Mechanical Ventilation in Obstructive Lung Disease

Asthma

  • In asthma:
    • The patient has a constriction of the bronchial smooth muscles in the airways:
      • Leading to reversible air trapping:
        • This is indicated in the picture:
          • Note that the bronchial muscles do not extend into the small airways
A normal cluster of alveoli with a normal capillary, delivering carbon dioxide (CO2) and picking up oxygen (O2).
  • Intubation of an asthmatic is a dreaded complication of this illness:
    • As asthmatics can deteriorate rapidly on the ventilator:
      • Without close monitoring and active management
    • The goal with a ventilated asthmatic:
      • Is to prevent breath-stacking or autoPEEP:
        • And the hemodynamic instability that can result
  • Clinicians should note that intubation of an asthmatic:
    • Should trigger even more active management with medications, rather than less
  • Intubated asthmatic patients should continue to receive aggressive treatment with:
    • Bronchodilators
    • Steroids
    • Magnesium
    • As well as deep sedation:
      • Possibly even neuromuscular blockade in the initial hours after intubation:
        • In an effort to relax the chest wall musculature and gain control of the situation
      • Please note that neuromuscular blockade:
        • Only works on skeletal muscle and therefore:
          • Will not bronchodilate smooth muscle in the airways
  • In addition:
    • It is very critical to be aware of the patient’s intravascular volume status:
      • As the excess positive pressure:
        • Can lead to hemodynamic collapse
    • Moreover, the excess pressure, including the auto-PEEP:
      • Can result in barotrauma:
        • Such as the development of a pneumothorax very quickly in this patient population
  • The ventilator screen below demonstrates the effects of reactive airways disease on pulmonary mechanics:
    • This patient had unexpected bronchospasm after being intubated:
      • Note the elevated peak inspiratory pressure (PIP) of 45 despite the relatively low tidal volume of 365:
        • The patient’s resistance was too high for her to even receive the full tidal volume, as the ventilator was only able to deliver 320 ml before stopping
  • Checking the plateau pressure (Pplat):
    • Confirmed that this was a resistance problem, rather than a pure compliance problem:
      • Her PIP was 39 at the time the inspiratory hold was performed:
        • But her Pplat was only 28:
          • The delta between 39 and 28 indicates a significant resistance component
  • This patient was treated with continuous bronchodilators with rapid improvement in the bronchospasm:
    • Her PIP returned to normal within minutes
  • Four ventilator maneuvers:
    • Increase expiratory time:
      • Decreasing the respiratory rate
      • Decreasing the I:E ratio
      • Decreasing the inspiratory time
      • Increasing the inspiratory flow
        • Of these, decreasing the respiratory rate is the most effective means:
          • To allow more time to exhale
  • The figure shows a picture of 30 seconds with two patients, set with the same I:E ratio of 1:2:
    • The first patient:
      • Has a rate of 10 breaths per minute:
        • Allowing 6 seconds per breath cycle
    • The second patient:
      • Has only 3 seconds per breath cycle:
      • Given the respiratory rate of 20
    • The blue represents inspiration, the red the time for exhalation:
      • Note that even with the same I:E ratio:
        • The lower rate offers a substantially longer time to exhale
  • In looking further at this diagram:
    • One can imagine the effects of changing the I:E ratio, the inspiratory flow, or the I time:
      • The following figure shows a hypothetical example of the effects of these changes in a patient on volume control:
        • In a given patient, the exact values will vary, but the purpose of the illustration is to show the relationship among the parameters of:
          • I:E ratio, inspiratory time, and inspiratory flow
  • In addition to a slow respiratory rate, a low I:E ratio, a short inspiratory time and/or a fast inspiratory flow rate:
    • Asthmatics should also be ventilated with:
      • Low tidal volumes:
        • Considering that the larger the tidal volume:
          • The more the patient has to exhale
  • In monitoring an intubated asthmatic, looking for air trapping is key:
    • In the ventilator tracing below, note that the flow tracing, in the middle, does not return to the baseline before the next breath. (Red arrows):
      • This represents that the patient is still exhaling when the next breath is given:
        • Leading to air trapping:
          • Seeing this pattern on the ventilator can be an early clue that the patient is air trapping
  • In this patient:
    • You could first decrease the respiratory rate, or increase sedation if the patient is over-breathing
    • The I:E ratio is only 1:2:
      • So changing the I time to make a ratio of 1:3 or 1:4 is also appropriate
    • Also continued treatment with bronchodilators:
      • To decrease the bronchospasm associated with this disease:
        • Will also mitigate the excess auto-PEEP
  • Recall that to quantify the pressure exerted by air trapping:
    • One should check for autoPEEP:
    • B y checking an expiratory hold button on the mechanical ventilator
  • The intrinsic PEEP is 11, and the total PEEP is 12:
    • This indicates that the patient was only set on 1 of PEEP (an unusual – and not recommended – setting, used in this circumstance for demonstration purposes only.)
  • Thus, to set the ventilator for an asthmatic, select:
    • A low tidal volume:
      • 6 to 8 mL/kg of predicted body weight
    • The respiratory rate should be low:
      • Less than 20 breaths per minute:
        • Oten around 10 breaths per minute
    • The I:E ratio should be changed to:
      • 1:3 or less
    • PEEP should be set at:
      • 5 cmH2O
    • The FiO2 should be down-titrated as tolerated
    • These patients continue to receive:
      • Heavy sedation
        • Possibly NMB if required
      • Continuous bronchodilators
      • Close monitoring for breath stacking and autoPEEP:
        • AutoPEEP should be monitored periodically or after any ventilator change:
          • With an expiratory hold
      • Arterial blood gases (ABGs) should be checked:
        • To ensure that the patient is being adequately ventilated
  • Permissive hypercapnia:
    • Is the concept of tolerating a PaCO2 greater than 40mmHg and a pH greater than 7.20 to 7.25:
      • For the sake of achieving another goal:
        • In the case of asthma:
          • The goal is to allow time to exhale and prevent air-trapping:
            • Permissive hypercapnia is a reasonable strategy:
              • Especially early in ventilating the asthmatic
  • Initial Ventilator Settings in Asthma:
    • Tidal Volume:
      • 6 to 8 ml/kg PBW
    • Respiratory Rate:
      • 6 to 14 breaths/minute:
        • Allowing for permissive hypercapnia
    • PEEP:
      • ~ 5 cmH2O
    • FiO2:
      • Decrease as tolerated
    • SpO2 ≥ 92%
  • The following ventilator screen demonstrates these settings:
    • The patient is set at 6ml/kg at 350 mls, with a respiratory rate of 14, a PEEP of 5, and a FiO2 40%:
      • Note, however, that the patient is not synchronous with the ventilator and is taking large tidal volumes:
        • This can be a very dangerous situation, leading to worsening air-trapping and possibly hemodynamic compromise:
          • This patient needs to be deeply sedated and neuromuscular blockade administered if needed:
        • Additionally, the patient should continue to receive bronchodilators and all other appropriate medical treatments

