Respiratory Failure & Mechanical Ventilation

Volume Support Mode of Ventilation:

It is essentially a pressure support mode but the pressure is adjusted to target a preset volume. If the patient is apneic, the mode will change to PRVC.

Control logic and feedback loop for pressure-regulated volume control

PRVC is based on the concept of adaptive control in which the ventilator automatically adjusts the pressure limit of a breath to meet an operator-set volume target over several breaths. PRVC is a control mode of ventilation with a dual control on the volume and pressure. All breaths are patient- or machine-triggered, volume-controlled with pressure regulation and time-cycled. The breaths delivered at preset tidal volume, minute volume and preset rate during preset inspiratory time. The ventilator automatically adjusts the inspiratory pressure control level to changes in the mechanical properties of the lung/thorax on a breath-by-breath basis. The pressure change is 2-3 cm H2O each time and the pressure dos does not exceed 5 cm H2O below the pressure alarm (limit) level set on the ventilator even if the targeted volume is not achieved, an alarm message is then displayed showing the target volume is not achieved.

The ventilator always uses…

Breaths in Pressure controlled CMV are patient-triggered, pressure targeted and time-cycled (assisted breath) or time-triggered (machine), pressure targeted and time-cycled (controlled breath). The operator will set inspiratory pressure (Pinsp), respiratory rate (f), FiO2, inspiratory time (Tinsp), PEEP, and Slope. The flow will be decelerating waveform. If the patient is not breathing, all breaths will be controlled, and the trigger timer is set based on the set rate (60 sec/rate). Once the patient starts to breath and reaches the sensitivity level, the breath will be assisted with the set inspiratory pressure and terminated after the set inspiratory time is elapsed. The set rate will function then as a backup rate, if the trigger timer is reached and the patient did not initiate a breath, the machine will deliver a mandatory breath.

Breaths in volume-controlled CMV are patient-triggered, volume targeted and time-cycled (assisted breath) or time-triggered (machine), volume targeted and time-cycled (controlled breath). The operator will set tidal volume, flow rate, respiratory rate (f), FiO2, inspiratory time (Tinsp), PEEP, and Slope. If the patient is not breathing, all breaths will be controlled, and the trigger timer is set based on the set rate (60 sec/rate).

Once the patient starts to breath and reaches the sensitivity level, the breath will be assisted with the set tidal volume and terminated after the set inspiratory time is elapsed. The set rate will function then as a backup rate, if the trigger timer is reached and the patient did not initiate a breath, the machine will deliver a mandatory breath.

Intermittent mandatory ventilation (IMV) was an earlier version of the more advanced SIMV. In this mode of ventilation, a preset respiratory rate is delivered at a specified time interval. For a patient receiving 10 breaths per minute, a breath is delivered every six seconds regardless of the patient's efforts. The theoretical disadvantage of this form of ventilation is that the patient may take a spontaneous breath and could receive a machine delivered breath at the same time or during expiration, causing hyperinflation and high peak airway pressures. SIMV is said to avoid this problem by monitoring the patient's respiratory efforts and delivering breaths in response to the patient's inspiratory efforts. The patient can breathe spontaneously in between the mandatory breaths and those breaths can be pressure supported.

SIMV is similar to IMV in that it will still deliver a minimum number of breaths, despite the potential lack of inspiratory effort…

Pressure support is only applied to spontaneous breaths, the trigger of the breath could be either pressure or flow (sensitivity). In pressure support ventilation, all the breaths are supported breath type and are initiated by the patient. Once the breath is triggered, the ventilator will deliver the pressure support at the limit of the set level above the CPAP/PEEP and the breath will be cycled off when the patient's inspiratory flow declines to a value determined by the clinician (for example, 25% of peak inspiratory flow). In PSV the volume and the flow are both variable and determined by the resistance, compliance, inspiratory effort, and level of pressure support; in addition, the inspiratory time is variable as well.

Pressure support ventilation is a pressure preset mode in which each breath is patient’s triggered, and machines supported. It provides a means of a positive pressure that is synchronized with the inspiratory…

Continuous positive airway pressure (CPAP) is the use of continuous positive pressure to maintain a continuous level of positive airway pressure in a spontaneously breathing patient. It is functionally similar to positive end-expiratory pressure (PEEP), except that PEEP is an applied pressure against exhalation and CPAP is a pressure applied by a constant flow. The ventilator does not cycle during CPAP, no additional pressure above the level of CPAP is provided, and patients must initiate all of their breaths above the level of CPAP (Figure 9). The breath types are all spontaneous with a sinusoidal flow waveform.