange. A PEEP higher than the critical closing pressure prevents alveolar collapse and can markedly improve the arterial Po2
in patients with a lowered functional residual capacity, as in acute respiratory failure.
Effects of the application of positive end-expiratory pressure (PEEP) on the alveoli. A, Atelectatic alveoli before PEEP application. B, Optimal PEEP application has reinflated alveoli to normal volume. C, Excessive PEEP application overdistends the alveoli and compresses adjacent pulmonary capillaries, creating dead space with its attendant hypercapnia. From Pierce, 1995.
pulmonary artery wedge pressure
(PAWP) (pulmonary capillary wedge pressure
(PCWP)) intravascular pressure, reflecting the left ventricular end diastolic pressure, measured by a swan-ganz catheter
wedged into a small pulmonary artery to block the flow from behind.
pulse pressure the difference between the systolic and diastolic pressures. If the systolic pressure is 120 mm Hg and the diastolic pressure is 80 mm Hg, the pulse pressure is 40 mm Hg; the normal pulse pressure is between 30 and 40 mm Hg.
the pressure inwards exerted by the walls of the urethra, which must be counteracted in order for urine to flow through; see also urethral pressure profile
water vapor pressure the tension exerted by water vapor molecules, 47 mm Hg at normal body temperature.
wedged hepatic vein pressure the venous pressure measured with a catheter wedged into the hepatic vein. The difference between wedged and free hepatic vein pressures is used to locate the site of obstruction in portal hypertension; it is elevated in that due to cirrhosis, but low in cardiac ascites or portal vein thrombosis.