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.
pressure (presh'ur) [L. pressura]
1. A compression.
2. Stress or force exerted on a body, as by tension, weight, or pulling.
3. In psychology, the quality of sensation aroused by moderate compression of the skin.
4. In physics, the quotient obtained by dividing a force by the area of the surface on which it acts.
airway opening pressure
The pressure at the access point to a patient's airway, (nose, mouth, or for a mechanically ventilated patient, the trachea). When this pressure exceeds the alveolar pressure, gases tend to move into the lower airways and open the alveoli. When this pressure is less than the alveolar pressure, gases tend to move out of the lungs and into the atmosphere. See: alveolar pressure
Air pressure in the alveoli and bronchial tree. When this pressure is positive, it is higher than atmospheric pressure; when negative, less. Gases flow from higher to lower pressures: when alveolar pressures are higher than atmospheric pressure, respiratory gases tend to be exhaled. When alveolar pressures are less than atmospheric pressures, gas flows into the lungs. Synonym: intrapulmonic pressure
The pressure of the blood in the arteries. For a normal young person at physical and mental rest and in sitting position, systolic blood pressure averages about 120 mm Hg; diastolic pressure about 80 mm Hg. A wide range of normal variation is due to constitutional, physical, and psychic factors. For women, the figures are slightly lower. For older people, they are higher. Normally there is little difference in the blood pressure recorded in the two arms. See: blood pressure
The pressure of the weight of the atmosphere; at sea level it averages about 760 mm Hg.
bilevel positive airway pressure Abbreviation: BiPAP
A type of continuous positive airway pressure in which inspiratory and expiratory pressure differ from each other.
The pressure exerted on the teeth during biting. Synonym:
blood pressure See: blood pressure
The blood pressure in the capillaries.
central venous pressure Abbreviation: CVP
The pressure within the superior vena cava. It reflects the pressure under which the blood is returned to the right atrium. The normal range is between 5 and 10 cm H2O. A high CVP indicates circulatory overload (as in congestive heart failure), whereas a low CVP indicates reduced blood volume (as in hemorrhage or fluid loss). CVP can be estimated by examining the cervical veins or the dorsal veins of the hand if the neck and hand are at the level of the heart. Those veins are well filled if CVP is normal or high, and tend to collapse if it is low.
The pressure of the cerebrospinal fluid. This varies with body position but is normally about 100 to 180 mm H2O when the spinal canal is initially entered during lumbar puncture with the patient lying on his or her side.
continuous positive airway pressure Abbreviation: CPAP
A method of ventilatory support applied to the spontaneously breathing patient in which airway pressure is maintained above atmospheric pressure throughout the respiratory cycle. CPAP can be applied by way of a nasal mask, a face mask, or an endotracheal tube. It can be used to treat congestive heart failure, acute pulmonary edema, obstructive sleep apnea syndrome, and other conditions. A potential adverse effect of CPAP is barotrauma to the lungs.
coronary perfusion pressure Abbreviation: CPP
The blood pressure in the aorta during diastole minus the blood pressure during right atrial diastole. For ICU patients it is an indicator of the adequacy of blood flow through the epicardial coronary arteries, e.g., during CPR. Patients whose CPP is > 15 mm Hg during CPR are more likely to regain spontaneous circulation than patients whose CPP is lower.
The application of manual pressure onto the cricoid cartilage during intubation and mechanical ventilation. This technique helps to occlude the esophagus and prevent the entry of air into the gastrointestinal tract during ventilation. It also diminishes the chances for regurgitation from the stomach and aspiration of gastric contents.
The pressure exerted by a vapor at its critical temperature in an evacuated container.
The gas pressure used to inflate and hold in place the balloon surrounding a tracheal tube or laryngeal airway.
CAUTION!To avoid damage to the trachea, the pressure should be carefully monitored and adjusted. Cuff pressure should be sufficient to prevent aspiration of secretions around the cuff.
1. In respiratory physiology, the difference between the inspiratory pressure and the expiratory pressure.
2. In cardiovascular physiology, the difference between the arterial pressure at the source of blood flow, and the pressure at the target tissue.
effective osmotic pressure
That portion of the total osmotic pressure of a solution that determines the tendency of the solvent to pass through a membrane, usually one that is semipermeable. The tendency is for the solvent to pass from a solution containing a high concentration of the solute to the side of the membrane with the low concentration. Synonym: threshold pressure
Blood pressure in a ventricle of the heart at the end of diastole.
