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Pulm > 8 Ventilation and Perfusion > Flashcards

8 Ventilation and Perfusion Flashcards

1
Q

Ventilation

A
  • The repetitive movement of gas into and out of the lungs
  • Delivers the oxygen and removes the carbon dioxide that is exchanged across the alveolar-capillary interface
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2
Q

Lung volumes and capacities

  • Tidal volume (VT)
  • Residual volume (RV)
  • Expiratory reserve volume (ERV)
  • Inspiratory reserve volume (IRV)
A
  • Tidal volume (VT)
    • Volume of gas inhaled (or exhaled) during a breath
    • ~500ml in a resting adult
    • Increases as needed to meet the metabolic demands of the body (e.g. during exercise)
  • Residual volume (RV)
    • Amount of gas remaining in the lungs after a maximal expiration
    • Determined primarily by the inward pressure generated by the expiratory muscles and by the outward elastic recoil of the respiratory system
    • ~1.5L in a normal adult
  • Expiratory reserve volume (ERV)
    • Volume of gas that can be forced from the lungs starting at the end of a normal tidal expiration
  • Inspiratory reserve volume (IRV)
    • Volume of gas that can be inhaled during a maximal inspiration starting at the end of a normal tidal inspiration
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3
Q

Lung volumes and capacities

  • Functional residual capacity (FRC)
  • Total lung capacity (TLC)
  • Vital capacity (VC)
  • Inspiratory capacity (IC)
A
  • Functional residual capacity (FRC)
    • Volume remaining in the lungs at the end of a passive expiration
    • Represents the equilibrium position of the respiratory system
      • The point at which the inward elastic recoil of the lungs is balanced by the outward recoil of the chest wall
    • Sum of the residual and the expiratory reserve volumes
      • FRC = RV + ERV
  • Total lung capacity (TLC)
    • Volume in the lungs at the end of a maximal inspiration
    • Determined by the maximum force generated by the inspiratory muscles and by the inward elastic recoil of the lungs and chest wall
  • Vital capacity (VC)
    • Volume of gas that can be exhaled during a maximal effort beginning at the end of a maximal inspiration
    • Sum of the IRV, ERV, and VT
    • Difference between TLC and RV
      • VC = TLC - RV
  • Inspiratory capacity (IC)
    • Amount of gas that enters the lungs during a maximal inspiration beginning at the end of a normal tidal expiration
    • Sum of the IRV and VT
      • IC = IRV + VT
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4
Q

Partial pressure of respiratory gases

  • In a gas mixture, the pressure exerted by a gas is equal to…
  • Dry air composition of O2, N2, & CO2
  • Total barometric pressure (PB) at sea level
  • Partial pressures of O2, N2, & CO2
A
  • In a gas mixture, the pressure exerted by a gas is equal to…
    • The product of its fractional concentration (F) and the total pressure of all the gases in the mixture
    • Pgas = Ptotal x Fgas
  • Dry air composition of O2, N2, & CO2
    • FO2 = 0.21 (21%)
    • FN2 = 0.79 (79%)
    • FCO2 = 0.0004 (0.04%)
  • Total barometric pressure (PB) at sea level
    • 760 mmHg
  • Partial pressures of O2, N2, & CO2
    • PO2 = PB x F?
    • PO2 = 760 x 0.21 = 160 mmHg
    • PN2 = 760 x 0.79 = 600 mmHg
    • PCO2 = 760 x 0.0004 = 0.3 mmHg
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5
Q

Partial pressure of respiratory gases

  • Partial pressure of the water vapor (PH2O)
  • When air enters the conducting airways of the lungs…
  • Calculation of partial pressure of inspired gas in the airways (PIgas)
  • Partial pressures of O2, N2, & CO2
A
  • Partial pressure of the water vapor (PH2O)
    • ~47 mmHg at body temperature
  • When air enters the conducting airways of the lungs…
    • It is heated and humidified
    • Total pressure exerted by dry gas decreases
    • Partial pressure of each gas falls
  • Calculation of partial pressure of inspired gas in the airways (PIgas)
    • PIgas = (PB – PH2O) x Fgas
  • Partial pressures of O2, N2, & CO2
    • PIO2 = (760 – 47) x 0.21 = 150mmHg
    • PIN2 = (760 – 47) x 0.79 = 150mmHg
    • PICO2 = (760 – 47) x 0.0004 = 150mmHg
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6
Q

