Test 8 Flashcards
Positive Pressure improves
Alveolar Recruitment and Oxygenation, excessive pressures may damage the lung tissue
-May also result in other potentially serious unwanted effects, such as reduced venous return of blood to the heart
Inspiratory limb and expiratory limb
Inspiratory- facilitates breath delivery
Expiratory- which directs exhaled gases toward the expiratory port on the vent for measurement
The circuit is comprised of large bore corrugated tubing, which much like the lungs has
Compliance and Elastance
Tube compliance, combined with positive pressure generated during breath delivery, causes
the circuit to bulge or stretch, resulting in reduced gas delivery to the pt.
- This amount is an estimated 3-5ml of volume per cmH2O pressure. Thus at a PIP of 25cmH2O about 100ml are lost to the compressible circuit
- Most modern vents measure and compensate for this compressible vol.
Ve and Va
Minute ventilation and alveolar ventilation, the main factors that facilitate the rate of removal of Carbon Dioxide from the blood
Alveolar Ventilation
determines the true amount of gas that reaches the alveoli to participate in gas exchange. It also controls for the amount lost through factors such as dead space and , if mechanically ventilated, the amount of compressible volume
Deadspace is defined as
areas of ventilation where there is no perfusion of blood to promote gas exchange.
-An example of this is the volume of air that remains in the conducted airways that never reaches the alveoli for gas exchange
Anatomical deadspace, and VA
Anatomical deadspace is estimated as 1ml/ilb of predicted body weight
- Va can be estimated as: (Vt- deadspace volume) x RR
- Excessive tubing or various adapters that are added to the vent circuit further increase the deadspace volume, impacting the Va
A 200-lb PBW male patient is being mechanically ventilated in volume control ventilation. His RR is 20 bpm, and his Vt is 550 mL, resulting in a PIP of 25 cm H2O.
What is the patient’s estimated alveolar ventilation?
Also recall that 25 cm H2O equals roughly 100 mL of compressible volume retained in the circuit. A 200-lb PBW patient has an anatomic dead space of 200 mL (1 mL/lb).V̇a = (550 – 300) x 20. V̇a = 5000 mL or 5.0 L.
Hypoxemia
Low levels of O2 in the arterial blood
-is most commonly assessed through examination of ABG results and interpretation of the partial pressure of arterial oxygen (PaO2)
Normal PaO2
80-100mmHg
Primary factors that impact oxygenation during MV include
- FiO2
- Mean Airway Pressure (Paw), which itself is influenced by PEEP and I:E ratio
- Lung function
The primary determinants of PaCO2 are
the production of carbon dioxide (VCO2) by the body and alveolar ventilation
While we cannot directly control the amount of carbon dioxide the body produces, we can impact Va by manipulating various MV settings that increase the Ve such as
Vt
RR
I-time
E-time
Pulse oximeters provide an estimated
percentage of hemoglobin saturated with oxygen
For normal healthy pts ETCO2 correlates well with PaCO2, but as
deadspace to Vt ratio (Vd/Vt) increases, the correlation becomes less accurate and inconsistent
Normal spontaneous breathing is the result of
a negative pressure gradient between the intrathoracic cavity and the atmosphere, created by the contraction of the diaphragm
-MV is the opposite: positive pressure
VILI
is the result of overdistention of the alveoli
Commonly measured pressures in lungs include
PIP
Pplat
PEEP
AutoPEEP
Compliance
- Elastance of the lung
- Obstructive diseases such as Emphysema increases the cl
- Restrictive diseases such as pulmonary fibrosis decreases it
During volume control ventilation
- There is an inverse relationship between Cl and airway pressures such as PIP and Pplat
- there is a direct relationship between Raw and PIP
During Pressure control ventilation
- There is an inverse relationship between Cl and delivered volumes
- There is an inverse relationship between Raw and Vt
Increases in Raw are commonly due to
secretions in the airway or bronchospasm of the smooth muscle lining the airway
Air trapping
in the lungs can be caused by an obstructive disease process or inadequate vent settings
AutoPEEP
additional pressure above the PEEP set on the MV, and the combination of the two represents total PEEP
-Increases the WOB and reduces venous return and cardiac output
following will occur in volume control ventilation during a decrease in compliance?
Vt will remain the same, PIP will increase
Pressure Volume Curve
used to aid in ventilator parameter selection, minimizing the physiologic impact of auto-PEEP and high airway pressure
- Can be examined using the inspiratory curve of the pressure-volume loop
- Normally football shaped in the healthy lung
“bird beak” in pressure volume curve idicates
alveolar overdistension
Two points that can assist in setting PEEP and targeting a Pplat threshold level that maintains alveolar recruitment while avoiding overdistension
- the lower inflection point (LIP)
- The upper inflection point (UIP)
LIP
lower inflection point (LIP)- represents the pressure at which a large number of alveoli are recruited. Setting the PEEP at this level helps improve oxygenation and prevents alveolar collapse
UIP
Upper inflection point (UIP)- represents the point at which a large number of alveoli are overdistended. Using volume or pressure control strategies that maintain Pplat below this threshold helps prevent alveolar overdistension and VILI
Stress index method
evaluates the level of PEEP to avoid overdistension and underrecruitment of alveoli
-Used during constant flow tidal volume delivery (square flow waveform)
An upwardy concave sloping pressure-time curve suggests
Improved Cl and is scored as a stress index of less than 1, indicating additional potential for recruitment and increase levels of PEEP
A pressure-time curve lacking concavity is scored as
a stress index equal to 1 , meaning the amount of pressure and volume is ideal to minimize overdistension and maximize recruitment of alveoli
A downwardly concave sloping pressure-time curve is scored as
a stress index of greater than 1, representing alveolar overdistension and the need to decrease PEEP and/or tidal volume
Respiratory Muscle load represents
the amount of work required to meet the pts physiologic needs. This load can be increased due to a variety of different factors
- Minute ventialtion
- Increased Resistive load
- Increased elastic load
What increases minute ventilation in respiratory muscle load? Resistive load? Elastic load?
- Minute ventilation: Pain and anxiety, Sepsis, Increased deadspace, excessive feeding
- Increased Resistive Load: Bronchospasm, Secretions, Small artificial airways
- Increased Elastic Load: Low lung compliance, Low chest wall compliance, AutoPEEP
Respiratory Muscle Capacity represents
the ability of the respiratory muscles to perform the work needed to meet or exceed the respiratory muscle load.