What are the effects of mechanical ventilation?

ARTICLE REVIEW

ARDS associated haemodynamic instability is a major driver of mortality. Shock occurs due to pulmonary hypertension, impact of positive pressure ventilation on right ventricular (RV) function, and commonly septic shock. 

The first half of this review by Veillard-Baron et al. highlights the physiological impact of mechanical ventilation in ARDS on haemodynamic status. The second half describes key steps in haemodynamic management.

Key features:

Fluid management

1.    Determination of tissue perfusion:

• Ultrasound evaluation or inferior vena cave dimension
• Pulse pressure variations
• CVP monitoring in response to intervention

2.    Cautious fluid resuscitation during shock to avoid RV failure.

3.    Fluid conservative protocol as per ‘FACTT Lite’ during the absence or after resolution of shock.

Examination for acute cor pulmonale and RV support

1.    RV failure is independently associated with mortality in ARDS.

2.    ACP is reported in 20-25% of ARDS patients.

3.    Once fluid repletes, further fluid expansion is useless and possibly harmful.

4.    Norepinephrine can improve RV perfusion and function especially in the context of increased RV wall stress.

5.    More data is required for the use of levosimendan.

6.    NO and inhaled prostacyclin have comparable effects on oxygenation, but reduce pulmonary vascular resistance and improve ventilation/perfusions matching.

Factors associated with RV failure in the ARDS patient

1.    Causes of RV overload in ARDS:

a.    Lung derecruitment

b.    Lung overdistension

c.    Hypoxic and hypercapnic pulmonary vasoconstriction

2.    Risk factors for RV failure in ARDS:

a.    Pneumonia as a cause.

b.    PF ratio ≤ 150mmHg

c.    Driving pressure ≥ 18cmH2O

d.    PaCO2 ≥ 48mmHg

3.    All four risk factors lead to a >60% risk whereas absence of all presents a <10% risk of RV failure.

Respiratory manoevres to reduce RV wall stress

1.    PEEP optimisation to improve oxygenation whilst preventing PEEP-induced lung overdistension and subsequent impairment of pulmonary circulation.

2.    Avoid respiratory modalities that promote high mean airway pressures and lung overdistension e.g. HFOV, vigorous spontaneous ventilation, asynchronous ventilation and pendelluft.

3.    Prone ventilation produces more uniform ventilation and unloads the RV, even restoring RV function in those with documented RV failure. Still only recommended in severe ARDS.

Extracorporeal support

1.    vvECMO improves blood oxygenation thereby reducing pulmonary hypertension and RV wall stress.

2.    ECCO2R facilitates lung protective ventilation whilst avoiding severe hypercapnea, but has yet to be shown effective in clinical trials.

3.    Haemodynamic compromise on vvECMO should be treated with assessment of cardiac function using echocardiography and supplemented with vasopressors and inotropes as appropriate.

4.    va-ECMO implementation should be considered in the presence of refractory cardiogenic shock.

Take home message

Overall, this review highlights the pathophysiological basis of cardiorespiratory interactions that occur during ARDS. International experts offer an evidence-based management strategy to optimise pulmonary blood flow and haemodynamic status.

Article review was submitted by Brijesh Patel on behalf of the ESICM NEXT Committee.

Reference

Vieillard-Baron, M. Matthay, J. L. Teboul, T. Bein, M. Schultz, S. Magder, and J. J. Marini. Experts’ opinion on management of haemodynamics in ARDS patients: Focus on the effects of mechanical ventilation. Intensive Care Medicine. Review
Volume 42, Issue 5 / May, 2016; Pages 739 – 749