1 Why might a patient require mechanical ventilation?

There are three conditions for which mechanical ventilation (MV) may be required:

The decision to provide MV should be based on clinical examination and assessment of gas exchange by arterial blood gas analysis. This decision must be individualized because arbitrary cutoff values for PO₂, PCO₂, or pH as indicators of respiratory failure may not be germane to all patients. Physiologic derangements necessitating the need for MV include primary parenchymal disorders such as pneumonia, pulmonary edema, or pulmonary contusion or systemic disease that indirectly compromises pulmonary function such as sepsis or central nervous system dysfunction. The principal goal of MV in the setting of respiratory failure is to minimize the potential for ventilator-induced injury and support gas exchange while the underlying disease process is reversed.

2 Which is the most common mode of ventilation, volume or pressure control?

From a classification standpoint neither is a true mode of ventilation. To be precise, conventional modes of MV control either volume or pressure. The mode of ventilation is a function of the types of breaths delivered (e.g., either mandatory and/or spontaneous breaths) and how the timing of each breath is determined. Volume control provides breaths that are volume constant and pressure variable. Pressure control provides breaths that are pressure constant and volume variable. It is the response of the ventilator to the patient’s effort that determines the mode.

3 What are the most commonly used modes of positive-pressure ventilation?

Currently there are nine modes that are based on using either volume or pressure as the control variable:

VC-CMV, volume control–continuous mandatory ventilation

VC-A-C, volume control–assist control (ventilation)

VC-IMV or VC- SIMV, volume-control intermittent or synchronized intermittent mandatory ventilation

PC-CMV, pressure control–continuous mandatory ventilation

PC-A-C, pressure control–assist control (ventilation)

PC-IMV or PC-SIMV, pressure control–intermittent mandatory ventilation or pressure control–synchronized intermittent ventilation

PSV, pressure support ventilation

4 Does PC-CMV permit the patient to interact with the ventilator?

Use of a term such as PC-CMV frequently leads to confusion to the uninitiated. The term controlled MV implies that the patient is receiving a neuromuscular blocking agent and is prevented or locked out from triggering the ventilator. Therefore, when speaking of volume control or pressure control, more descriptive terms such as volume-targeted or pressure-targeted ventilation may minimize any misunderstanding.

5 How does VC-A-C mode work?

The VC-A-C mode delivers a set minimum number of mandatory breaths and also allows the patient to trigger (or assist) additional breaths. Each breath (mandatory or assisted) is associated with a preset flow rate to deliver a preset tidal volume (V_t). AC-A-C may cause respiratory alkalosis more often and may promote auto-positive end-expiratory pressure (PEEP) because the patient receives a full set V_t with every breath, even when tachypneic.

6 Do VC-A-C and VC-SIMV differ?

Both modes deliver mandatory breaths at a preset frequency, a preset V_T, and a preset inspiratory gas flow rate. Between machine-initiated breaths in the VC-A-C mode, the patient can trigger the ventilator and receive an assisted breath at the set V_t. In the VC-SIMV mode the ventilator generates a timing window based on the set frequency of the mandatory breaths, and attempts to synchronize the delivery of the breath in concert with the patient’s spontaneous effort. In between the mandatory breaths, while in the SIMV mode, the patient is free to initiate a totally spontaneous effort and generate a V_t compatible with the patient’s inspiratory muscular effort. Flow rates of gas will vary during these spontaneous breaths and will be based on the patient’s efforts and demands.

7 When initiating mechanical ventilation, how do you decide on VC-A-C over VC-SIMV?

If the intent of MV is to provide full ventilator support, provision of a sufficient level of alveolar ventilation is obligatory. The preference of VC-A-C over VC-SIMV, assuming that minute ventilation demands (set frequency and set V_t) are being met, is really a point of style or preference. However, once the patient has achieved cardiopulmonary stability and the goal is now to initiate partial ventilator support, the options for VC-A-C vs. VC-SIMV are very different. A patient on a set respiratory frequency of 4 breaths/min in the VC-A-C mode with a total respiratory rate of 18 breaths/min is still receiving full ventilator support. In contrast, in the VC-SIMV mode a patient with the same set frequency is considered to be in a partial mode of support and must be capable of supporting the bulk of his or her own gas exchange demands.

8 What other variables are associated with conventional modes of MV?

Simplistically ventilators are classified as either a pressure, volume, or flow controller. Current intensive care unit (ICU) ventilators can mix pressure control with flow control, all within a single breath. Next there are the phase variables, events that take place during a ventilator cycle. These phase variables control how the breath is

Triggered (i.e., patient or machine)

Limited (i.e., pressure or volume)

Cycled from inspiration to exhalation (i.e., pressure, volume, flow, or time)

9 What is pressure support ventilation?

PSV augments a patient’s spontaneous inspiratory effort with a clinician-selected level of positive airway pressure. Inspiration is completed when the patient’s spontaneously generated peak flow rate decreases below a minimum level or a percentage of the initial inspiratory flow, typically less than 25%. Therefore PSV is a purely patient-triggered, pressure-limited, and flow-cycled mode of ventilation. PSV allows patients to establish their own respiratory rate (RR), vary their peak flow rate based on breath–to-breath demands, and enhance delivered V_t. Taken together the RR, peak flow, and V_t determine the inspiratory to expiratory (I:E) ratio. It is much more physiologic and comfortable for the patient to control the I:E ratio than to impose a fixed I:E ratio that will not respond favorably to increased patient demands.

10 How does pressure control ventilation differ from pressure support ventilation?

PCV, unlike PSV, is a pressure-limited, time-cycled mode of ventilation. The V_t generated results from the clinician-determined inspiratory time and the applied airway pressure and are predominantly influenced by flow resistance and respiratory system compliance. Practically speaking, PCV is used as a mode of ventilation when full ventilatory support is necessary, whereas PSV is optimal for providing partial ventilatory support.

11 What are trigger variables?

All modern ICU ventilators constantly measure one or more of the phase variables (i.e., pressure, volume, flow, or time) (Table 21-1). Inspiration occurs when one of these variables reaches a preset value. Clinically this is referred to as triggering the ventilator. The following conditions are necessary to initiate a breath under each individual variable:

Pressure triggering: requires patient-initiated effort to decrease circuit pressure below a preset value (e.g., −2 cm H₂O below baseline end-expiratory pressure is common).

Flow or volume triggering: again requires patient effort that results in a drop in the flow rate or volume of gas that is continually present within the circuit.

Time triggering: does not require patient effort but occurs when the set respiratory rate on the ventilator becomes due.

TABLE 21-1 Breath Types for Volume and Pressure Modes of Ventilator Operation

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