Partial pressure of respiratory gases

  • Once gas enters the alveoli…
  • The partial pressure of gases in the alveoli…
  • For practical purposes, we can assume that the mixed alveolar PCO2 (PACO2)…
A
  • Once gas enters the alveoli…
    • Diffusion occurs leading to a drop in the PO2 and an increase in PCO2
  • The partial pressure of gases in the alveoli…
    • Cannot be measured, but they can be estimated
  • For practical purposes, we can assume that the mixed alveolar PCO2 (PACO2)…
    • Is equal to the PCO2 of arterial blood (PaCO2)
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7
Q

Partial pressure of respiratory gases:
Alveolar air equation

  • PAO2
  • PA-aO2 gradient
A
  • Used to calculate the average alveolar PO2 (PAO2) assuming “ideal” conditions (i.e. no mismatching of ventilation and perfusion)
    • PAO2 = [(PB – PH2O) * FIO2] – [PACO2 / R]
    • PACO2 = PaCO2 = 40 mmHg
    • R = VCO2 / VO2 = 0.8
    • PAO2 = [(760 - 47) * 0.21] - [40 / 0.8] = 100 mmHg
  • Used to derive the PA-aO2 or the A-a gradient
    • Difference between the calculated alveolar and the measured arterial PO2 (always exists)
    • Normally 8-12 mmHg
    • Increases in the presence of ventilation-perfusion mismatching, shunt, and diffusion impairment
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8
Q

Partial pressure of respiratory gases

  • Respiratory exchange ratio (R)
  • Partial pressure of O2, CO2, & N2 in the alveoli
A
  • Respiratory exchange ratio (R)
    • Volume of CO2 that enters the alveoli divided by the volume of O2 that diffuses into the pulmonary capillary blood over a given period of time (VCO2/VO2)
      • Ratio of CO2 produced to O2 consumed by the tissues
    • Assumed to be 0.8
      • There is more O2 leaving the lung than CO2 entering it
      • PO2 will fall by more than the increase in PACO2
      • It will decrease by PACO2/R
  • Partial pressure of O2, CO2, & N2 in the alveoli
    • PAO2 = 100 mmHg
    • PACO2 = 40 mmHg
    • PAN2 = 573 mmHG
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9
Q

The oxygen cascade

  • Wordy explanation
  • Numeral explanation
A
  • Wordy explanation
    • Air comes in at 160
    • Once it enters our conducting airways, it’s 150
    • Then it drops to 100 by the time it gets to the alveoli
    • Difference b/n alveolar gas and arteriolar blood: A-a gradient
    • As the blood is carried to the tissues, the PO2 really drops
    • By the time it gets to the capillaries, there’s very little PO2
      • Estimated mitochondria PO2 = 5
  • Numeral explanation
    • Air
      • PO2 = 160
      • PCO2 = 0
    • Airways
      • PO2 = 150
      • PCO2 = 0
    • Alveoli
      • PO2 = 100
      • PCO2 = 40
    • Arterial
      • PO2 = 95
      • PCO2 = 40
    • Mixed venous
      • PO2 = 40
      • PCO2 = 46
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10
Q

Dead space

  • Dead space
  • Alveolar dead space
  • Physiologic dead space
A
  • Dead space
    • A significant portion of each tidal breath never reaches the alveoli
    • Volume of gas that fills the nose, mouth, pharynx, larynx, and conducting airways
    • Since this gas is never in contact with pulmonary capillary blood, it does not participate in gas exchange
    • Varies with body (and airway) size
    • Although it can be measured, for practical purposes it is often estimated as 1ml per pound of ideal body weight
  • Alveolar dead space
    • Another portion of the tidal breath that reaches alveoli that either
      • Receive no blood flow
      • Are under-perfused relative to the amount of ventilation they receive
  • Physiologic dead space
    • Sum of the anatomic and alveolar dead space
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11
Q

Volumes (L)

