Mechanical Ventilation Flashcards
2 major drawbacks of positive pressure ventilation?
It causes hypotension and barotrauma.
Negative pressure ventilators (…personal iron lung-ish things) don’t cause hypotension and barotrauma. Why aren’t they more commonly used?
The tidal volume you can generate is limited.
You can’t control the airway.
(they’re really only used in neuromuscular diseases, and sometimes COPD)
You can set a volume or set a pressure with the respirator. Which is preferred for acute respiratory failure?
Volume-cycled, positive pressure ventilation is preferred.
Is hypoxemia alone reason to go to mechanical ventilation?
No. Try oxygen first. Mechanical ventilation and/or intubation is indicated when 100% oxygen by face mask is inadequate.
5 indications for mechanical ventilation (MV)?
Hypercapnea.
Hypoxemia (but.. it’s not actually that good at fixing it.)
Decrease work of breathing (probs most important).
In shock, to spare CO.
Airway protection
Does MV improve gas exchange?
No, not per se. But if you add in PEEP, it may.
MV increases length of hospital stay, costs, and complications.
Sure does.
Review: Equation for work of breathing?
W = integral ( P deltaV)
How can you change pCO2 with a ventilator?
By changing the alveolar ventilation - by adjusting the rate and tidal volume.
What’s the deal with peak flow and patient comfort?
Patients are more comfortable with a shorter inspiration, longer expiration - which is achieved with a higher peak flow.
But higher peak flow -> higher pressure, which can be bad)
Why does increased peak flow increase airway pressure?
Higher peak flow leads to turbulent flow -> more pressure.
How can MV actually improve hypoxia? (it’s not with increased fiO2)
Positive end-expiratory pressure (PEEP) - keeps alveoli open during expiration.
(PEEP allows for lower fiO2, too, which is great)
When giving MV to a hypercapnic patient, is normal PCO2 the goal?
No. You don’t want to be too aggressive… Just aim for safe.
How does MV improve PCO2?
By increasing (worsening) V/Q due to increased dead space via…
Radial traction on airways.
High pressures divert blood flow -> dead space in zone 1 lung.
Decreased venous return.
What’s the first treatment for hypotension associated with MV?
Fluid bolus - get more preload to the heart.
just like in cardiac tamponade… because, similarly, the hyper-inflated lungs are compressing the heart
4 adverse effects of MV?
Hypotension.
Barotrauma.
Respiratory distress & auto-PEEP.
Acute lung injury.
Complications of barotrauma? (4 things)
Pneumothorax.
Pneumomediastinum.
Pneumopericardium.
Subcutaneous emphysema.
What are the two most important values for figuring out the cause of respiratory distress of a patient on MV?
Peak and plateau pressures.
really the difference between them is what you care about
What lung function variable(s) is/are reflected in peak pressure?
Both compliance and airflow.
How do you measure plateau pressure?
Occlude the airway momentarily -> pressure will plateau.
What lung function variable(s) is/are reflected in plateau pressure?
Compliance only.
How do you calculate the pressure contributed by airflow?
Peak pressure - plateau pressure.
This will reflect pressure generated by airflow.
DDx of MV respiratory distress with unchanged P(peak) - P(plateau)?
(i.e. with normal airflow pressure)
Pulm edema. Pulm embolus. PNA. PTX. Anxiety, pain. Auto-PEEP.
DDx of MV respiratory distress with wide P(peak) - P(plateau)?
(i.e. with increased airflow pressure)
Something must be blocking airflow: Endotracheal tube problems. Tubing kinked Secretions. Bronchospasm.
What is auto-PEEP?
Physiologic causes?
Pressure due to air trapped by insufficient expiratory time.
Caused by high tidal volumes, Starling resistors.
(must suck harder to generate further breaths… can trigger respiratory distress.)
Can MV cause ARDS?
Sure can.
How can MV be altered to reduce risk of ARDS?
Use low-stretch MV (lower tidal volumes, plateau pressures).
Ventilate the sickest patients less!
4+ aspects monitoring MV?
Clinical: secretions, mental status.
CXR and ABGs.
Daily spontaneous breathing trials (switch off machine while still intubated).
Static compliance (Cstat)
What’s static compliance?
Cstat is the change in lung volume divided by change in distending pressure.
You really should really really know the equation for static compliance (Cstat).
Cstat = Tidal Volume / (Plateau pressure - PEEP pressure)
