Volume-controlled ventilators

Figure 18.2 shows the flow and pressure waveforms for volume-controlled ventilation. In this illustration, the inspiratory flow waveform is chosen to be a half sine wave. In Figure 18.2a, f is the inspiration duration, t the exhalation period, and Q the amplitude of inspiratory flow. The ventilator delivers a tidal volume equal to the area under the flow waveform in Figure 18.2a at regular intervals (t + Q set by the therapist. The resulting pressure waveform is shown in Figure 18.2b. It is noted that during volume-controlled ventilation, the ventilator attempts to deliver the desired volume of breath, irrespective of the patient s respiratory mechanics. However, the resulting pressure waveform, such as the one shown in Figure 18.2b, will be different depending on the patient s respiratory mechanics. Of course, for safety...

FIGURE 18.2 (a) Inspiratory flow for a mandatory volume-controlled ventilation breath and (b) airway pressure resulting from the breath delivery with a nonzero PEEP. 

Figure 18.3 shows a plot of the pressure and flow during a mandatory pressure-controlled ventilation. In this case, the respirator raises the airway pressure and maintains it at the desired level, Pj, which is set by the therapist, independent of the patient s respiratory mechanics. Although the ventilator maintains the same pressure trajectory for patients with different respiratory mechanics, the resulting flow trajectory, shown in Figure 18.3b, will depend on the respiratory mechanics of each patient. As in the case of mandatory volume-controlled ventilation, the total volume of delivered breaths is monitored to ensure that patients receive adequate ventilation. 

The therapist entry for pressure-controlled ventilation is shown in Figure 18.8 (lower left-hand side). In contrast to the volume-controlled ventilation, where Qj(t) was computed directly from operators entry (Equations 18.1 through 18.3), the total desired flow is generated by the closed-loop airway pressure controller shown in Figure 18.8. This controller uses the therapist-selected inspiratory pressure, respiration rate, and the 1 E ratio to compute the desired inspiratory pressure trajectory. The trajectory serves as the controller reference input. The controller then computes the flow necessary to make the actual airway pressure track the reference input. Assuming a proportional-plus-integral controller, the governing equations are... 

Volume-controlled ventilation A mandatory mode of ventilation where the volume of each breath is set by the therapist and the ventilator delivers that volume to the patient independent of the patient s respiratory mechanics. 

Volume-controlled ventilation—the patient receives a specific volume of gas delivered to the lungs at set time intervals with pressure limit. 

More rare today are volume controllers. The distinguishing feature of a volume controller is that it measures directly the volume it delivers. Therefore the only ventilators that qualify as true volume controllers are those whose drive mechanisms allow the direct measure of volume. These machines measure volume change as the displacement of a piston, bellows, or similar mechanism. The few volume-controlled ventilators still available in the market are listed in Table 3. To our knowledge, there are no new volume-controlled ventilators being manufactured. 

Other Applications Very small, very low-flow, and relatively high-velocity exhaust inlets, similar to LVHV nozzles, have been used successfully to control fumes from electric soldering irons." " Some investigations have been made into small, point-control exhaust ventilation for aerosols generated by high-speed dental tools. However, such low-volume point-control ventilation systems have not seen widespread use. 

In thermal models, the ventilation airflow rates normally arc input parameters, to be defined by the user or to be calculated by the program on the basis of a nominal air exchange or flow rate) and some control parameters (demand-controlled ventilation, variable air volume flow ventilation systems), in airflow models, on the other hand, room air temperatures must be defined in the input (see Fig. 11.49). 

Halothane (Fluothane) depresses respiratory function, leading to decreased tidal volume and an increased rate of ventilation. Since the increased rate does not adequately compensate for the decrease in tidal volume, minute ventilation will be reduced plasma PaCOz rises, and hypoxic drive is depressed. With surgical anesthesia, spontaneous ventilation is inadequate, and the patient s ventilation must be controlled. 

Controlling ventilation rates to limit volumes of combustible mixtures present in a facility after a fuel release. 

Owing to the need for monitoring both pressure and volume, in more recent years, new modes that combine several aspects of the volume- and pressure-controlled ventilation are devised. These modes are generally referred to as dual-control modes. Although these modes are relatively new and not all of their clinical outcomes are known, they utihze more of the power and flexibility that new ventilator hardware and software offer (Lellouche and Brochard, 2009). Two dual-control modes are described below. 

