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Name the process that exchanges gases between the external environment and the alveoli.
- Ventilation
- The term ventilation is defined as the process that exchanges gases from the external environment and the alveoli
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*tidal volume (VT)
- If a patient's IBW is 70 Kg, what tidal volume would we set if we wanted 6 mL/Kg? 70 Kg*6mL/Kg=420
- Defined as the volume of air that normally moves into and out of the lungs in one quiet breath
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minute volume
The volume of air inhaled/ exhaled from a persons lungs in one minute
Given the respiratory rate 16 and tidal volume of 500 mL, what is the Minute Ventilation? 8L/min
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anatomic deadspace (VDanat) and state its normal value
- The volume of gas in the conducting airways (nose, mouth, pharynx, larynx, and lower airway)
- Normal value: 1mL/lb of ideal body weight (ex. 150 lbs = 150 mL)
Moreover, because of the anatomic dead space, the gas that does enter the alveoli during each inspiration (alveolar ventilation) is actually a combination of anatomic dead space gas (non fresh gas) and gas from the atmosphere (fresh gas).
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alveolar deadspace
- What will explain a patient that has significantly increased minute volume, but no change in PaCO2?
- Occurs when the alveolus is ventilated but not perfused with pulmonary blood, thus the air that enters the alveolus is not effective in terms of gas exchange because there is no pulmonary capillary flow
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physiologic deadspace
Sum of the anatomic dead space and alveolar dead space
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alveolar ventilation (VA), and be able to calculate minute alveolar ventilation (volume)
Only the inspired air that reaches the alveoli is effective in terms of gas exchange, i.e. alveolar ventilation
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hyperventilation and hypoventilation
- Hyperventilation: When a person has a decrease in CO2 levels <35 mmHg.
- Increased alveolar ventilation that causes the PACO2 and therefore the PACO2 to decrease.
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- Hypoventilation: When a person has an in crease in CO2 levels >45 mmHg. Decreased alveolar ventilation that causes the PACO2 therefore the PACO2 to increase.
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Biot's breathing
Biots breathing is best described as rapid deep breaths followed by periods of apnea
Short episodes of rapid, uniformly deep inspirations, followed by 10-30 seconds of apnea
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Kussmaul's breathing
Kussmaul’s breathing is best described as rapid deep breathing
Both an increased depth and rate of breathing. This ventilatory pattern cause PaCO2 and PACO2 to decline and the PACO2 and PaO2 to increase
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Cheyne-Stokes breathing
- Cheyne – Stokes breathing is best described as waxing and waning of respirations followed by periods of apnea
- Ten to 30 seconds of apnea, followed by a gradual increase in the volume and frequency of breathing, followed by a gradual decrease in the volume of breathing until another period of apnea occurs.
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Minute volume (VE)
- VE = VT x RR
- VT: Tidal Volume
- RR: Respiratory Rate
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Alveolar minute ventilation
- VA = (VT – VD) x breaths/min
- VA: minute alveolar ventilation
- VT: Tidal volume
- VD: dead space ventilation
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Deadspace minute ventilation
VD = (VT – VA) x breaths/min
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***I:E ratio
The normal I:E ration is usually about 1:2, that is, the time required to inhale a normal breath is about one-half the time required to exhale the same breath.
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IBW (ideal body weight)
- MALE: 106 + (6 lbs x every inch over 5 feet) = weight in pounds/2.2 = kg
- FEMALE: 105 + (5 lbs x every inch over 5 feet) = weight in pounds/2.2 =kg
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VT based on IBW.
Weight in kg x volume of air
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Describe where aspirated contents will most likely come to rest in the lung.
Right middle and lower lobes
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State the average pleural pressure at the end of a passive exhalation.
760 mmHg
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Discuss how pleural pressures will vary from the base of the lung to the apex of the lung.
- They decrease as the move up the lungs
- The negative pleural pressure at the apex of the lung is usually greater (-7 to -10 cm H2O) than at the base of the lung (-2 to -3 cm H2O)
- This means that there is less pressure at the apex of the lung than there is at the base of the lung.
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Carefully read and understand the section on Pressure Gradients during Ventilation 64-70.
- A gas or a liquid always moves form an area of high pressure to an area of low pressure.
- Gas will always move down its pressure gradient
- When atmospheric pressure is greater than intra-alveoli pressure, air moves down the gas pressure gradient and inspiration occurs (gas moves from the atmosphere to the alveoli)
- When intra-alveolar pressure is greater than the atmospheric pressure, gas moves down the pressure gradient and expiration occurs (air flows from alveoli to atmosphere)
- Boyles law: States that a volume of gas varies inversely proportional to its pressure at a constant temperature (P1 x V1 = P2 x V2)Applying boyles law to mechanics of ventilation: When the thoracic cavity increases in volume caused by the downward contraction of the diaphragm during inspiration, the pressure in the thoracic cavity decreases
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Describe lung "compliance'
- The ease of lung distention
- Lung compliance is defined as how readily the elastic force of the lungs accepts a volume of inspired air
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State the normal value of total static compliance (lungs and thorax together).
