Crit Care Clin 23 (2007) 161–167

Mode of Mechanical Ventilation:Volume Controlled Mode

Shin Ok Koh, MD, PhDDepartment of Anesthesiology and Pain Medicine, Anesthesia and Pain Research Institute,

Severance Hospital, Yonsei University College of Medicine, 134 Shinchon-dong,

Seodaemungu, Seoul 120-752, Korea

It is important to understand the goals of mechanical ventilation (MV).The primary goal of ventilator support is the maintenance of adequate,but not necessarily normal, gas exchange, which must be achieved with min-imal lung injury and the lowest possible degree of hemodynamic impair-ment, while avoiding injury to distant organs such as the brain.

Modes of MV are described by the relationships between the varioustypes of breaths and by the variables that can occur during the inspiratoryphase of ventilation. Each mode of ventilation is distinguished by how it ini-tiates a breath (trigger), how it sustains a breath (limit), and how it termi-nates a breath (cycle); these are referred to as phase variables. There aretwo basic modes of ventilation: ventilation limited by a pressure targetand ventilation limited to the delivery of a specified volume. Volume-targeted ventilation modes can be categorized as follows: patient triggeror time trigger, flow-limited, volume-cycled assist/control, or synchronizedintermittent mandatory ventilation (SIMV) modes.

Volume-controlled mode

In the volume-controlled mode, each machine breath is delivered with thesame predetermined inspiratory flow–time profile. Because the area undera flow–time curve defines volume, the tidal volume (VT) remains fixed anduninfluenced by the patient’s effort. Volume-controlled ventilation (VCV)with constant (square wave) inspiratory flow is the most widely used breathdelivery mode. Alternative flow–time profiles such as decelerating or sinu-soidal inspiratory flow waveforms sometimes are used in the hope of reduc-ing the risk of barotraumas. Pressure is the dependent variable in the modesof ventilation in which volume is the target. Because pressure will vary in

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volume-targeted modes of ventilation, careful monitoring and assessment ofrespiratory system compliance and resistance are necessary.

Assist/control ventilation in volume-controlled mode

Assist/control ventilation (ACV) is a mode in which patients are allowedto trigger the ventilator to receive an assisted breath from the device (Fig. 1).ACV is associated with the least amount of work of breathing and thereforeis used widely during the acute phase of severe respiratory failure. A com-mon problem associated with ACV is respiratory alkalosis in patientsbreathing at high respiratory rates and a significant decrease in venousreturn and cardiac output. Although all modes of MV can decrease venousreturn, mean airway pressures can be higher in ACV.

For early management of patients with acute lung injury (ALI) or acuterespiratory distress syndrome (ARDS), in ARDS network centers, volume-assist control was the most commonly selected mode of ventilation (56%overall), and volume-targeted ventilation was used in most patients(82%). SIMV or SIMV with pressure support (PS) was used more oftenin patients who had a PaO2/FiO2 (P/F) ratio of 201 to 300 than in patientswho had ARDS. The use of pressure control was uncommon (10% overall),as was the use of permissive hypercapnia (6% of patients who had ARDSand 3% of patients who had a P/F ratio of 201 to 300) and the use of pos-itive end expiratory pressure (PEEP) greater than 10 cm H2O [1].

Synchronized intermittent mandatory ventilation

During SIMV in volume-controlled mode, a specified number of volume-preset breaths are delivered every minute. In addition, the patient is free tobreathe spontaneously between machine breaths from the reservoir or totake breaths augmented with PS. Unless the patient fails to breathe spontane-ously, machine breaths are delivered only after the ventilator has recognizedthe patient’s effort, such that ventilator and respiratory muscle activities aresynchronized (Fig. 2). Because all intermittent MV (IMV) circuits now aresynchronized, the terms IMV and SIMV are used interchangeably.

Volume-targeted ventilation for neonates

Unfortunately, traditional volume-controlled ventilation is not feasible insmall newborns because of unpredictable loss of VT to gas compression in

Fig. 1. Airway pressure curve of assist control ventilation (ACV). Solid lines represent mechan-

ical breath cycle; dotted line represents spontaneous breaths.

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the circuit, stretching of the tubing, and variable leakage around the uncuf-fed endotracheal tube (ETT). Therefore microprocessor-based modificationsof pressure-limited, time-cycled ventilation were developed to try to com-bine the advantage of pressure-limited ventilation with the ability to delivera more constant VT. Three devices widely used in neonatal ventilation offersome form of volume-targeted ventilation, and each of the available modeshas advantages and disadvantages. Clinical data validating the performanceof these modes are limited. Pressure-regulated volume control (PRVC)is a pressure-limited time-cycled mode that adjusts inspiratory pressure totarget a set tidal volume based on a compliance calculation from the pres-sure plateau of an initial volume-controlled breath. The breath-to-breathchange in peak inspiratory pressure (PIP) is limited to 3 cm H2O to avoidovershoot.