  • Tidal volume (VT)
  • Dead space
  • Dead space volume (VD)
  • Alveolar volume
A
  • Tidal volume (VT)
    • Volume of gas that enters the lungs during a single breath
  • Dead space
    • A significant portion of each tidal breath never reaches the alveoli
    • Volume of gas that fills the nose, mouth, pharynx, larynx, and conducting airways
    • Since this gas is never in contact with pulmonary capillary blood, it does not participate in gas exchange
    • Varies with body (and airway) size
    • Although it can be measured, for practical purposes it is often estimated as 1ml per pound of ideal body weight
  • Dead space volume (VD)
    • Amount of gas entering the physiologic dead space
    • The volume of each breath that does not participate in gas exchange
    • Anatomic + alveolar dead space
  • Alveolar volume (VA)
    • Volume of gas that actually reaches the alveoli and participates in gas exchange during a tidal breath
    • Difference between tidal volume and dead space volume
      • VA = VT - VD
    • If take a breath in of 500 ml and have a dead space of 200 ml, then true alveolar volume that participates in gas exchange is 300 ml
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12
Q

Ventilations (L/min)

  • Minute ventilation (VE)
  • Dead space ventilation (VD)
  • Alveolar ventilation (VA)
A
  • Minute ventilation (VE)
    • Total volume of gas that enters or leaves the lungs each minute
    • Product of tidal volume and respiratory rate (RR)
      • VE = VT x RR
  • Dead space ventilation (VD)
    • Volume of gas that enters and leaves the physiologic dead space each minute
    • VD = VD x RR
  • Alveolar ventilation (VA)
    • Volume of gas entering and leaving the lungs each minute that participates in gas exchange
      • VA = VA x RR
    • Difference between minute ventilation and dead space ventilation
      • VA = VE – VD
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13
Q

The influence of alveolar ventilation on PACO2

  • The partial pressure of carbon dioxide in the alveolar gas (PACO2) is determined by…
  • CO2 removal from alveoli vs. alveolar ventilation
  • PACO2 vs. VA
  • PACO2 equations
  • PaCO2 equations
A
  • PACO2 and PaCO2 are directly related to…
    • The rate at which CO2 enters the alveoli
  • The partial pressure of carbon dioxide in the alveolar gas (PACO2) is determined by…
    • The rate at which CO2 is produced by the tissues (VCO2)
    • The rates at which CO2 enters and leaves the alveoli
    • –> proportional to the partial pressure of CO2 in mixed venous blood (PvCO2)
    • –> –> depends on the rate of CO2 production by the tissues (VCO2)
      • PACO2 α VCO2
  • CO2 removal from alveoli vs. alveolar ventilation
    • The rate at which CO2 is removed from the alveoli is directly proportional to alveolar ventilation
  • PACO2and PaCO2 vs. VA
    • PACO2 and PaCO2 are inversely related to the rate at which CO2 is removed from the alveoli
      • Determined by alveolar ventilation (VA)
    • PACO2 α 1 / VA
  • PACO2 equations
    • PACO2 α VCO2 / VA
    • PACO2 = K x VCO2 / VA
  • PaCO2 equations
    • Alveolar and arterial PCO2 are essentially equal
    • PaCO2 = K x VCO2 / VA
    • PaCO2 = K x VCO2 / (VE - VD)
    • PaCO2 = K x VCO2 / VE [1 – (VD / VT)]
      • As VD/VT rises, VE must increase to maintain the same PaCO2
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14
Q

The influence of alveolar ventilation on PAO2

A
  • Unlike PACO2 (and PaCO2), alveolar ventilation does not directly affect PAO2
    • Instead, its effect is indirect and produced by changes in PACO2
  • By examining the alveolar air equation, it is evident that PAO2 and PACO2 must move in opposite directions
    • When PACO2 increases, PAO2 decreases
    • When PACO2 decreases, PAO2 increases
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15
Q