- Spontaneous breathing: 0.1 L/cm H20 or 100mL
- Ventilated patient: 40-60 mL
- Critical patients: 20mL
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Given the following information, answer questions a - c about a patient receiving mechanical ventilation:
Delivered volume (vol. delivered by ventilator)
Peak airway pressure (PIP)
Static (plateau) airway pressure (Pplat)
Airway pressure at rest (baseline or PEEP)
Inspiratory flow rate in L/min
What is the patient's total static compliance in ml/cm H2O?
What is the patient's total dynamic compliance in ml/cm H2O?
Calculate the patient's airway resistance in cm H2O/L/sec.
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State the normal range for airway resistance.
- .5 to 1.5 cm H2O/L/sec in adults
- Airway resistance can be defined as the pressure difference between the mouth and the alveoli divided by the flow rate
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State the region where most of the airway resistance is found (upper or lower airways).
- The nose
- Upper airway, specifically the nose
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State whether airway resistance increases or decreases as the bronchi branch toward the alveoli. Explain why this change occurs.
It decreases due to the large cross sectional area of the alveoli
Airway resistance will decrease as the bronchi branch toward the alveoli because this the region that has a very large cross-sectional area.
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Describe the change in airway resistance that is associated with the presence of mucus secretions, bronchospasm, or mucosal edema.
- Air way resistance will increase in the presence of mucus secretions, bronchospasms, and mucosal edema because each on of these decreases the radius of the airway.
- Mucus secretions: Can cause an increase in airway resistance, most noticeable during expiration
- Bronchospasm: Occurs when the airways spasm, and contract. This makes it hard to breathe and causes wheezing.
- Mucosal edema: This is the swelling or build of fluid in the mucosal layer, the tissue that lines the body’s interior.
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If a patient has a significantly increased minute volume, but no change in PaCO2, state a possible explanation for the lack of change in PaCO2.
This patient has an increase in alveolar dead space. Alveolar dead space is when an alveolus is ventilated but not perfused (not gas exchange occurs). A patient who has increased breaths per minute but no change in their PaCO2 levels shows that there is no gas exchange occurring at the alveoli. An example of this is a pulmonary embolus, when a clot travels through the venous system and lodges in the pulmonary arteries.
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Discuss Hooke's Law.
Force and stretch are directly related within the physiologic limits.
- Based on Hooke’s law, if we apply too much pressure or volume to the lung what is likely to happen? Alveolar rupture
- Elastance is the natural ability of matter to respond directly to force and to return to its original resting position or shape after the external force no longer exists.
- Hookes law states that when a truly elastic body, like a spring, is acted on by 1 unit of force, the elastic body will stretch 1 unit of length
- When Hookes law is applied to the elastic properties of the lungs, volume is substituted for length and pressure is substituted for force.
- Thus, over the normal physiologic range of the lungs, volume varies directly with pressure.
- Therefor, the greater the force, the greater the stretch.
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Explain the relationships that are defined by Poiseuille's Law.
- Of all the factors that affect the required driving pressure, which factor has the greatest effect? (defined by Poiseuille’s law) [radius]
- Poiseuilles law arranged for flow: Equation states that flow is directly proportional to P and r4 and inversely proportional to l and n.
- In other words, flow will decrease in response to a decrease in pressure and rube radius; conversely, flow will increase in response to an increase in pressure and tube radius
- Flow will increase in response to decrease tube length and fluid viscosity; conversely, flow will decrease in response to increased tube length and fluid viscosity
- FLOW IS PROFOUNDLY AFFECTED BY THE RADIUS OF THE TUBE; V is a function of the fourth power of the radius, therefor if you decrease the radius of the tube by one-half, you reduce gas flow by 1/16 of its original flow
- Poiseuilles law arranged for pressure: Equation states that pressure is directly proportional to V, l and n, and inversely proportional to r4.
- In other words, pressure will increase in response to a decreased tube radius and decrease in response to a decreased flow rate, tube length and viscosity; the opposite is also true, pressure will decrease in response to an increased tube radius and increase in response to an increased flow rate, tube length and viscosity.
- PRESSURE IS PROFOUNDLY AFFECTED BY THE RADIUS OF THE TUBE, meaning if flow remains the same, decreasing the radius of the tube by one half of its previous size requires an increase in pressure 16 times its original level.
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Explain what will happen to pressure in an alveolus if the radius decreases, but surface tension remains the same. Explain what will happen to the alveolus. (LaPlaces Law)
According to LaPlaces law: By decreasing the radius of the alveolus, the pressure will ultimately increase. If the radius is decreased to the point of critical point, the alveolus will collapse into each other.
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***Describe the role of surfactant in preventing alveolar collapse of small alveolar units.
- During exhalation, when the alveolus decrease in size, the proportion of DPPC (the primary surface- tension lowering chemical in surfactant) and alveolar surface tension increases;
- and during inhalation, when the alveolus increase in size, the relative amount of DPPC to alveolar surface area decreases (this is because alveolar surfactant molecules don’t change when the size of the alveolus change), which decreases the effect of the DPPC molecules and causes the alveolar surface tension to increase (during inhalation, surfactant is spread thinner across the alveoli allowing surface tension to increase)
- In the absence of pulmonary surfactant, the alveolar surface tension increases to the level it would naturally have (50dynes/cm) and the distending pressure necessary to overcome the recoil forces of the liquid film coating the small alveoli is high.
- When the distending pressure of the small alveoli falls below the critical closing pressure, the liquid molecular force pulls the alveolar walls together.
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