Brian Locke

Basic Ventilator Chalk Talk

==Patient Scenario: Blurb 1==

Respiratory Failure:

The pulmonary system has two primary roles, each of which can fail:

  1. Oxygenation of the blood
  2. Removal of CO2, which is a result of ventilation.

Consider: How do we get information about the respiratory systems performance in each of these?

  • Oxygenation: SpO2 is usually sufficient to estimate PaO2 (which can be directly measured on an ABG). When this fails, we call it Hypoxemic Respiratory Failure
  • Ventilation: a blood gas to measure CO2, ideally arterial. When it fails, we call it Ventilatory Failure

Note: I prefer the term ventilatory failure to the commonly used "Hypercapneic respiratory failure" because it is possible to have failure of ventilation without hypercapnia. Can you think how?

Answer: Imagine a respiratory system that is stressed (ie. requiring compensation) by a severe metabolic acidosis (e.g. DKA) but is unable to rise and meet the demand. More here

==Patient Scenario: Blurb 2, put on HFNC==

What things does intubation allow that other modes (e.g. HFNC, NIPPV) do not in respiratory failure?

  • application of higher levels of PEEP (especially important if portions of lung need to be recruited, or opened up)
  • allowance for controlled ventilation
  • airway control

Ventilator Modes:

The most common are ACVC, ACPC, or CMV) and PS.

1 important concept is the concept of a dependent variable: for each mode, you set some parameters and the lung has certain characteristics like stiffness. Based on the interaction between the settings and the lung's physiology

For each mode: What causes a breath to start? What causes the breath to stop? What determines the how fast a breath goes in? What would happen to the respiratory system if the lungs became much stiffer (less compliant)?

AC-VC, Assist Control Volume Control

  • Breath start either when the patient starts the breath (an assist breath) or a the ventilator triggers a breath (a controlled breath)
  • The breath stops after the set volume is reached
  • The breath is given at an inspiratory pressure and over an "i-time" that are set
  • Volume (set) = i-time (set) * flow. Flow results from pressure (dependent variable) and lung stiffness (called complaince). Thus, in ACVC, you must pay attention to make sure that you're pressures (Peak and Plateau, see below) aren't too high. If the lung becomes stiffer (less compliant), this will manifest as increasing pressure.

AC-PC Assist Control Pressure Control

  • Breaths start either when the patient starts the breath (an assist breath) or a the ventilator triggers a breath (a controlled breath)
  • The breath stops after the set time is reached.
  • The breath is given at a set inspiratory pressure.
  • The I-time and flow are set (flow is set because the pressure is set and the compliance of the lung is what it is), thus it is the volume that is the uncontrolled variable. Stiffer lungs would show up as decreased tidal volume (or conversely, compliant lungs might lead to injuriously large volumes on normally reasonable pressures)

PS Pressure Support

  • Breaths start when the patient triggers them (there is no ventilator set rate, the patient must be initiating breaths - meaning, not comatose)
  • The pressure support stops when flow decreases
  • The breath is given with the set pressure support
  • Same as in AC-PC, the tidal volume is the dependent variable. Additionally, one must watch the RR as the patient controls this entirely.

Ventilator Parameters:

What ventilator parameters effect oxygenation? What parameters effect ventilation?

Things that effect oxygenation:

  • FiO2
  • PEEP (by recruiting more lung and improving V/Q mismatch)
  • parameters of the respiratory cycle (e.g. things that make it so that more of the cycle is at a higher pressure = more lung units recruited.)

Things that effect ventilation:

  • Respiratory rate (*see below)
  • Tidal volume
  • Deadspace (though this not directly controlled, but a function of the lungs and how the patient is being ventilated).

How does deadspace influence alveolar ventilation?

Minute ventilation (total amount of air being ventilated/going in mouth; RR * Tidal volume) - Deadspace ventilation (amount not participating in gas exchange) = alveolar ventilation *(amount of air that is participating in gas exchange).

How do you get information on the amount of deadspace? The deadspace fraction

==Patient Scenario: Blurb 3==

How do we estimate initial parameters?

What are the practical constraints in how we can adjust ventilator settings?

What limits our up-titration of each of the major parameters? RR TIdal Volume (Vt) FiO2 PEEP

VILI

Note: mechanical ventilation does not fix any disease process. It only supports the lungs (and respiratory system) until they heal themselves. Thus, our MAJOR goal is to avoid doing damage to the lungs.

  • Volutrauma - tidal volumes too large
  • Barotrauma - pressures too high
  • Atelectatrauma - collapsing and opening alveoli

##Peak and Plateau Pressure

How do we measure pressure in the lungs? Separate dynamic pressure (from resistance to flow through the airways) and static pressure (pressure in the lungs when there is no flow). Peak = reflects airway plus alveoli pressure

Plateau = just reflects alveoli pressure (transpulmonary aka alveolar - pleural pressure at lung inflation to total lung capacity). Try to keep below 30-35 mmHg. (or driving pressure in obesity)

High peak + normal plateau pressure localizes to airway or tubing

High peak + high plateau pressure localizes to high alveolar pressure (low lung compliance, pneumothorax, abd process, mainstem bronchus). Note: obesity causes this pattern without add'n stress (because pleural pressure is similarly elevated = same driving pressure. Pleural pressure can be estimated by esophageal probe. Transpleural pressure correlates with barotrauma. Generally, need higher PEEP to recruit)

Protective Lung Ventilation

Permissive Hypercapnea

#Weaning

Test of mastery: why is it that a patient who is on ACPC with a inspiratory pressure of X and is taking ALL ASSISTED BREATHS might do ok, but then fail when switched to pressure support at the same inspiratory pressure?

Ans: the difference in assisted breaths on ACPC and PS is how the ventilator decides when to stop the breath. In ACPC, the support will continue for the set time. In PS, the support will only continue until the flow drops off. Thus, if the patient is unable to maintain inspiration (either due to inadequate strength, or insufficient drive to breath), they will end up taking rapid, shallow breaths in PS.