Modes of Positive-Pressure Ventilation

Modes of PPV are described by the relationships among the various types of breaths and by the variables that can occur during the inspiratory phase of ventilation. Each mode of ventilation is distinguished by how it initiates a breath (trigger), how it sustains a breath (limit), and how it terminates a breath (cycle), which are referred to as "phase variables" ( Fig. 75-6 ). Modes of PPV can also be distinguished by whether the patient's breath is mandatory, spontaneous, or a combination of the two. The names given the various modes of ventilation do not describe their function but rather appear to be ploys to establish uniqueness for marketing. The variety of names has led to confusion about how the newer modes of ventilation differ from the conventional modes utilized in the past.

There are two basic types of goals for the modes of ventilation: ventilation limited by a pressure target and ventilation limited to the delivery of a specified volume. Formerly, mechanical ventilators could control only one

Figure 75-6 Criteria for determining the phase variables during a positive-pressure breath on a mechanical ventilator. (Adapted from Tobin MJ: Principles and Practice of Mechanical Ventilation. New York, McGraw-Hill, 1994.)

Pressure-Targeted Versus Volume-Targeted Modes of Ventilation

There are distinct differences, advantages, and disadvantages in pressure- and volume-targeted strategies ( Table 75-6 ). Pressure-targeted modes of ventilation allow the clinician to control the peak inspiratory pressure (PIP) and the inspiratory time, or I:E ratio. Flow is delivered in a decelerating fashion and varies from breath to breath. The initial peak flow is rapidly reached at the beginning of the breath and then decreases throughout inspiration, maintaining the peak pressure until a preset inspiratory time is met. Figure 75-7 illustrates the important control variables when applying a pressure-targeted ventilation strategy. It is hypothesized that pressure-targeted ventilation allows a more even distribution of ventilation in the lung while using the variable (decelerating) flow profile, and that there is better maintenance of mean airway pressure.^[75] It has also been suggested that patients are more comfortable breathing spontaneously while on pressure-targeted ventilation. This may be due in part to the constant adaptation of peak flows and to the rate of deceleration that occurs between breaths. Because tidal volume is a dependent variable, inconsistent alveolar minute ventilation can occur. When using pressure-targeted ventilation, the clinician must be aware that the tidal volume delivered depends on changes in lung and chest wall compliance and airway resistance.

In volume-targeted modes of ventilation, the controlled variables are tidal volume, which is a function of

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Figure 75-7 Control variables and their interconnection for modes of pressure-targeted ventilation. (Adapted from Chatburn RL, Lough MD: Mechanical ventilation. In Lough MD, Doershuk CF, Stern RC [eds]: Pediatric Respiratory Therapy. Chicago, Year Book, 1985, pp 148–191.)

Pressure is the dependent variable in the modes of ventilation in which volume is the target. Because pressures will vary in volume-targeted modes of ventilation, careful monitoring and assessment of respiratory system compliance and resistance is necessary.

Specific Modes of Ventilation

Today, continuous mandatory ventilation (CMV) is synonymous with assist/control ventilation (ACV), in which patients are allowed to "trigger" the ventilator to receive an assisted breath from the device. Other synonymous terms include continuous mechanical ventilation, controlled mandatory ventilation, and controlled mechanical ventilation. In this mode every breath is "mandatory" and is delivered at a frequency, volume, or pressure and at an inspiratory time set by the clinician. In between machine-initiated breaths, the patient can initiate and receive a mandatory breath set on the ventilator. This is considered a full-support mode. Other parameters that can be set by the clinician on PPV include peak inspiratory flow (as opposed to inspiratory time), flow waveform, and I:E ratio ( Fig. 75-9 ).

ACV is the most commonly used mode of mechanical ventilation in the world,^[76] although many institutions

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Figure 75-8 Interrelation between variables used to ensure constant minute ventilation in a volume-targeted ventilator. (Adapted from Chatburn RL, Lough MD: Mechanical ventilation. In Lough MD, Doershuk CF, Stern RC [eds.]: Pediatric Respiratory Therapy. Chicago, Year Book, 1985, pp 148–191.)

A common problem associated with ACV is respiratory alkalosis in patients breathing at high respiratory rates. Alkalemia shifts the oxyhemoglobin curve to the left, making oxygen delivery to the tissues more difficult. Alkalemia also lowers the seizure threshold^[77] and has been associated with cardiac arrhythmias. Other potential deleterious affects of this mode of ventilation include a hemodynamically significant decrease in venous return and in cardiac output. All modes of PPV can decrease venous return; however, mean airway pressures can be higher in ACV, so there may be an increased incidence of this complication with this mode.

In patients with acute lung injury, it has been shown that tidal volumes should be set in the range of 4 to 6 mL/kg patient body weight to protect the lung against ventilator-induced injury.^[78] Because the tidal volume is fixed at less than a patient's usual demand, the patient attempts to take larger and more frequent breaths, leading to dyssynchrony. These patients often require large quantities of sedation and analgesics in an effort to improve patient-ventilator interaction. (The use of low tidal volumes in patients with acute lung injury is discussed in greater detail in Chapter 74 .).

Figure 75-9 Airway pressure tracings of the four standard volume-preset modes. Thick solid lines represent ventilator breaths; thick dotted lines represent spontaneous breaths; and thin dotted lines refer to what the spontaneous pattern would have been without the ventilator breaths. IMV, intermittent mandatory ventilation; SIMV, synchronized IMV.

