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Control Mode (C) Control mode mechanical ventilation is the simplest form of mechanical ventilation from the ventilator's standpoint. There is no spontaneous breathing and only the ventilator can determine when breaths will be delivered (generally at a rate set by the practitioner). As you might expect, this is not a very friendly mode of ventilation for the awake patient and it is normally only used with sedation and/or paralysis. It is considered poor practice to lock a patient's spontaneous efforts away from an atmosphere to breath. In control mode, if one sets a rate of 12 breaths per minute then the ventilator will deliver a tidal volume breath every 5 seconds regardless of what the patient is doing. Assist Mode (A) In assist mode, a mechanical breath is only delivered when the ventilator senses that the patient is attempting to take a spontaneous breath. The ventilator continuously senses the pressure in the breathing circuit and delivers a breath when that pressure falls below a pre-set level. The advantages of this mode are that there is very little work of breathing and the patient gets to set the amount of ventilation delivered. The disadvantages are that the patient must have an intact respiratory drive (that will not cause hyperventilation or hypoventilation) and the patient must have adequate muscle strength to drop the pressure in the ventilator circuit. Thus, there is no protection against apnea. Since every patient effort is rewarded by the delivery of a full tidal volume mechanical breath, a patient can potentially be hyperventilated very easily with this mode if the patient doesn't slow his/her respiratory rate down in response to a lowered carbon dioxide tension. Assist-Control Mode (A/C) Continuous Mechanical Ventilation (CMV) This mode is a combination of control mode and assist mode. The practitioner sets a control rate which is the minimum number of breaths per minute that the ventilator will deliver. If the ventilator is set to a rate of 10 breaths per minute then the ventilator will wait a maximum of 6 seconds before delivering a mechanical breath. In this mode, the ventilator will also deliver a breath any time the patient makes a spontaneous ventilatory effort sufficient to drop the circuit pressure below a pre-set level. This means that you have the benefits of assist mode without the danger of apnea. Each time the patient initiates a mechanical breath, the ventilator's control mode timer is reset. If the patient breaths faster than the pre-set control, or backup rate, then changes to that backup rate will have no effect on the number of breaths delivered by the ventilator. This can make weaning difficult in this mode since there's no graceful way to gradually withdraw ventilatory support. Like assist mode, a patient can easily hyperventilate using this mode if the carbon dioxide drive is not intact as both the mechanically initiated and patient initiated breaths contain a full tidal volume of inspired gas. Intermittent Mandatory Ventilation (IMV/SIMV) This is a combination of Control mode and spontaneous breathing. On the surface it sounds like Assist-Control mode but with IMV, spontaneous tidal volumes are totally patient delivered and determined and are without any mechanical assistance. If the IMV rate is set to 10 breaths per minute, the ventilator will deliver a mechanical tidal volume every 6 seconds regardless of the patient's spontaneous efforts. If the patient wants to breath extra spontaneous breaths between the mechanical breaths then humidified oxygen is available to the circuit for the patient to breath. IMV is convenient for weaning as the patient takes on more and more of the total minute volume responsibility as the IMV rate is gradually decreased. When the IMV rate is set to zero then the patient is breathing totally spontaneously. SIMV stands for synchronized intermittent mandatory ventilation.^PIt is a modification of basic IMV in which the ventilator keeps track of patient efforts in an attempt to synchronize the time cycled mechanical breaths with patient's spontaneous breaths (if they are present) in order that breath stacking might be prevented. Empirically, one would expect that SIMV would result in greater patient comfort and safety but several studies have failed to show any objective advantage of SIMV or IMV. SIMV is a built-in non-optional feature of many modern ventilators however and it is presented here only for explanatory purposes. Pressure Support (PS) Pressure support is a means of reducing the work of spontaneous breathing. It can be used either alone or in combination with conventional mechanical ventilation. In this mode, the ventilator monitors either the breathing circuit pressure or the patient's spontaneous inspiratory flow rates or both and delivers enough flow to the circuit, during the patient's spontaneous inspiratory effort, to maintain the airway pressure at a pre-set level. When the patient's inspiration ceases^P(or nearly ceases), the ventilator will cut off the addition of supplemental flow to the circuit to allow for a passive exhalation. In this mode, the patient determines how much gas will be inspired but the pressure support reduces the amount of effort required of the patient to receive that volume. Pressure support is frequently added to the spontaneous breaths taken during the IMV or SIMV modes of mechanical ventilation though it can be used as the only mode in use if the patient has an intact respiratory drive. In some ways, pure pressure support breathing is similar to assist mode in that all breaths are patient initiated. The difference is that in assist mode, the tidal volume is preset and the airway pressure varies with the patient's effort and lung mechanics while in pressure support the tidal volume varies with patient effort and the airway pressure is pre-set. Pressure Control Ventilation (PCV) All of the ventilator modes discussed thus far (other than pressure support) are forms of volume limited ventilation. This means, that all of the mechanical breaths were delivered to a pre-set tidal volume. In volume limited ventilation, the volume is held constant while airway pressure is allowed to vary with lung mechanics. Pressure Control Ventilation^P(PCV) is a form of pressure limited ventilation where the airway pressure is held constant while the tidal volume varies with lung characteristics. For this mode, the rate, pressure limit, and inspiratory time are pre-set by the practitioner. Breaths are initiated at a pre-set rate (time cycled) and gas is added to the patient breathing circuit until a pre-set pressure is reached. At this point, the gas flow is reduced to the minimum flow required to hold the pressure at the pre-set level until the inspiration timer expires. This mode of ventilation, while used commonly in infants, is still considered investigational in adults and is used in cases of the Adult Respiratory Distress Syndrome (ARDS) where fine control of the mean airway pressure is desired and other modes have failed to adequately ventilate and/or oxygenate the patient.