V_t or TV is the volume of a single breath. Conventional goal TV is approximately 7 ml per kg ideal body weight (IBW). Note that IBW is more a function of patient height than weight or overall size. TVs may be reduced (to 4 to 6 ml per kg IBW) in certain circum stances to minimize ventilator-induced lung injury (VILI) associated with excessive airway pressure and overdistention of functional lung units.

The airway conduits do not exchange gas and therefore represent anatomical dead space that accounts for a fixed volume of each tidal breath. The remaining volume in each breath participates in gas exchange and constitutes alveolar ventilation. As TV is reduced, anatomical dead space makes up a proportionally larger portion of each breath. It is important to increase minute ventilation through enhanced respiratory rate (RR) to balance the decrease of effective alveolar ventilation with TV reduction.

RR or frequency (f) is simply the number of breaths per minute. Usual starting RR is 12 to 20 breaths per minute in adults. Higher rates are typical in neonates, infants, and small children.

Given our attention to low-TV ventilation, even in patients without lung injury, minute ventilation is typically modified by increasing RR rather than TV. In addition to compensating for the relative proportion of dead space mentioned above, enhanced RR may be used to provide compensation for metabolic acidosis or enhanced carbon dioxide (CO₂) production (e.g., fever/hyperthermia, sepsis, and hypermetabolic conditions).

In reactive airways diseases, the concept of permissive hypercapnia refers to the use of a low RR (8 to 10 breaths per minute), which allows for a prolonged expiratory time, combined with low TVs (6 to 7 ml per kg IBW) to diminish the risk of hyperinflation.

Fractional concentration of inspired oxygen (FiO₂) ranges from the concentration of oxygen in room air (0.21 or 21%) to that of pure oxygen (1.0 or 100%). When initiating mechanical ventilation, start with an FiO₂ of 100% and reduce the oxygen based on pulse oximetry.

Inspiratory flow rate (IFR) is the rate at which a TV is delivered during inspiration. In an adult, this is typically set at 60 L per minute. Cases of reactive airways disease may require peak IFR to be increased to 90 to 120 L per minute to shorten the inspiratory time (Ti) and thus increase expiratory time and diminish dynamic hyperinflation.

Positive end-expiratory pressure (PEEP) provides a static pressure to the airways during inspiration and expiration, and is typically set at a minimum of 5 cm H₂O. PEEP increases functional residual capacity, total lung volumes, and total lung pressures. When a patient is unable to meet oxygenation goals using an FiO₂ >50%, PEEP can be progressively increased to augment mean airway pressure and oxygenation. Excessive PEEP can cause overdistention and contribute to VILI and compromised venous return with consequent hemodynamic deterioration.

Peak inspiratory pressure (PIP) and plateau pressure (P_plat): The PIP, the greatest pressure reached at any point during the inspiratory phase, is a function of the ventilator circuitry, endotracheal tube (ETT), peak flow, and the patient’s lung and thoracic compliance. It is useful for rapidly assessing the patient when acute change has occurred, but does not accurately reflect the risk of VILI. The risk for VILI is best represented by the P_plat, mea sured at the end of inspiration during an inspiratory pause. The pause enables equilibra tion of pressures between the ventilator and lung units to measure the P_plat of the system. P_plat correlates best with the risk of VILI and the current recommendation is to maintain the P_plat <30 cm H₂O. P_plat >30 cm H₂O are best managed by reducing either TVs or PEEP.

The trigger is the event that initiates inspiration: either patient effort or machine-initiated positive pressure.

The limit refers to the airflow parameter that is used to regulate inspiration: either airflow rate or airway pressure.

The cycle terminates inspiration: either a delivered set volume (volume cycled ventilation), a delivered pressure over a set time period (pressure cycled ventilation [PCV]), or termination of inspiratory effort by the patient (pressure support [PS] ventilation).

The best mode in a given circumstance depends on the needs of the patient.

Control mode ventilation (CMV) is almost exclusively relegated to the operating room in sedated and paralyzed patients, but an understanding of this mode provides appreciation of the support provided through other modes. In CMV, all breaths are triggered, limited, and cycled by the ventilator. The clinician sets the TV, RR, IFR, PEEP, and FiO₂. The ventilator then delivers the prescribed V_t (the cycle) at the set IFR (the limit). Even if the patient wanted to initiate an additional breath, the machine would not respond. In addition, if the patient has not completely exhaled before initiation of the next breath, the machine would generate the required pressure to deliver the full V_t breath. For these reasons, CMV is only used in those patients who are sedated and paralyzed.

Assist control (AC) is the preferred mode for patients in respiratory distress. The clinician sets the V_t, RR, IFR, PEEP, and FiO₂. In contrast to all other modes, the trigger that initiates inspiration can be either patient effort or an elapsed time interval. When either occurs, the ventilator delivers the prescribed TV. The ventilator synchronizes set RRs with patient efforts, and if the patient is breathing at or above the set RR, then all breaths are patient initiated. The work of breathing is primarily limited to the patient’s effort to trigger the ventilator and can be altered by adjusting the sensitivity threshold.

Synchronized intermittent mandatory ventilation (SIMV with or without PS) is commonly misunderstood and can lead to excessive patient work of breathing. The physician sets the V_t, RR, IFR, PEEP, and FiO₂. Importantly, the trigger that initiates inspiration depends on the patient’s RR relative to the set RR. When the patient is breathing at or below the set RR, the trigger can be patient effort or elapsed time. In these cases, the ventilator operates similar to an AC mode. If the patient is breathing above the set RR, the ventilator does not automatically assist the patient efforts and the TV is determined by effort and resistance to airflow through the ETT and ventilator circuit. In these instances, work of breathing can be excessive.

Addition of PS to the SIMV mode provides a set inspiratory pressure that is applied during patient-initiated breaths, which exceed the set RR. Appropriate PS balances the inherent resistance of the artificial airways and supports the patient’s physiologic situation to limit undue work of breathing. Insufficient PS is associated with high RR and low V_t, also known as rapid, shallow breathing. Typically, RR is the best marker for the appropriate level of PS. RR should be maintained at <30 breaths per minute and ideally below 24 breaths per minute. SIMV provides no clear benefit over AC mode ventilation. Although previously used as a weaning mode wherein the set rate is progressively decreased to allow the patient to assume increased work of breathing, the absence of additional pressure support (PSV) substantially increases work of breathing and is frequently overtaxing. Spontaneous breathing trials using minimal pressure support ventilation (PSV), without SIMV, is the current standard approach to assess readiness for liberation from mechanical ventilation.

Continuous positive airway pressure (CPAP) is not a true mode of invasive mechanical ventilation. It is equivalent to PEEP in that it provides a static positive airway pressure throughout the entire respiratory cycle.

In a fashion similar to SIMV, PS can be added to CPAP to function as an assisted form of ventilation. In the CPAP-PS mode, the patient determines the RR, initiating and terminating each breath. The TV is dependent on patient effort and the degree of PS relative to the resistance of the airway circuit. This mode should never be used in patients at risk for hypoventilation or apnea because there is no mandatory backup rate to support the patient in case of failure.