Lecture 31 - Mechanical Ventilation Flashcards
2 forms of respiratory failure?
- Hypercapnic
2. Hypoxic
Causes of hypercapnic respiratory failure?
- Depressed drive
2. Overloaded muscles (excessive loads or weakened muscles)
Causes of hypoxic respiratory failure?
- Gas exchange abnormalities
- Low perfusion states
Aka: 5 physiologic causes of hypoxemia
Example of an excessive load causing the respiratory muscles to be overloaded?
Obstructive lung diseases that increase airway resistance, like asthma
2 ways of adjusting oxygenation on a mechanical ventilator?
- FiO2 (except for shunt)
2. PEEP
How does PEEP increase oxygenation?
Increases surface area of gas exchange as more alveoli are open at FRC
What 4 settings need to be set on a mechanical ventilator?
- FiO2
- Respiratory rate
- TV
- PEEP
At what FiO2 do we usually start patients who need a mechanical ventilator? Why?
100% FiO2 to avoid any hypoxic injury to the brain (unless patient needs ventilator because of a seizure)
How to determine what to set TV at to start?
Based on ideal body weight
How to determine what to set RR at to start?
Normal rate: 10-12 breaths/min
How to determine what to set PEEP at to start?
Start at physiologic PEEP of 5 cm of H2O
Way of adjusting PaCO2 levels on a mechanical ventilator?
Minute ventilation through RR
What will happen if the TV is kept constant on someone with a restrictive lung disease?
As time goes by, the lung will lose more and more compliance (more diseased lung, less healthy lung), and the TV will become too high for the lung which could get damaged unless the TV is adjusted
How to adjust RR to adjust minute ventilation and therefore PaCO2? Under what conditions in this equation valid?
°VeCO2 x 0.86 / (1- Vd/ Vt) = PA/a CO2 x °Ve
1- Vd/Vt = percent of volume that is alveolar
°VeCO2 = minute ventilation of CO2 = volume of CO2 being expired/min
Conditions: as long as minute production of CO2 (°VeCO2) and ratio of dead space to tidal volume (Vd/Vt) is constant
If a patient is on a ventilator at constant TV and °Ve, and the PaCO2 increased, what happened?
One of 2 things:
- Either the CO2 production rose = hypercatabolic state
- Or the volume of physiological dead space increased
Can you adjust PaCO2 by adjusting TV?
NOPE, only RR
What do volume targeted ventilators do?
Most common mode in which every breath you give the patient is the same volume with
pressure dependent on compliance
What do pressure targeted ventilators do?
Ventilator mode in which the pressure given is constant and the volume will depend on compliance
What 4 factors will cause peak pressure to increase?
- Decrease compliance
- Increased airway resistance
- Large tidal volume
- Higher flow
If volume on a pressure targeted ventilator goes up, what are the 2 possible causes?
- Increased compliance
2. Decreased airway resistance
If peak P on a volume targeted ventilator goes up, what are the 2 possible causes?
- Decreased compliance
2. Increased airway resistance
How to obtain plateau pressure on a mechanical ventilator? What is the plateau pressure equal to?
Program a one second inspiratory pause
Plateau pressure = end alveolar distending pressure
What pressure should never go over 30-35 cm H2O?
Plateau pressure = end alveolar distending pressure
2 factors affecting plateau pressure?
- Compliance
2. TV
2 possibilities if compliance is decreasing?
- Decreasing lung compliance
2. Decreasing chest wall compliance
What is alveolar space synonymous with?
Respiratory zone
What does somulent mean?
Abnormally drowsy
When given baseline and new values for arterial blood gas, how to determine acute from chronic respiratory acidosis/alkalosis?
- Calculate pH change compared to PaCO2 change from baseline to new values
- Calculate pH change compared to PaCO2 change from baseline to standard values
=> Could have an acute disorder on top of a chronic state
What happens right after you put a patient in chronic respiratory acidosis/alkalosis on a ventilator to fix his PaCO2? What happens once this goes away?
Short period of time during which the patient will be in metabolic acidosis/alkalosis that was there to compensate the respiratory primary disorder
Once the metabolic compensatory mechanisms dissipates, if all of the ABG values go back to STANDARD normal and the patient is extubated… There will be an issue!
Since we got rid of the metabolic compensation and we stabilized him at standard normals instead of his own, the patient goes right back into hypercapnic failure with acute respiratory acidosis/alkalosis
What are cc?
mL
What is tachypnea?
Abnormally rapid breathing = hyperventilation
What is the diagnosis if a patient has acute respiratory alkalosis and once fixed with 100% O2 the patient is still tachypneic?
Pulmonary embolism causing a V/Q imbalance (VD/VT has doubled) where the blood clot is and the patient is breathing fast to exhale all of the CO2 through one of the 2 lungs to maintain normal °VeCO2 => he needs to double his RR to maintain a constant PaCO2
