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Time
Pressure
The Basics ofMechanical Ventilation
4
Inspiratory hold maneuver
Plateau pressure (P
Driving pressure ( P)
Positive end-expiratory
pressure (PEEP)
Fig. 4.2 Plateau pressure and driving pressure—the airway pressure when inspiratory ow has
stopped and static equilibrium is reached is called the plateau pressure (P
sured using an inspiratory hold maneuver. The driving pressure (ΔP) is the difference between the
plateau pressure and the positive end-expiratory pressure
), which can be mea-
plat
89
)
plat
• Plateau pressure, or P
, is the pressure remaining in the airway at the end of
plat
inspiration when ow has stopped (has reached equilibrium) (Fig. 4.2). This
pressure can be measured on a ventilator by performing an end-inspiratory
breath hold.
• Driving pressure, or ΔP, is the difference between the P
and PEEP and reects
plat
the pressure that is distending the lungs during inspiration (Fig.4.2). Excessive
driving pressure contributes to ventilator-induced lung injury and is associated
with increased mortality for patients on mechanical ventilation.
Tidal volume, or Vt, is the volume of gas delivered with each breath by the venti-
lator. Lower tidal volumes on the order of 6–8mL/kg of ideal body weight are typically preferred, especially in lung injury and ARDS as higher tidal volumes in this
population are associated with higher mortality.
Respiratory rate (RR) is the number of breaths delivered by the ventilator per
minute. A respiratory rate is set on the ventilator by the clinician and represents the
minimum number of breaths per minute the patient will receive; however, if the
patient triggers additional breaths per minute, the observed respiratory rate will be
higher than the set respiratory rate. Modern ventilators provide support or assistance
for every breath triggered by the patient beyond the set respiratory rate.
Minute ventilation is the total volume of gas entering (or exiting) the lung per
minute, which is equivalent to the tidal volume multiplied by the respiratory rate.
The minute ventilation, sometimes referred to as the “minute volume,” is a helpful
metric of overall ventilation and can serve as a target for adequate ventilation especially when inadequate gas exchange or increased ventilatory demand is contributing to respiratory failure.
Compliance reects the ease with which lungs expand when a given pressure is
delivered, calculated as the tidal volume divided by the driving pressure. Low
90
T. Peck and R. M. Schwartzstein
compliance indicates stiffness of the respiratory system and can be caused by stiff
lungs or decreased coordination/stretch of the chest wall.
Airway resistance refers to the opposition to airow within the airway, including
both the patient’s anatomic airway and the articial tubing used to deliver breaths
into the patient (e.g., the endotracheal tube). Increased airway resistance can be
caused by airow obstruction in the patient’s airways as in bronchospasm and
inammation associated with asthma exacerbations or in the articial airway as in
kinking or mucus plugging of the endotracheal tube.
4.5 Modes ofMechanical Ventilation
In delivering breaths to patients receiving mechanical ventilation, the ventilator can
be set to different modes that provide different methods of respiratory support, each
with unique features, benets, and cautions (Table4.1). The ventilator mode should
be selected based on the phase of respiratory failure for a given patient (active vs.
resolving), specic aspects of the patient’s respiratory physiology, and effort to
minimize ventilator-induced lung injury. This section delves into three basic modes
of mechanical ventilation that are widely used in ICU environments, acknowledging
that this section is not exhaustive and there are many other modes used in patient
care for various purposes [4].
Table 4.1 Modes of mechanical ventilation
4 The Basics ofMechanical Ventilation
91
4.5.1 Volume Control Ventilation
Volume control ventilation (VCV) delivers a set tidal volume to the patient for a
specied minimum number of breaths per minute. The volume delivered is regulated by the ventilator by providing a predetermined ow of inspired gas during the
inspiratory time, after which ow ceases (volume=ow × time). The ow pattern
and inspiratory time can be set on the ventilator to alter the inspiratory:expiratory
(I:E) ratio which can be helpful in patients with obstructive lung disease in whom a
prolonged expiratory time can allow more gas to be exhaled. VCV is an “assistcontrol mode,
the patient is fully supported with the set inspiratory settings, even if a given breath
is beyond the set respiratory rate.
A major benet of volume control ventilation is that tidal volume is xed, thereby
allowing for good control of delivered volumes to avoid potentially injurious larger
tidal volumes. However, in this mode, inspiratory pressure is a dependent variable
(based on the delivered tidal volume and the respiratory system compliance) and
can cause a rise in transpulmonary pressure to excessive levels, contributing to
ventilatorof ow are both set on the ventilator, this can limit the patient’s ability to alter ow
breath to breath as is natural in spontaneous breathing and, consequently, can be
more uncomfortable for patients ventilated in this mode, contributing to discomfort
and patient-ventilator dyssynchrony.
” a method of ventilating a patient in which every breath triggered by
induced lung injury due to barotrauma.