COPD

  • There are two types of obstructive lung disease falling under the umbrella of COPD:
    • Namely:
      • Chronic bronchitis
      • Emphysema
    • While some patients may have one or the other:
      • Many will exist on the continuum
  • Chronic bronchitis can resemble the asthmatic schematic above:
    • With the notable exception that:
      • Muscles hypertrophy and are not entirely reversible
    • Additionally, chronic bronchitis is associated with:
      • Increased mucous production
  • Emphysema:
    • Is a disease of parenchymal destruction:
      • Not only is there loss of alveoli:
        • Resulting in decreased surface area, or decreased diffusion area (leading to an increased DLCO):
          • But the small airways:
            • Can become floppy:
              • Due to the loss of other tissues holding them open
  • Understanding the pathophysiology of COPD is important for considering how to best ventilate these patients:
    • It should be noted, however:
      • That most patients with COPD have:
        • Some mixing of elements of chronic bronchitis and emphysema:
          • These conditions exist on a spectrum rather than a dichotomy
  • Most patients with COPD are now managed:
    • With BPAP:
      • With improved outcomes over intubation:
        • However, on occasion:
          • A patient with COPD is not a candidate for BPAP or fails to improve with a trial of BPAP:
            • Mandating intubation and invasive mechanical ventilation
    • Many of the principles that apply in mechanical ventilation for asthma also apply in COPD:
      • Both are obstructive diseases, and in both processes:
        • The patients require adequate time to exhale:
          • Therefore:
            • Low tidal volumes
            • Low respiratory rates
            • Low I:E ratios:
              • Are appropriate:
                • However, a key difference involves the role of PEEP
    • Patients with COPD:
      • Are at high risk of developing autoPEEP:
        • Due to their obstructive disease:
          • They require additional time to exhale
      • However, the mechanism of obstruction can differ between asthma and COPD:
        • Especially COPD with emphysematous changes as illustrated above:
          • With the destruction of parenchyma, the small airways can collapse with exhalation:
            • Trapping air behind:
              • In this circumstance, this trapped air leads to autoPEEP
                • Increasing the set PEEP, to match the autoPEEP, is not necessarily an intuitive solution:
                  • However, as illustrated by the diagram below, increasing the PEEP to prevent collapse of these small airways:
                    • Can allow the patient to exhale more fully
  • Reexamine the tracing of the figure from the Asthma section:
    • Imaging that this patient has COPD:
      • If this patient has 11 of autoPEEP, or intrinsic PEEP, what PEEP would you select?
  • To match the autoPEEP:
    • 11 cmH2O would be an appropriate PEEP selection
  • Lastly:
    • Patients with COPD are often chronically hypoxemic:
      • Indications of chronic hypoxemia physical exam findings of chronic hypoxemia can be demonstrated with nail clubbing
      • Additionally, can include an elevated hemoglobin level on the CBC:
        • Indicating the patient’s compensation for their chronic lung disease
      • Because these patients are baseline hypoxemic, and ventilation is often a relatively greater issue for them than hypoxemia:
        • The oxygen saturation for a patient with COPD should be targeted at 88% to 92% in most circumstances:
          • This is increasingly important as more data demonstrating the risks of hyperoxia continue to accumulate
  • Initial Ventilator Settings in COPD:
    • Tidal Volume:
      • 6 to 8 ml/kg PBW
      • Respiratory Rate
        • 6 – 20 breaths/minute:
          • Allowing for permissive hypercapnia
      • PEEP
        • 5 to 15 cmH2O:
          • May need to match autoPEEP:
            • For patients with significant emphysematous physiology
      • Fi02:
        • Decrease as tolerated
      • SpO2 target:
        • 88% to 92%
  • This ventilator screen demonstrates a patient with COPD with severe dyssynchrony:
    • The PIP is 54:
      • Indicating severe pathology
    • The irregular waveforms:
      • Indicate the dyssynchrony
    • The patient is set at a respiratory rate of 16 but is breathing at 24
  • An expiratory hold was performed and demonstrated a:
    • Total PEEP of 29, with a set PEEP of 10
      • This indicates a high autoPEEP of 19:
        • Therefore, this is a very high-risk situation:
          • This patient was deeply sedated, NMB administered, and the ETT was disconnected from the ventilator to allow the patient to exhale
  • Once sedated and relaxed, the patient was placed back on the ventilator at a rate of 12, with frequent expiratory holds to check the autoPEEP

#Arrangoiz #Surgeon #Teacher #CancerSurgeon #MechanicalVentilation

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s