Chapter 2: Ventilation Flashcards
Name the process that exchanges gases between the external environment and the alveoli
Ventilation moves gases from the atmosphere to the alveoli for gas exchange
Define tidal volume and minute volume
Tidal volume(VT): the volume of gas moved into or out of the lungs in one minute
VT x RR = MV
Anatomic dead space (VDanat) and state it’s normal value
The volume of gas found in the conducting airways. Extends from the external nares to the terminal bronchioles
1mL/lb of IBW
Alveolar deadspace
Normally there isn’t any but it occurs when there is ventilation without perfusion
Physiologic deadspace
VDphys = VDana + VDalv
Measured using VD/VT = (PaCO2 - PECO2)/PaCO2
Alveolar ventilation (VA) and be able to calculate minute alveolar ventilation (volume)
~If VD/VT was 0.65 then:
-VA = VE x (1-0.65)
OR
- VA = VE - VD/min
Hyperventilation and Hypoventilation
~Eupnea occurs when the PaCO2 is 35 - 45 mm Hg
~Hyperventilation occurs when PaCO2 is <35 mm Hg ]
~Hypoventilation occurs when PaCO2 is >45
Biot’s, Kussmaul’s, and Cheyne-Stokes breathing
~Biot’s breathing is periods of deep rapid breathing interspersed with periods of apnea
~Kussmaul’s breathing is continuous hyperpnea rapid deep breathing typically associated with diabetic ketoacidosis
~ Cheyenne-Stokes breathing is characterized by waxing and waning ventilation interspersed with periods of apnea
Be able to calculate (see comprehensive worksheet)
-Minute volume (VE)
-Estimate VD
-Alveolar minute ventilation
-Deadspace minute ventilation
-I:E ratio (typically 1:2 or less in spontaneously breathing patients… but in patients with COPD, Asthma, CF require longer to breathe out due to increased expiratory raw. So I:E ratios 1:4, 1:5, 1:6)
-Itime
-RCT (amount of time it takes to complete one respiratory cycle, to breath in and out)
RCT= IT + ET or 66/RR
So if a patient is breathing 20 times a minute their RCT = 60/20 or 3 seconds
Or IT is 1.5 s and ET is 3sec… RCT = 4.5 s, RR = 60/4.5 = 13b/min
- IBW (ideal body weight)
-VT based on IBW
Describe where aspirated contents will most likely come to rest in the lung.
In the middle lobe and the right lower lobe
State the average pleural pressure at the end of the passive exhalation
(-) 5cm H2O
Discuss how pleural pressures will vary from the base of the lung to the apex of the lung
(-)2 at the base of the lung to -10 at the lung apex
Understand section of Pressure Gradients during ventilation
KEY POINTS
- Gas always moves along its pressure gradient from high pressure to low
- During spontaneous breathing:
~inspiration: pressures in the alveoli fall, generating a pressure gradient with the atmosphere. Atmospheric pressure pushes gas into the lungs
~expiration: passive recoil of the lungs, largely secondary to surface tension forces, cause the lungs to get smaller and increase alveolar pressures. This results in alveolar pressures exceeding atmospheric pressures and gases out of lungs
-Positive pressure ventilation:
~ inspiration: pressures in excess of atmospheric pressure is generated at the mouth. This pressure gradient causes gases to flow into the lungs. If volumes are small, increasing the pressure gradient will result in INCREASED VOLUMES
~Expiration: same as spontaneous breathing. Passive recoil of the lungs, largely secondary to surface tension forces, cause the lungs to get smaller and increase alveolar pressures leading gases to flow out of the lungs
Describe lung compliance
Lung compliance is defined as the ease of lung distention. The more compliant the lungs are the easier to expand
Units used are mL/cwp or L/cwp
State the normal value of total static compliance (lungs and thorax together)
Normal lung compliance
-spontaneous breathing patient: ~100mL/cwp
-ventilated patient ~ 40-60 mL/cwp
-critical value <20 mL/cwp
Given the following information, answer questions a-c about a patient receiving mechanical ventilation.
A. What is the patients total static compliance in ml/cm H2O
B. What is the patients total dynamic compliance in ml/cm H2O
C. Calculate the patients airway resistance in cm H2O/L/sec
Delivered volume (volume 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
State the normal range of airway resistance
0.5-1.5 cwp/L/s in spontaneously breathing patients
6-8 cwp/L/s in spontaneously breathing patients in intubated patients
State the region where most of the airway resistance is found (upper or lower airways)
- The nose is the highest
- the small airways are the lowest due to their huge cross-sectional area
State whether airway resistance increases or decreases as the bronchi branch toward the alveoli
Branching always causes turbulence and turbulence is associated with increased resistance. The lower you go in the airway the lower the overall resistance.
Describe the change in airway resistance that is associated with the presence of mucus secretions, bronchospasm, or mucosal edema
These all reduce the radius of the airway which results in an exponential rise in the raw
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
There is increased alveolar dead space ventilation and/or decreased alveolar ventilation. This might be caused by something like pulmonary embolism
- PaCO2 =VCO2/VA
Discuss Hooke’s law
The greater the force the greater the stretch within physiologic limits. If the force surpasses the physiologic limit barotrauma will occur
Explain the relationships that are defined by Poiseuille’s Law
- The big relationships here are the effects of length and radius on flow and pressures
*if length increases there is a direct increase resistance so: * if flow is held constant it will take double the pressure to maintain flow * if pressure is held constant flow will decrease * if radius is decreased there is an exponential rise in raw * if flow is to held constant it will take a huge increase ( r4) in the pressure to maintain the flow * if the pressure is held constant it will result in a huge decrease in flow as the rise in raw in response to the change in radius (r4) results in a large fall in flow for the same pressure
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)
- Based on LaPlace’s Law , the smaller the radius the higher the pressure. Because alveoli are all interconnected this would result in smaller alveoli collapsing into the bigger alveoli
Describe the role of surfactant in preventing alveolar collapse of small alveolar units
As alveolar size decreases the surfactant comes closer together and effectively shields the air / liquid interphase so that surface tension is reduced significantly. This stabilizes the smaller alveoli preventing atelectasis. On inspiration as the alveoli expand surfactant is spread thinner and surface tension rises contributing to lung recoil. As the lung recoils alveoli get smaller and again the surfactant effectively covers the air/ liquid interphase, reducing STand stabilizing the alveoli