Regional distribution of alveolar ventilation

  • In normal subjects, who are either seated or standing, the volume of air reaching the alveoli…
  • Although a single value is often used to describe the pressure in the pleural space, in an upright subject, pleural pressure…
  • Since the pressure in all alveoli is zero (atmospheric) at the end of a passive exhalation, the transpulmonary pressure…
  • During inspiration, trans-pulmonary pressure…
  • Since alveoli in the non-dependent regions already have a relatively high volume, they are…
A
  • In normal subjects, who are either seated or standing, the volume of air reaching the alveoli…
    • Increases from the apex to the base of the lungs
    • This causes ventilation to increase from the top to the bottom of the lungs
    • This can be explained by gravity-induced changes in pleural pressure
  • Although a single value is often used to describe the pressure in the pleural space, in an upright subject, pleural pressure…
    • Progressively increases (becomes less negative) between the top (apex) and bottom (base) of the lungs
  • Since the pressure in all alveoli is zero (atmospheric) at the end of a passive exhalation, the transpulmonary pressure…
    • Must be higher in the upper lung zones than in the lower zones
    • This means that at end-expiration, alveolar volume is high at the lung apex and progressively falls in the more dependent regions of the lungs
  • During inspiration, trans-pulmonary pressure…
    • Increases equally regardless of alveolar location
  • Since alveoli in the non-dependent regions already have a relatively high volume, they are…
    • Less compliant than alveoli in the lower lung zones
    • This means that for the same change in trans-pulmonary pressure, the amount of fresh gas entering the alveoli is greater in the dependent than in the nondependent regions of the lungs
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16
Q

Perfusion:
Sources of blood flow to lungs

  • Bronchial circulation
    • Bronchial arteries
    • A large proportion of the deoxygenated bronchial venous blood drains into…
  • Pulmonary circulation
    • Pulmonary arteries
    • As compared with the systemic circulation, the pulmonary arteries have…
A
  • Bronchial circulation
    • Bronchial arteries
      • Carry oxygenated blood from the left ventricle
      • Originate either directly from the aorta or from intercostal arteries
      • Supply oxygen to several structures in the thorax including the conducting airways, the visceral pleura, and the esophagus
    • A large proportion of the deoxygenated bronchial venous blood drains into the pulmonary veins, thereby producing an anatomic right to left shunt
  • Pulmonary circulation
    • Pulmonary arteries
      • Carry the mixed venous blood from all the tissues of the body
      • Provide oxygen to the distal airways and alveoli
    • As compared with the systemic circulation, the pulmonary arteries have…
      • Both thinner walls and larger lumens
      • Less vascular smooth muscle
      • No muscular vessels analogous to systemic arterioles
17
Q

Perfusion:
Important differences between the pulmonary and systemic circulations

  • Pulmonary vascular resistance (PVR)
  • Pulmonary vascular resistance distribution
  • Pulmonary arteries
A
  • Pulmonary vascular resistance (PVR) is normally much less than (approximately one-tenth) that of the systemic circulation
  • Pulmonary vascular resistance is fairly evenly distributed between the arteries, capillaries, and veins, whereas arteries and arterioles account for approximately 80% of the systemic vascular resistance
  • Pulmonary arteries are much more distensible and compressible than their systemic counterparts
18
Q

Factors affecting pulmonary vascular resistance:
Active vs. passive factors

  • Active factors
  • Passive factors
A
  • Active factors
    • Pulmonary vascular resistance can be influenced by both neural and humoral factors, which alter vascular smooth muscle tone and vessel caliber
  • Passive factors
    • The distensibility, compressibility, relative lack of smooth muscle, and low intravascular pressure that characterize the pulmonary circulation cause PVR to be strongly influenced by a variety of extra-vascular or “passive” factors that have no effect on vascular smooth muscle tone
19
Q

Factors affecting pulmonary vascular resistance:
Active factors:
Neural factors

A
  • The pulmonary circulation is supplied with both sympathetic and parasympathetic innervation
  • Parasympathetic stimulation causes vascular dilation and a decrease in PVR
  • Increased sympathetic activity leads to vasoconstriction and an increase in PVR
20
Q

Factors affecting pulmonary vascular resistance:
Active factors:
Humoral factors

  • Pulmonary vasoconstriction is caused by…
  • Pulmonary vasodilationis caused by…
  • As PAO2 falls…
  • Alveolar hypoxia
  • Hypoxic-induced vasoconstriction
A
  • Pulmonary vasoconstriction is caused by…
    • Catecholamines, (e.g. dopamine, norepinephrine, and epinephrine), histamine, PGE2, PGF2α, and thromboxane
  • Pulmonary vasodilationis caused by…
    • Acetylcholine, PGE1 and PGI2 (prostacyclin)
  • As PAO2 falls…
    • Vasoconstriction occurs in the pre-capillary vessels
    • The mechanism for this increase in vascular tone is unknown, but the effect is not mediated through the autonomic nervous system
  • Alveolar hypoxia
    • The most important humoral factor
    • May have a direct effect on vascular smooth muscle
    • May cause the release of other humoral mediators such as histamine, serotonin, or prostaglandins
  • Vasoconstriction may occur…
    • Locally (e.g. atelectasis)
    • Diffusely (e.g. hypoventilation, high altitude)
  • Hypoxic-induced vasoconstriction
    • Clearly a beneficial response
    • Decreases blood flow to regions of the lung that are poorly ventilated
    • Unfortunately, this response is relatively weak due to the lack of vascular smooth muscle
21
Q