Intermittent mandatory ventilation (IMV), also referred to as "intermittent demand ventilation," is a partial-support mode. The patient receives mandatory (machine) breaths at a set frequency and volume or a set pressure and inspiratory time. Between mandatory breaths, the patient can breathe spontaneously from either a demand flow or a continuous flow system. The original version of the IMV mode is now considered obsolete. Most modern ventilators operate in an SIMV mode. In this updated version of IMV, the machine creates timing windows around the scheduled mandatory breaths in order to synchronize each machine's breath with the patient's inspiratory effort, which might vary the machine cycle times slightly. If no inspiratory effort is detected within the time window, the machine delivers a mandatory breath at the scheduled time (see Fig. 75-9 ).

IMV was first used in adult patients as a means of discontinuing ventilatory support. Although IMV improves patient/ventilator interaction at low (rate) levels, patients can expend an unanticipated amount of energy, which may contribute to failure in weaning.^[79]

Pressure-support ventilation (PSV) is a "spontaneous" mode of breathing in which the patient's inspiratory

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PSV allows the patient to control his or her respiratory rate, inspiratory time, and inspiratory flow rate. The tidal volume achieved is a function of the respiratory system compliance and resistance. Several factors may influence the effects of pressure-support ventilation, including the level of inspiratory pressure support and the pressure rise time. PSV has become a widely used mode during the weaning of patients who require prolonged durations of mechanical ventilation and has become popular during NIPPV. Bi-level positive airway pressure (BiPAP) is nearly identical to PSV, but the terminology used to describe the settings are different and it is commonly used to deliver NIPPV on a small machine designed just for that mode of ventilation, a BiPAP machine (discussed later).

CPAP is the mode of conventional ventilation that offers the least amount of support. Like PSV, it is classified as a spontaneous breathing mode and is used as a noninvasive interface. CPAP has a set level of pressure that is maintained throughout the respiratory cycle during spontaneous breathing. It has been used synonymously with PEEP, expiratory positive airway pressure (EPAP), and continuous positive-pressure breathing (CPPB). This mode is typically used to assess extubation readiness in an intubated patient.

Dual-Control Modes

Modes that combine the positive attributes of volume- and pressure-targeted strategies are designed for use in patients with disease processes in which pulmonary mechanics vary and/or in which ventilator dyssynchrony occurs. These devices, popularly referred to as dual-control modes, do not control both parameters (pressure and volume) simultaneously; rather, the modes switch from one to the other, based on a measured input variable. The device operates as a timed-cycle, pressure-limited ventilator using a clinician-selected tidal volume as an input variable to automatically adjust the pressure limit, based on changes in respiratory system compliance and/or resistance.

There are currently two techniques for providing dual-control PPV: those that switch the control variable "breath to breath" and those that switch the control parameter "within the breath." Table 75-7 lists some specific names of dual-control devices.

Pressure-regulated volume control (PRVC) is an example of a dual-control mode that switches the control parameter "breath to breath." In this mode, the clinician sets a desired tidal volume, respiratory rate, and maximum pressure limit, using the high inspiratory pressure alarm setting. The ventilator then delivers a manufacturer-determined number of "test breaths" to estimate the total respiratory system compliance. Following this test phase, the ventilator selects the pressure limit to deliver the

TABLE 75-7 -- Partial list of available dual-control modes of positive-pressure ventilation

Adaptive Pressure Ventilation (APV)	Volume-Assured Pressure-Support Ventilation (VAPSV)
Auto-Flow	Volume Control Plus (VC+)
Pressure Augmentation	Volume Support
Pressure-Regulated Volume Control (PRVC)
Variable Pressure Control

designated tidal volume. The subsequent breaths will adjust the pressure limit up or down, depending on the relationship of the peak inspiratory pressures, the exhaled tidal volumes, and the peak inspiratory pressure limit. ( Fig. 75-10 ). The volume-support mode is a spontaneous

Figure 75-10 Characteristic pressure volume and flow waveforms for dual-control modes that switch the control variable "within" the breaths. A, The set tidal volume has been reached before flow has decelerated to the set flow limit; therefore, the breath continues in pressure control until the flow cycle threshold is achieved. B, The switch from pressure control to volume control occurred because flow decelerated to the set flow limit before the set tidal volume was met. This result could be caused by a decrease in patient effort or by a change in respiratory system compliance relative to the set pressure limit. (Adapted from Wilkins RL, Stoller JK, Scanlan CL: Fundamentals of Respiratory Care. St Louis, Mosby, 2003.)

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Volume-assured pressure-support ventilation (VAPSV) is an example of a dual-control mode of ventilation that switches the control parameter "within the breath." This mode was initially described by Amato and colleagues^[80] as an alternative to conventional volume-target ACV.

As the name implies, this mode is very similar to pressure support in its function, however, this mode uses two flow sources that are activated simultaneously when the patient initiates a breath. The first source provides a constant flow pattern set by the clinician. The second source provides flow to reach a preset pressure limit set by the pressure-support control (demand flow). It is the second flow source that rapidly delivers gas into the circuit until the preset pressure limit is reached. This flow then tapers off slowly, maintaining constant pressure. If the delivered tidal volume is greater than the preset minimum tidal volume, the breath terminates like a pressure-supported breath would during an act of spontaneous breathing. If the delivered tidal volume is less than the preset minimum tidal volume, the set (constant) flow from the first source continues the breath until the tidal volume is delivered ( Fig. 75-11 ). Note that although a minimum tidal volume is guaranteed with this mode, the tidal volume target may be exceeded, depending on the patient's demand.