The volume-assured PS mode is a hybrid mode that seeks to ensure thatthe desired VT is reached. Each breath starts as a pressure-limited breath,but if the set VT is not reached, the breath converts to a flow-cycled modeby prolonging the inspiratory time with a passive increase in PIP. Thismay result in a prolonged inspiratory time, leading to expiratory asyn-chrony. Targeting of tidal volume also is based on inspiratory tidal volumeand is therefore susceptible to error in the presence of significant ETT leak-age. The volume guarantee (VG) option regulates inspiratory pressure usingexhaled VT measurement to minimize artifacts caused by ETT leakage. Theoperator chooses a target VT and selects a pressure limit up to which theventilator operating pressure (the working pressure) may be adjusted [2,3].

Setting the mechanical ventilator in volume-controlled mode

The mechanical output of a ventilator operating in the volume-controlledmode is defined uniquely by four settings: shape of the inspiratory flow pro-file, VT, machine rate, and a time variable in the form of either the inspira-tory-to-expiratory (I:E) ratio, or the inspiratory time (Ti) [4].

Inspiratory flow pattern

In patients who had ALI, use of the ventilator mode with different inspi-ratory flow patterns and I:E ratios altered the nonlinear volume pressure be-havior of the lung. This change was greatest with pressure control inverse

Fig. 2. Airway pressure curve of synchronized intermittent mandatory ventilation (SIMV).

Solid lines represent mechanical breath cycle; dotted line represents spontaneous breaths.

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ratio ventilation (PCIRV) compared with VCV, and lowest with pressurecontrol ventilation (PCV), despite minimal differences in gas exchange andhemodynamics [5].

In modern ventilators, the inspiratory flow waveforms can be altered.The constant square wave shape is probably the most widely used flow pat-tern, followed by linearly decelerating flow (DF). DF can reduce peak inspi-ratory pressure, resulting in a more even distribution of inspired air andbetter oxygenation than constant flow (CF). The DF pattern may enhancefilling of the alveoli with the longest inspiratory time constant. Hence inpatients with chronic obstructive pulmonary disease (COPD), combiningincreased flow rate with the DF pattern could be expected to lead to con-stant recruitment of the alveoli and reduced pulmonary hyperinflation, re-sulting in better oxygenation and a more even distribution of ventilation.Changing the ventilator in volume-controlled mode with a DF or CFprofile, however, had no significant cardiorespiratory effect in intubatedCOPD patients mechanically ventilated for acute respiratory failure [6].

The patient’s work of breathing (WOB) during assisted ventilation isreduced when inspiratory flow from the ventilator exceeds patient flowdemand. Patients in acute respiratory failure often have unstable breathingpatterns, and their requirement for flow may change from breath to breath.VCV traditionally incorporates a preset ventilator inspiratory flow that re-mains constant even under conditions of changing patient flow demand. Incontrast, PCV incorporates a variable decelerating flow waveform witha high ventilator inspiratory flow as inspiration commences. In the settingof ALI and ARDS, PCV significantly reduced patient WOB relative toVCV. This decrease in patient WOB was attributed to the higher ventilatorypeak inspiratory flow of PCV [7].

Tidal volume

A recent study conducted by ARDSnet showed that reducing the tidalvolume from 12 mL/kg to 6 mL/kg reduced mortality by over 20% [8].The recent modest reduction in clinician-prescribed tidal volume mayhave resulted from heightened concerns regarding ventilator-associatedlung injury.

In patients who have ARDS, MV with a low tidal volume results in de-creased mortality, and therefore an increased use of MV with low tidal vol-ume is expected. Even MV with a low tidal volume mode, however, inducesproinflammatory and profibrinogenic responses with a nondependent pre-dominance for interleukin-1b(IL-1b) and procollagen III (PC III) mRNAexpression in supine, ventilated, previously normal rats. A possible explana-tion for increased mediator expression with low tidal volume is lung hetero-geneity, which may cause alveolocapillar distension in the nondependentregion and repetitive opening and closing of distal lung in the dependentlung region, rendering the lung more susceptible to ventilator-induced

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lung injury (VILI). In addition, passive MV is not a physiologic condition,and it may induce a proinflammatory and profibrinogenic response [9].

Respiration rate

When VT and an end-expiratory volume have been decided, the mechan-ical backup rate should be selected considering the patient’s actual ratedemand, anticipated ventilatory requirement, and the impact of the rate set-ting on breath timing.

