• OXYGENATION and VENTILATION are the two main components of mechanical ventilation. Oxygenation is the process of oxygen intake. Ventilation is getting rid of carbon dioxide.
• Small circuit leaks will have little effect on minute ventilation when using a pressure-cycled ventilator.
  • Small leaks will not cause a change in tidal volume and/or minute ventilation with pressure-cycled ventilators because cycling will be delayed until the pressure limit is met. Hanging-bellows ventilators are no longer approved for use in the anesthesia circuit.
• Venous return can be decreased by INTERMITTENT POSITIVE PRESSURE VENTILATION or POSITIVE END-EXPIRATORY PRESSURE. IPPV increases the intrathoracic pressure on inhalation and PEEP during exhalation.
• ASCENDING BELLOWS refers to the bellows’ ascending movement on exhalation. If the circuit becomes disconnected or if a significant leak exists, the bellows would not fill properly and would cue the CRNA that an issue needs to be resolved.
• DESCENDING BELLOWS refers to the bellows’ descending movement on exhalation. In this ventilator type, the bellows would be able to fill even with disconnection/air leak and would inherently be more difficult to ascertain a problem.

MODES OF VENTILATION

VOLUME-CONTROL VENTILATION

• The fixed parameter is volume. Peak airway pressure is directly related to airway resistance and inversely related to lung compliance.

PRESSURE-CONTROL VENTILATION

• The fixed component is pressure. In this mode, the flow matches the set peak airway pressure. Tidal volume is directly proportional to lung compliance and inversely to airway resistance.

PRESSURE-CONTROL VOLUM GUARANTEE VENTILATION

• This is a recent development in GE- Datex- Ohmeda ventilators; autoflow in Drager ventilators. This mode allows for change in inspiratory pressure based upon the respiratory system’s compliance.

ASSISTED AND SUPPORTED MODES

• These modes are most often used for critically-ill patients; they allow the work of breathing to be shared between the patient and ventilator. These modes help to decrease coughing, bucking, and straining because the patient-ventilator dyssynchrony is limited.

ASSIST-CONTROL VENTILATION

• The ventilator is setup to assist a patient-assisted breath by delivering a predetermined volume. The ventilator provides a backup rate; meaning, as long as the patient breathes more than the control rate, a controlled breath is not delivered.

PROPORTIONAL ASSISTED VENTILATION

• The volume given by the ventilator is adjusted according to the patient’s effort. A bigger inspiratory effort from the patient means the volume delivered will be greater. More like volume-controlled ventilation in assisted mode.

PRESSURE SUPPORT VENTILATION

• This assisted mode helps the patient’s inspiratory effort reach a pressure that is set by the clinician.

INTERMITTENT MANDATORY VENTILATION

• The mandatory ventilator breaths are set by either volume or pressure at a specific rate and inspiratory time. A set amount of breaths is delivered to the patient while allowing the patient to breathe independently. There is a risk of barotrauma.

SYNCHRONIZED INTERMITTENT MANDATORY VENTILATION

• A modification to IMV where the breathing cycle is monitored in a set time interval. Mandated breaths are delivered as long as the patient has not attempted breathing.

SPONTANEOUS BREATHING WITHOUT SUPPORT

• As the name suggests, the patient is breathing without support from the ventilator. This is also called continuous positive airway pressure (CPAP). It is a weaning mode.