Ventilator Consequences

Part of: Mechanical Ventilation

Complications from Intubation

• Dental trauma
• Aspiration
• Laryngeal damage
• Bronchospasm
• Oesophageal intubation
• Right main bronchus intubation

Physiological Consequences

Effects on Gas Exchange

• At low and medium levels of PEEP: ↑ alveolar recruitment → larger surface area for gas diffusion → ↑ oxygenation
• At high levels of PEEP: alveolar dead space increases dramatically
  • PEEP has a greater effect on normal compliant alveoli than on stiff/fluid-filled alveoli
  • At high PEEP, healthy alveoli become overdistended → ↑ resistance to blood flow → blood shunts towards poorly ventilated alveoli → ↑ physiologic dead space → worsened V/Q mismatch
  • High PEEP can also increase pulmonary vascular resistance, worsening intracardiac R-L shunts

Effects on Haemodynamics

• Preload: PEEP can decrease preload because:
  • ↑ CVP → ↓ venous return to RA
  • ↑ RV afterload → ↓ blood exiting RV
  • ↑ RV afterload → leftward displacement of the IV septum → impairs LV diastolic filling
• LV Afterload: PEEP decreases $P_{\text{transmural}}$ which leads to decreased afterload (as per law of Laplace)

$$\underset{\text{(AKA afterload)}}{\text{LV Wall Tension}}\approx \frac{P_{\text{transmural}}\times R}{\text{LV wall thickness}}$$

• The effect on cardiac output and blood pressure is highly dependent on volume status:
  • Hypovolaemic or euvolaemic → ↓ preload significantly → ↓ cardiac output and blood pressure
  • Hypervolaemic → cardiac output increases, blood pressure may increase or remain unchanged

Effects on Monitoring

• High levels of PEEP impair the usual means of assessing cardiac preload (i.e. CVP and PCWP)
• Alternative strategies include:
  • Calculating ${\text{PCWP}}_{\text{adjusted}}$
    • With normal lung compliance: ${\text{PCWP}}_{\text{adjusted}}\approx {\text{PCWP}}_{\text{measured}}-\frac{\text{PEEP}}{2}$
    • With abnormal lung compliance: ${\text{PCWP}}_{adjusted}\approx {\text{PCWP}}_{\text{measured}}-\frac{\text{PEEP}}{4}$
  • Right ventricular end diastolic volume
  • Intrathoracic blood volume
  • Respiratory variation in aortic blood velocity

Effects on Cerebral Perfusion

$$\underset{\begin{array}{c}\text{Cerebral}\\ \text{Perfusion Pressure}\end{array}}{\mathrm{CPP}}=\text{MAP}-\text{ICP}$$

• PEEP can in some circumstances decrease cerebral perfusion (variable)
  • PEEP increases ICP due to decreased venous return

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Auto-PEEP (aka Intrinsic PEEP)

• Auto-PEEP is positive airway pressure that occurs at the end of expiration due to incomplete exhalation
• Consequences include:
  • ↑ Risk of barotrauma
  • ↓ Venous return → ↓ cardiac output → hypotension
  • Worsened V/Q mismatch
  • Patient-ventilator dyssynchrony
  • ↑ Patient’s work of breathing

Mechanisms

Mechanism	Examples
High minute ventilation	High $V_{\text{T}}$ and/or high respiratory rate
Expiratory flow limitation	High airway resistance in COPD
Expiratory resistance	Kinked ET tube, patient-ventilator dyssynchrony

Detection

• Examine for audible air flow extending to the end of expiration on auscultation

$$\text{Auto-PEEP}=\begin{array}{c}\text{End-expiratory}\\ \text{Alveolar Pressure}\end{array}-\begin{array}{c}\text{Applied}\\ \text{PEEP}\end{array}$$

• Quantify using an end-expiratory breath hold — the rise in pressure above the applied PEEP represents the auto-PEEP

Figure 9. Auto-PEEP absent

Figure 10. Auto-PEEP present

Treatment

Mechanism	Treatment
High minute ventilation	↓ $V_{\text{T}}$, ↓ respiratory rate, or ↓ I:E ratio (permits permissive hypercapnoea)
Expiratory flow limitation	Bronchodilators, secretion management, ↑ applied PEEP
Expiratory resistance	Upsize ET tube, ↑ sedation, paralytics

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Patient-Ventilator Dyssynchrony

• Dyssynchrony is a state in which the respiratory cycle of the patient does not always match that of the ventilator
• General signs of possible dyssynchrony: increased HR, increased RR, decreased $S_{\text{p}}\mathrm{O}{}_2$, increased expiratory muscle activity, coughing, agitation and visible inspiratory effort without triggering the ventilator

Phase	Type	Fix
Inspiration	Trigger Delay — extra delay between patient effort and ventilator response	↓ trigger pressure, ↓ sedation, correct electrolytes, bronchodilators, ↑ ETT size, correct auto-PEEP
Inspiration	Missed Trigger — insufficient effort to reach threshold	Same as above
Inspiration	Auto-Triggering — non-patient signals trigger a breath	↓ triggering sensitivity, address noise
Inspiration	Double Triggering — two breaths delivered for one effort	Usually consequence of premature termination; ↑ inspiratory time
Inspiration	Flow Dyssynchrony — flow rate set too low for patient demand (volume control)	↑ inspiratory flow rate, change flow pattern, change mode
Expiration	Delayed Termination — ventilator inspires longer than patient wants	↑ cycling threshold (earlier), change to time cycling
Expiration	Premature Termination — ventilator ceases before patient has finished inspiring	↓ cycling threshold (later), change to time cycling

Figure 11. Flow dyssynchrony. Note the concave or scalloped appearance of the inspiration waveform

• Delayed termination identified by a sharp spike in airway pressure at the end of inspiration

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Ventilator-Associated Pneumonia (VAP)

• Any pneumonia that occurs >48 hours after intubation

Figure 12. Mechanism of ventilator associated pneumonia