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

• 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

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