The pressure in the lungs when exhalation is complete.
The pressure in the lungs during the exhalation of a breath.
expiratory positive airway pressure Abbreviation: EPAP
The amount of positive pressure in excess of the barometric pressure that is used during invasive or noninvasive ventilation while the patient is exhaling.
The average pressure in the atria or the ventricles at the end of diastole.
The pressure exerted by a fluid within a closed system.
increased intracranial pressure
An elevation of the pressure of the cerebrospinal fluid. In healthy people intracranial pressures range between 0 and 10 mm Hg. Pressures higher than 20 mm Hg increase the risk of compression or herniation of the brain or brainstem.
The pressure in the lungs during the inhalation of a breath.
Pressure within the abdominal cavity, such as that caused by descent of the diaphragm.
The pressure of the cerebrospinal fluid in the subarachnoid space between the skull and the brain. The pressure is normally the same as that found during lumbar puncture.
intraocular pressure Abbreviation: IOP
Fluid pressure inside the eye, normally 12 to 21 mm Hg.
intrapleural pressurePleural pressure.
intrapulmonic pressureAlveolar pressure.
intrathoracic pressurePleural pressure.
The pressure within the ventricles of the heart during different phases of diastole and systole.
jugular venous pressure Abbreviation: JVP
Back pressure exerted into the jugular veins from the right side of the heart.
To estimate jugular venous pressure, have the patient lie on his or her back with the chest, neck, and head elevated 30 degrees above the horizontal. Measure the vertical height of the fullness seen as the jugular vein fills. In the healthy (and in the dehydrated) little or no blood is seen in the jugular veins, and they appear to be flat. In right-sided heart failure, the column of blood distends the jugular veins to a height of 5 to 6 cm or more.
maximum inspiratory pressureMaximum inspiratory force.
Any pressure less than that of the atmosphere, or less than that pressure to which the initial pressure is being compared.
Osmotic pressure exerted by colloids in a solution.
opening pressure Abbreviation: OP
The pressure of the cerebrospinal fluid that is detected just after a needle is placed into the spinal canal. It is normally 100 to 180 mm H20.
The force with which a solvent, usually water, passes through a semipermeable membrane separating solutions of different concentrations. It is a colligative property, i.e., it depends on the number of particles, not on the nature of the particles, dissolved in solution. It is measured by determining the hydrostatic (mechanical) pressure that must be opposed to the osmotic force to bring the passage to a standstill.
In a gas containing several different components, the pressure exerted by each component.
partial pressure of exhaled carbon dioxide Abbreviation: PETCO2
The carbon dioxide content of an expired breath. In a patient with normal ventilation and perfusion, it should exceed 40 mm Hg.
peak inspiratory pressure
The maximum pressure in the lungs that is achieved at the peak of inhalation.
The average pressure in the alveoli during the brief pause that follows an inhaled breath.
The pressure in the pleural space, e.g., during mechanical ventilation or critical illness. It is normally lower than atmospheric pressure and therefore is sometimes called a negative pressure. It is rarely measured, except during mechanical ventilation, thoracentesis, or critical illness. Synonym: intrapleural pressure; intrathoracic pressure
Pressure greater than atmospheric or greater than the pressure to which the initial pressure is being compared.
positive end-expiratory pressure Abbreviation: PEEP
In respiratory medicine, a method of holding alveoli open during expiration. This is done by gradually increasing the expiratory pressure during mechanical ventilation. When PEEP is used, it is important to monitor the hemodynamic status of the patient because PEEP reduces venous return to the heart and cardiac output. The goal is to achieve adequate arterial oxygenation, without using toxic levels of oxygen and without compromising cardiac output.