Inspiratory-to-expiratory ratio

The setting of timing variables, in conjunction with VT and PEEP, deter-mines the volume range over which the lungs are cycled during ventilation.A long TI, a high TI/TTOT, and a low mean inspiratory flow all promote ven-tilation with an inverse I:E ratio. Long pause times favor the recruitment ofpreviously collapsed or flooded alveoli and offer a means of shortening ex-piration independent of rate and mean inspiratory flow. Although alveolarrecruitment is a desired therapeutic endpoint in the treatment of patientswho have ARDS, one should consider that keeping the lungs expanded athigh volumes or pressures for some time may damage relatively normalunits and may have adverse hemodynamic effects.

Ventilator mode and ventilator-induced lung injury

MV, although life sustaining, can be harmful to the diseased lung, espe-cially when high ventilatory volumes and pressures that cause lung overdis-tension are used. This observation led the author to think that ventilatorystrategies designed to avoid exposure of the lung to high pressure or volumemight improve outcome. Consequently, it was recommended that underconditions in which lung overdistension is likely to occur, tidal volumeand airway pressure should be limited, accepting the attendant increase inarterial carbon dioxide levels. Theoretically, pressure-limited ventilationcan be provided equally well by either pressure target modes that limit air-way pressure to preset levels or by volume-cycled ventilation with tightly setpressure alarms and close monitoring of plateau pressure.

Many clinicians prefer PCV, because it is easy to control peak airwaypressure and keep peak inspiratory pressure below critical limits, thus pos-sibly reducing volutrauma. Davis and colleagues [10] have demonstrated animprovement in oxygenation and pulmonary mechanics in ARDS patientswho were switched from VCV to PCV while VT, inspiratory time andPEEP were held constant. The finding was thought to reflect an increasein mean airway pressure. The downside, however, is that the high peak in-spiratory flow of PCV may aggravate lung injury because of greater shearforces than the lower peak inspiratory flow of VCV. Indeed, a rabbit model

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revealed that the high peak inspiratory flow in PCV induced significantlymore severe lung damage than low peak inspiratory flows in VCV [11].

Recognition that volume, rather than pressure, is the critical determinantof VILI has focused attention on the need to better control the delivered VT.The author investigated the ventilator strategy that was most effective atreducing VILI. There are three basic mechanisms of VILI:

Volutrauma, expansion of alveoli because of high ventilation pressureAtelectrauma, shear stress induced injury caused by unstable alveoli

recruiting and derecruiting (R/D) with each breathBiotrauma and inflammatory injury that occurs secondary to the tissue

damage caused by both volutrauma and atlectotrauma [12]

Ventilator mode and outcome

The increased incidence of extrapulmonary organ failure in patients ofthe VCV group was associated strongly with a higher mortality. The devel-opment of organ failure likely was not related to the ventilatory modes.There was no difference in outcome in patients with ARDS who were ran-domized to PCV or VCV [13].

Volume-control ventilation type of noninvasive pressure ventilation

In clinical practice, pressure-type noninvasive pressure ventilation(NIPPV) generally is preferred over volume-type NIPPV in patients whohave home MV, and pressure-type NIPPV has replaced volume NIPPV.Pressure and volume ventilation NIPPV were equivalent with respect tonocturnal and daytime physiology, and the resulting daytime function andhealth status in chronic respiratory failure caused by chest wall deformity[14]. Nocturnal volume- and pressure-limited NPPV had similar effectson gas exchange and sleep quality in patients who had hypercapnic chronicrespiratory failure [15]. To date, no differences in the relative advantagesor disadvantages of either type of NPPV have been demonstrated.

Summary

Mechanical ventilation can be harmful to the diseased lung, especiallywhen it involves high ventilatory volumes and pressures that cause lungoverdistension. Ventilatory strategies designed to avoid exposing the lungto high pressure or volume might, therefore, improve outcome. The best ap-proach to MV for patients who have ALI or ARDS has been controversial.The consensus conference recommendation is to limit tidal volume and endinspiratory airway pressure and to accept permissive hypercapnia. Theoret-ically, pressure-limited ventilation can be equally well-provided by pressuretarget modes that limit airway pressure to preset levels or by volume-cycled

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ventilation with tightly set pressure alarms and close monitoring of plateaupressure. Physicians caring for patients early in the course of ALI/ARDS inARDS network centers favored volume-targeted ventilation. Using phasevariables, volume-targeted ventilation may be characterized as patient- ortime-triggered, flow-limited, volume-cycled assist/control, or SIMV (IMV)modes. The method by which MV is provided to reduce the inspiratory pla-teau pressure, by decreasing either VT on VCV or inspiratory pressure onPCV, did not influence mortality independently. The mortality of ARDSwas associated strongly with the development of multiple organ failure.

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