Chapter 36: Ventilator Management

INTRODUCTION

Ventilator management or mechanical ventilation management includes the indication of mechanical ventilation, choosing the ventilator mode, initiation of mechanical ventilation, prevention of complications, the assessment of adequacy of therapy, and readiness for liberation.

Normal inspiration is performed by the expansion of the thorax or chest cavity when the muscles of inspiration contract. Contraction of the diaphragm results in it descending and enlarging the vertical size of the thoracic cavity. The external intercostal muscles contract and raise the ribs slightly to increase the circumference of the thorax. The contraction of these muscles is the “work” required to inspire. The intrapleural pressure becomes more negative during inspiration in relation to atmospheric pressure. This negative intrapleural pressure goes from –5 cmH₂O at the end of expiration to –10 cmH₂O at the end of inspiration. The negative intrapleural pressure is transmitted to the alveolar space. The transpulmonary pressure or the gradient across the lung widens. As a result, the alveoli have a negative pressure during spontaneous inspiration. Pressure at the mouth is still atmospheric. Air flows from the mouth into the alveoli, and the alveoli expand. The volume of gas builds up in the alveoli, and the pressure returns to zero. The airflow stops. This is the end of inspiration, and no more gas moves into the lungs.

Normal exhalation is passive and does not require any work. The muscles relax, the diaphragm moves upward to its resting position, and the ribs return to their normal position. The volume of thoracic cavity decreases and air is forced out of the alveoli. The thoracic volume decreases to resting, and the intrapleural pressure returns to –5 cmH₂O. The pressure inside the alveolus increases during exhalation and becomes slightly positive (i.e., +5 cmH₂O). Pressure is lower at the mouth than inside the alveoli. The transairway pressure gradient causes air to move out of the lungs. Exhalation ends when the pressure in the alveoli and the mouth is equal.

This chapter covers the basics of ventilator management. Many Emergency Departments have ventilators ready to use (Figures 36-1 and 36-2). They are applied to the patients who require endotracheal intubation before they are admitted. Refer to a text for more detailed information regarding managing a ventilator.

ANATOMY AND PATHOPHYSIOLOGY

COMPLIANCE

Monitoring changes in compliance is a valuable means of assessing changes in the patient’s condition during mechanical ventilation.¹ Compliance usually is measured under conditions of no gas flow. It is referred to as a static compliance ...