Factors affecting pulmonary vascular resistance:
Passive factors:
Lung volume

  • Alveolar vessels
    • Primarily…
    • Affected by…
    • During inspiration…
    • As lung volume falls below FRC,…
  • Extra-alveolar vessels
    • Exposed to…
    • During a spontaneous inspiration,…
    • During forced expiration below FRC,…
  • Both the alveolar and extra-alveolar vessels
  • A complex relationship exits between lung volume and PVR
A
  • Alveolar vessels
    • Primarily the pulmonary capillaries
    • Affected by alveolar volume
    • Resistance varies directly with lung volume
      • During inspiration, the increase in alveolar volume causes the capillaries to be compressed and elongated, thereby increasing the resistance of these vessels
    • As lung volume falls below FRC, this process is reversed, and PVR decreases
  • Extra-alveolar vessels
    • Exposed to pleural pressure
    • During a spontaneous inspiration, pleural pressure falls
      • This increases vascular trans-mural pressure and vessel diameter and causes vascular resistance to decrease
    • During forced expiration below FRC, pleural pressure rises and resistance increases
  • Both the alveolar and extra-alveolar vessels
    • Contribute significantly to total resistance
  • A complex relationship exits between lung volume and PVR
    • PVR is lowest near FRC and progressively rises with both increasing and decreasing lung volume
22
Q

Factors affecting pulmonary vascular resistance:
Passive factors:
Cardiac output

  • In normal subjects, significant increases in cardiac output (e.g. during exercise)…
  • Intra-vascular pressure is determined by…
  • Increases in cardiac output must be balanced by…
  • As flow increases, pressure…
  • Both of these processes lead to…
A
  • In normal subjects, significant increases in cardiac output (e.g. during exercise)…
    • Have very little effect on pulmonary artery pressure
  • Intra-vascular pressure is determined by…
    • The product of resistance and flow
  • Increases in cardiac output must be balanced by…
    • A fall in PVR and little change in arterial pressure
    • This decrease in vascular resistance is not mediated by alterations in vascular tone, but is instead due to two passive processes
      • Recruitment: increase blood flow –> open up vessels that are closed to begin with near the top of the lungs –> reduce resistance
      • Distention: distend all other vessels –> increase their radius –> reduce resistance
    • ​Overall: greater CO –> lower pulmonary vascular resistance –> maintain normal pulmonary arterial pressure (even during very active exercise)
  • As flow increases, pressure…
    • Rises transiently
    • Opens or recruits capillaries and other small vessels that had been closed due to insufficient intra-vascular pressure
    • This increase in vascular pressure also causes distention of the pulmonary vasculature
  • Both of these processes lead to…
    • A fall in PVR and minimize any flow-induced increase in pulmonary vascular pressure
23
Q

Factors affecting pulmonary vascular resistance:
Passive factors:
Gravity

A
  • Intravascular pressure is higher in dependent than in non-dependent lung regions
  • The higher the intravascular pressure, the greater the vascular distention and the lower the PVR
  • Due to the weight of the liquid, the pressure within a column of water (or blood) is higher at the bottom than near the top
  • Similarly, intra-vascular pressure increases in the dependent (bottom) portions of the lungs
  • In an upright subject, this pressure gradient causes progressive vascular distention toward the lung bases and narrowing near the apices
24
Q

The regional distribution of pulmonary blood flow

  • Pulmonary blood flow increases in the _ portions of the lungs
  • In a seated or standing subject, blood flow…
  • In a supine subject, flow increases to the…
  • When a subject lies on his or her side, blood flow increases to the…
A
  • Pulmonary blood flow increases in the dependent (bottom) portions of the lungs
    • This effect is mediated by gravity-induced increases in intra-vascular pressure, which, in turn, lead to vascular distention, decreased resistance, and increased flow
  • In a seated or standing subject, blood flow is greatest at the lung bases (bottom) and least at the apices (top)
  • In a supine subject, flow increases to the anatomically dependent (dorsal) portions of the lungs
  • When a subject lies on his or her side, blood flow increases to the dependent lung
25
Q