CAUTION!The patient must be carefully monitored to allow observation for undesired side effects such as pneumomediastinum, subcutaneous emphysema, and pneumothorax.
positive end-expiratory pressure, auto Abbreviation: auto-PEEP
A complication of mechanical ventilation in which the ventilator does not permit the patient sufficient time to exhale. This causes air to be trapped in the lungs, particularly the alveoli. If continued, auto-PEEP causes respiratory muscle fatigue and can cause rupture of the lung (i.e., pneumothorax). Auto-PEEP may be corrected by increasing exhalation time, decreasing the ventilator rate, or switching the ventilation mode so that the patient's spontaneous respiratory pattern governs the inspiratory and expiratory times.
posterior cricoid pressure
Pressure applied by firmly placing the thumb and index finger on the lateral aspects of a patient's cricoid ring to occlude the esophagus. Once cricoid pressure is applied, it should not be discontinued until control of the airway (as by intubation to achieve a patent airway) has been established. Synonym: Sellick's maneuver
pulmonary artery occlusive pressurePulmonary artery wedge pressure
pulmonary artery wedge pressure Abbreviation: PAWP
Pressure measured in the pulmonary artery after catheterization. The catheter is positioned in the pulmonary artery, and the distal portion of the catheter is isolated from pressure behind it in the artery by inflating a balloon with air. This allows the catheter to float into a wedged position and permits sensing of transmission of pressures ahead of the catheter (in the pulmonary capillary bed) by the transducer. Because no valve is present between this location and the left atrium, the measurement reflects left atrial pressure, and, in the presence of a competent mitral valve, the measurement provides an indication of left ventricular end-diastolic pressure. The balloon is then passively deflated after measurements of wedge pressure are completed. Elevated wedge pressures are found characteristically in patients with congestive heart failure or fluid overload. Synonym: pulmonary artery occlusive pressure
; wedge pressure
See: Swan-Ganz catheter
The nurse prepares and sets up the transducer equipment to monitor pulmonary artery pressure and PAWP according to institutional protocol and the manufacturer's instructions. The transducer is balanced and calibrated as required (every 4 to 8 hr). Hemodynamic status is monitored, and findings are documented, including pulmonary artery pressure (normally 20 to 30 mm Hg systolic and 8 to 12 mm Hg diastolic) every hour as directed. To measure PAWP every 1 to 4 hr as directed, the nurse inflates the balloon with 0.75 to 1.5 cc of air depending on balloon size (the balloon is never inflated with fluid) while watching for change in waveform (indicating wedging) and assessing for balloon rupture (lack of resistance on inflation, with absence of wedging). If this occurs, the wedging procedure is discontinued (because of concern for air embolism), and therapy is managed based on pulmonary artery diastolic pressures. Pulmonary artery wedge pressure is read, documented (normally 4 to 12 mm Hg), and correlated to clinical findings and other hemodynamic values, and any abnormal findings are reported. The nurse then removes the syringe and permits passive deflation of the balloon while observing for reappearance of pulmonary artery pressure waveform. If the balloon remains inflated, the patient is at risk for pulmonary artery necrosis. The patient should be positioned on the right side and encouraged to take deep breaths and to cough as the nurse mobilizes the right arm. If the balloon remains wedged, the physician should be notified. Fluid and diuretic therapy are adjusted based on PAWP and other values as prescribed. Impedance cardiography may be employed as an alternative to invasive monitoring with a pulmonary artery catheter.
The difference between systolic and diastolic pressures. The systolic pressure is normally about 40 points greater than the diastolic. A pulse pressure over 50 points or under 30 points is considered abnormal.
The idea that an institution's quantity of productivity is more important than its quality. In health care institutions, the pressure to move patients quickly from one status or setting to another, or to maximize bed occupancy, may sometimes take precedence over attention to detail and the provision of optimal care. To counteract such pressures, health care professionals must communicate clearly and effectively with each other, follow established safe practices, have methods for reporting errors, and have protocols that link providers of different organizational status.
Pressure that tends to dissolve a solid present in a solution.
1. The pressure in the circulatory system between pulses.
2. Pressures that are present in a system when any variables that may impact the system are minimized.
systolic pressureSystolic blood pressure.
threshold pressureEffective osmotic pressure.
Alveolar pressure minus pleural pressure. When normal transpulmonary pressures are exceeded, air leaks may develop.
The pressure of the blood within the veins. It is highest near the periphery, diminishing progressively from capillaries to the heart. Near the heart the venous pressure may be below zero (negative pressure) owing to negative intrathoracic pressure.
wedge pressurePulmonary artery wedge pressure.