Basic Invasive Mechanical Ventilation

Benjamin D. Singer, MD; Thomas C. Corbridge, MD

Disclosures

South Med J. 2009;102(12):1238-1245. 

In This Article

Ventilator Basics

Once the trachea has been successfully intubated and proper endotracheal tube placement has been verified by clinical and radiographic means, ventilator settings must be selected.[2] The first parameter to be chosen is the ventilator mode.[3] The mode determines how the ventilator initiates a breath, how the breath is delivered, and when the breath is terminated. Despite the availability of several new modes of ventilator support, time-tested modes such as assist-control (AC), synchronized intermittent mandatory ventilation (SIMV), and pressure support ventilation (PSV) are the most commonly used and the focus of this review.

Assist-control

Assist-control is a commonly used mode of mechanical ventilation in medical intensive care units. A key concept in the AC mode is that the tidal volume (VT) of each delivered breath is the same, regardless of whether it was triggered by the patient or the ventilator. At the start of a cycle, the ventilator senses a patient's attempt at inhalation by detecting negative airway pressure or inspiratory flow. The pressure or flow threshold needed to trigger a breath is generally set by the respiratory therapist and is termed the trigger sensitivity.[4] If the patient does not initiate a breath before a requisite period of time determined by the set respiratory rate (RR), the ventilator will deliver the set VT. For example, if RR is set at 12 breaths per minute and the patient is not initiating breaths, the ventilator will deliver a breath every 5 seconds; this is called time-triggering. Similarly, if RR is 15 breaths per minute, the ventilator will deliver a breath every 4 seconds. However, if the patient initiates a breath, the ventilator in AC mode will deliver the set VT; these breaths are patient-triggered rather than time-triggered.

Regardless of whether the breath is patient-triggered or time-triggered, the exhalation valve closes and the ventilator generates inspiratory flow at a set rate and pattern. The patient is limited to that flow rate and pattern during inhalation. Flow may be either constant (square waveform) or decelerating (ramp waveform) (Fig. 1).[5] A square waveform is generally selected when inspiratory time is to be minimized thus allowing more time for exhalation (ie obstructive lung diseases). Ramp waveforms are useful for ventilating a heterogeneous lung, such as in the acute respiratory distress syndrome (ARDS). Often the flow rate and pattern are selected to maximize patient comfort and patient-ventilator synchrony. Inspiratory flow lasts until the set VT is delivered at which time the breath is cycled-off (and so the term volume-cycled mechanical ventilation).

Figure 1.

Flow-pressure waveforms. The left tracing represents a constant or square waveform. When flow is delivered at a constant rate, resistive pressure remains fairly constant (reflecting constant flow) while distending pressure increases with delivery of the tidal breath. In the tracing on the right, a decelerating or ramp waveform is shown. Since flow is decreasing, resistive pressure decreases as distending pressure increases. The net effect is an essentially constant pressure during the tidal breath.

Thus, the AC mode is patient- or time-triggered, flow-limited, and volume-cycled. An important correlate to this mode is that the airway pressures generated by chosen ventilator settings are determined by the compliance of the respiratory system and the resistance of the airways.

When the exhalation valve opens, the patient is allowed to exhale passively or actively until the airway pressure reaches end-expiratory pressure. This pressure is typically set slightly higher than atmospheric pressure to prevent atelectasis, decrease inspiratory work of breathing, or improve gas exchange depending on the clinical scenario. This positive end-expiratory pressure (PEEP) is generated by a resistor in the exhalation port of the ventilator (Fig. 2).[6]

Figure 2.

Assist-control (AC) mode. Flow, pressure, and volume tracings of three separate breaths are presented. The first two breaths are initiated by the patient (patient-triggered) via a drop in airway pressure (circled). The breath is delivered by constant flow (flow-limited), shown as a rapid increase in flow to a preset level. Flow lasts until a preset tidal volume (VT) is reached (volume-cycled). The exhalation port of the ventilator then opens and the patient passively or actively exhales. In the third breath, the preset backup time limit is met (the patient did not initiate a breath) and the ventilator delivers the breath (time-triggered). Note that patient-triggered and time-triggered breaths deliver the same inspiratory flow and tidal volume in the assist-control mode.

AC mode has several advantages including low work of breathing, as every breath is supported and tidal volume is guaranteed.[7,8] However, there is concern that tachypnea could lead to hyperventilation and respiratory alkalosis. Breath stacking can occur when the patient initiates a second breath before exhaling the first. The results are high volumes and pressures in the system. Hyperventilation and breath stacking can usually be overcome by choosing optimal ventilator settings and appropriate sedation.

Synchronized Intermittent Mandatory Ventilation

Synchronized intermittent mandatory ventilation (SIMV) is another commonly used mode of mechanical ventilation (Fig. 3).[9,10] Like AC, SIMV delivers a minimum number of fully assisted breaths per minute that are synchronized with the patient's respiratory effort. These breaths are patient- or time-triggered, flow-limited, and volume-cycled. However, any breaths taken between volume-cycled breaths are not assisted; the volumes of these breaths are determined by the patient's strength, effort, and lung mechanics.[11] A key concept is that ventilator-assisted breaths are different than spontaneous breaths. Another important concept is that AC and SIMV are identical modes in patients who are not spontaneously breathing due to heavy sedation or paralysis. High respiratory rates on SIMV allow little time for spontaneous breathing (a strategy very similar to AC), whereas low respiratory rates allow for just the opposite.

Figure 3.

Synchronized intermittent mandatory ventilation (SIMV) mode. As in assist-control mode, mandatory breaths are patient-triggered, flow-limited, and volume-cycled. However, breaths taken between mandatory breaths (bracketed) are not supported. Rate, flow, and volume are determined by the patients strength, effort, and lung mechanics.

SIMV has been purported to allow the patient to exercise their respiratory musculature while on the ventilator by allowing spontaneous breaths and less ventilator support.[12,13] However, SIMV may increase work of breathing and cause respiratory muscle fatigue that may thwart weaning and extubation.

Pressure Support Ventilation

A common strategy is to combine SIMV with an additional ventilator mode known as pressure support ventilation (PSV) (Fig. 4).[14] In this situation, inspiratory pressure is added to spontaneous breaths to overcome the resistance of the endotracheal tube or to increase the volume of spontaneous breaths. PSV may also be used as a stand-alone mode to facilitate spontaneous breathing.

Figure 4.

Synchronized intermittent mandatory ventilation plus pressure support ventilation (SIMV + PSV) mode. The first and last breath tracings are identical to those seen in SIMV. However, during pressure-supported breaths (bracketed), the ventilator delivers a set inspiratory pressure which is terminated when the flow drops below a set threshold. Spontaneous breaths are patient-triggered, pressure-limited, and flow-cycled.

PSV mode is patient-triggered, pressure-limited, and flow-cycled. With this strategy, breaths are assisted by a set inspiratory pressure that is delivered until inspiratory flow drops below a set threshold. When added to SIMV, PSV is applied only to the spontaneous breaths taken between volume-guaranteed (volume-cycled) breaths. During PSV alone, all breaths are spontaneous. Airway pressures drop to the set level of PEEP during exhalation and rise by the amount of selected pressure support during inhalation. RR and VT are determined by the patient; there is no set RR or VT.

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