The zones of the lung

  • Alveolar vs. vascular pressure
  • Three lung zones are based on…
  • In Zone 1
  • In Zone 2
  • In Zone 3
A
  • Alveolar vs. vascular pressure
    • Since alveolar pressure is fairly uniform, and intra-vascular pressure varies due to the effect of gravity, alveolar pressure may exceed vascular pressure in the non-dependent (top) regions of the lungs
  • Three lung zones are based on…
    • The relationship between pulmonary arterial (Pa), pulmonary venous (Pv), and alveolar (PA) pressures
  • In Zone 1
    • PA > Pa > Pv
    • No blood flow occurs because the alveolar vessels are completely collapsed
  • In Zone 2
    • Pa > PA > Pv
    • Blood flow occurs, but the pressure gradient driving that flow is Pa – PA, not Pa – Pv
  • In Zone 3
    • Pa > Pv > PA
    • Blood flow is driven by the difference between pulmonary arterial and venous pressure
26
Q

The zones of the lung

  • The extent of each of these zones is dependent on…
  • For example, low intra-vascular pressures (e.g. from hypovolemia) and high alveolar pressure (e.g. forced expiration, positive pressure ventilation) will create larger…
  • On the other hand, low alveolar pressure and/or high intra-vascular pressure will increase…
  • Body position also determines…
  • For example, in an upright subject,…
  • In a supine subject…
A
  • The extent of each of these zones is dependent on…
    • The magnitude of pulmonary vascular and alveolar pressures
  • For example, low intra-vascular pressures (e.g. from hypovolemia) and high alveolar pressure (e.g. forced expiration, positive pressure ventilation) will create larger…
    • Zone 1 and 2 regions
  • On the other hand, low alveolar pressure and/or high intra-vascular pressure will increase…
    • Zone 3 regions
  • Body position also determines…
    • The extent and location of the lung zones
  • For example, in an upright subject,…
    • Zone 1 (usually minimal or absent) is at the apex
    • Zone 2 is in the midlung
    • Zone 3 is at the base
  • In a supine subject…
    • Zone 1 (if present) will be in the ventral portion of the lungs
    • Zone 3 will be in the dorsal regions
27
Q

Fluid flow across the pulmonary capillaries

  • Like all capillaries, the junctions between the endothelial cells of the pulmonary capillaries are…
  • The net movement of fluid across the pulmonary capillaries is described by the Starling equation
A
  • Like all capillaries, the junctions between the endothelial cells of the pulmonary capillaries are…
    • Permeable to fluids
  • The net movement of fluid across the pulmonary capillaries is described by the Starling equation
    • Qf = Kf [(Pc – Pi) – σ (πp – πi)]
    • Qf = net fluid flow
    • Kf = filtration coefficient (proportional to capillary permeability)
    • Pc = capillary hydrostatic pressure
    • Pi = hydrostatic pressure in the lung interstitium
    • σ = reflection coefficient (describes the ability of the capillary to retain solute)
    • πp = colloid osmotic pressure of the plasma
    • πi = colloid osmotic pressure of the lung interstitium
28
Q

Fluid flow across the pulmonary capillaries

  • Normally, there is a small amount of net fluid flow from the…
  • This fluid drains into the…
  • Under pathological conditions, pulmonary lymphatic drainage can…
  • Interstitial and alveolar pulmonary edema can potentially result from…
    • Most important
    • Less important
A
  • Normally, there is a small amount of net fluid flow from the…
    • Capillaries to the pulmonary interstitium
  • This fluid drains into the…
    • Pulmonary lymphatics and is removed from the lung
  • Under pathological conditions, pulmonary lymphatic drainage can…
    • Increase as much as tenfold
  • Interstitial and alveolar pulmonary edema can potentially result from…
    • Most important
      • Increased capillary hydrostatic pressure
      • Increased capillary permeability (increased Kf and/or decreased σ)
    • Less important
      • Impaired lymphatic drainage
      • Decreased interstitial hydrostatic pressure
      • Decreased plasma oncotic pressure
      • Increased interstitial oncotic pressure

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