This new mode is a form of pressure-controlled ventilation that simultaneously keeps track of the delivered tidal volume. Figure 18.4 shows characteristic changes of pressure, volume, and flow of inspiratory flow in this mode. As shown in the top panel of Figure 18.4, for each breath, (a) through (e), the ventilator controls the inspiratory pressure to a level that may vary from breath to breath. Specifically, the ventilator controls the pressure, but also monitors the delivered tidal volume and compares it with the desired tidal volume. If the actual delivered tidal volume matches the desired level, such as in (a), then the level of controlled pressure for the next breath will be the same. However, if the next breath produced a larger than desired tidal volume, such as in (b), then the controlled pressure will be reduced in the next breath (c). Similarly, if the tidal volume falls short, such as in (d), then the controlled pressure in the next breath, (e), will be raised to... 

FIGURE 18.4 Patterns respiratory desired pressure and resulting volume and flow for adaptive pressvue control ventilation mode. Breath illustration (a) through (e) shows how the applied inspiratory pressure is automatically adjusted to achieve the desired tidal volume. 

In a microprocessor-based ventilator, the mechanisms for delivering mandatory volume- and pressure-controlled ventilation have mostly common components. The primary difference lies in the control algorithms governing the delivery of breaths to the patient. 

In a microprocessor-controlled ventilator (Figure 18.8), the electronically actuated valves open from a closed position to allow the flow of blended gases to the patient. The control of flow through each valve depends on the therapist s specification for the mandatory breath. That is, the clinician must specify the following parameters for the delivery of volume-controlled mandatory ventilation breaths (1) respiration rate (2) flow waveform (3) tidal volume (4) oxygen concentration (of the delivered breath) (5) peak flow and (6) PEEP, as shown in the lower left side of Figure 18.8. It is noted that the PEEP selected by the therapist in the mandatory mode is only used for the control of exhalation flow this... 

Pressure-controlled ventilation—a constant pressure applied to the airway and the lungs fill according to their compliance and the set pressure (volume = compliance x pressure). 

Pressure-controlled ventilation with volume guarantee—a set pressure is delivered to the breathing circuit, but that pressure is altered by the ventilator until the set tidal volume is achieved. 

The reactor auxiliary systems are similar to those found on other PWRs and typically include a primary water volume control and inventory system, a primary water purification system, radioactive liquid and gaseous effluent treatment systems, and a ventilation system. At low power levels, many of these systems may be required only on an intermittent basis and would be valved out during periods of autonomous operation. 

Some ventilators offer a volume guarantee feedback option (pressure-regulated volume control and volume assist). 

Abbreviations VC, volume control VA, volume assist PC, pressure control PA, pressure assist PS, pressure support PR, pressure release Sp, spontaneous unassisted SIMV, synchronized intermittent mandatory ventilation. 

Regardless of whether a ventilator uses positive pressure or negative pressure, the trans-pulmonary pressure gradient determines the tidal volume. A ventilator that is a pressure controller delivers a preset pressure and this variable is unaffected by changes in limg compliance or resistance. A positive pressure ventilator applies pressure inside the chest to expand it using a noninvasive interface, or an artificial airway. 

If flow is measured and used to deliver a preset volume, then a ventilator is considered to be a flow controller (1). In most of the cases, volume preset ventilation is provided by ventilators that actually measure flow and use flow over time to deliver a preset volume. These machines maintain an approximately constant volume in the face of varying lung mechanics. The most common flow-controlled ventilators are listed in Table 2. As indicated in the table, most of these machines can also provide pressure ventilation. 

In the first condition inspiratory time can be increased by augmenting the tidal volume and decreasing inspiratoiy flow in the volume-controlled modes of mechanical ventilation (17). In pressure-support mode, inspiratoiy time can be prolonged by the delay in reaching the target inspiratoiy flow as a consequence of the intentional air leak (19). 

Bi-level NIV may be used as a first-line treatment, with supplemental oxygen (27). Expiratory airway pressure is titrated to control h5q)opneas and apneas, and inspiratory airway pressure is added to control Paco2. If bi-level NIV fails, nasal volume ventilation may be used (29). In many patients with OHS and predominant OSA, once hypercapnia has improved (which may take several weeks) nCPAP may be used (29). Thirteen obese patients (n = 13) with a BMI > 35, aged 28-69 years with severe OSAS and hypercapnia (8.2 0.3 kPa) and failing to respond to initial CPAP therapy, were treated via a nasal nocturnal volume-cycled ventilator, which was tolerated by all patients. Significant improvements in daytime arterial blood gas levels were obtained after 7 to 18 days of nasal intermittent positive pressure ventilation (29) in 10 of the 13 patients three months later, 12 of the 13 patients could be converted to nCPAP therapy and one patient remained on NIV. In another study (37), the same results were observed after three months of home nocturnal bi-level NIV in seven patients, three of whom had severe obesity. 

This chapter describes the aerodynamic principles, models, and equations that govern the flow and the contaminant presence and transport in a designated volume of a work room. The purpose of local ventilation is to control the transport of contaminants at or near the source of emission, thus minimizing the contaminants in the workplace air. 

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