Also, because the rate and pattern
4.5.2 Pressure Control Ventilation
Pressure control ventilation (PCV) delivers breaths with a set inspiratory pressure
for a predetermined inspiratory time, during which there is less control of ow by the
ventilator; the patient determines the ow based on how much effort they exert during inspiration. With no xed amount of ow during the inspiratory time, there is no
specic tidal volume delivered, and tidal volume can vary breath to breath based on
patient effort, airway resistance, and respiratory system compliance—this is a potential hazard in patients who would benet from low tidal volume ventilation with set
tidal volumes. Because the patient is better able to regulate the inspiratory ow, this
mode is considered more comfortable than volume control ventilation. Like volume
control ventilation, the inspiratory:expiratory (I:E) ratio can be predetermined in
pressure control ventilation because the inspiratory time is set by the clinician.
4.5.3 Pressure Support Ventilation
Pressure support ventilation (PSV) is used for patients who can initiate breaths on their
own, with each of these breaths being supported with a set level of pressure delivered
by the ventilator. Clinicians commonly use this mode for patients with resolving
92
respiratory failure as a step toward liberation from mechanical ventilation. In PSV,
patients have more control over their breathing pattern than with other ventilator
modes; they can vary their respiratory rate, tidal volume, inspiratory time, and inspiratory ow. There is no minimum respiratory rate setting below which the ventilator
would provide a control breath as in other modes; however, the ventilator monitors
patients for episodes of apnea and will transition to a backup control mode of ventilation if the patient has an episode of apnea lasting a set duration of time (e.g., 20seconds).
Major benets of pressure support ventilation are that it allows for minimization
of the amount of respiratory support the patient receives from the ventilator and is
typically the most comfortable ventilator mode due to the patient’s ability to alter
their breath-to-breath respiratory pattern. There are potential downsides, however.
Because the tidal volume is determined by the patient and is not limited to any specic volume, the patient can easily receive large tidal volumes that are potentially
injurious. Additionally, patients may experience increased work of breathing to
achieve their physiologically necessary minute ventilation if PSV settings are providing inadequate support; the ventilator does not assess patient effort, so clinical
oversight to ensure adequate ventilator support is important.
T. Peck and R. M. Schwartzstein
4.6 Patient-Ventilator Interactions
Mechanical ventilation, while lifesaving, can become detrimental if the patient and
ventilator are not in sync [5]. This mismatch, termed patient-ventilator dyssynchrony
(PVD), arises when the ventilator’s delivered breaths do not coincide with the patient’s
breathing efforts or demands. PVD can lead to a vicious cycle: increased patient work
of breathing, discomfort, and potential ventilator-induced lung injury (VILI). Some
forms of PVD reect almost reex-type interactions between the patient and machine,
while others may be the consequence of patient breathing discomfort associated with
the breathing parameters prescribed by the ventilator settings. By understanding the
different types of PVD, healthcare providers can contribute to optimizing ventilator
settings and ensuring non-harmful patient- ventilator interaction. In some cases,
adjustments in sedation and analgesia may also be indicated, and pharmacologic neuromuscular blockade may even be required in cases of severe and clearly harmful
dyssynchrony until other changes are made to promote safe ventilation. The various
types of PVD can be broadly categorized based on the phase of the respiratory cycle
where the mismatch occurs: trigger, ow, and cycle dyssynchrony [6].
4.6.1 Trigger Dyssynchrony
Trigger dyssynchrony involves issues with initiating a breath. An ineffective trigger
occurs when the patient’s effort fails to initiate a ventilator breath. Conversely, double triggering happens when a single patient effort triggers two ventilator breaths in
4 The Basics ofMechanical Ventilation
quick succession. Auto-triggering arises when the ventilator misinterprets intrinsic
airway uctuations or ventilator circuit artifacts as patient effort, delivering unintended breaths. Finally, reverse triggering occurs when a ventilator breath delivered
before the patient’s expiration is complete, interrupting ongoing exhalation.
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4.6.2 Flow Dyssynchrony
Flow dyssynchrony disrupts the inspiratory ow pattern. When the ventilator’s
delivered ow rate does not meet the patient’s inspiratory demand, it can lead to
patient effort to achieve a sufcient breath volume, increasing work of breathing
and causing harmful swings in transpulmonary pressure. This can also cause patient
discomfort, leading to increased need for sedation.
4.6.3 Cycle Dyssynchrony
Cycle dyssynchrony pertains to issues with breath termination. Premature cycling
occurs when the ventilator ends inspiration before the patient completes inhalation,
causing discomfort and potentially leading to a breath stacking, in which a second
breath is delivered before the preceding exhalation is completed, which can cause
volume accumulation and volutrauma. Conversely, delayed cycling happens when
the ventilator fails to terminate inspiration despite the patient attempting to exhale,
potentially leading to high airway pressure and barotrauma.
4.7 Complications ofMechanical Ventilation
While a lifesaving intervention, mechanical ventilation is not without its risks.
Complications can arise from various factors, including ventilator settings, duration
of ventilation, patient-ventilator interactions, and underlying patient condition.
Here is a closer look at some of the potential complications associated with mechanical ventilation:
• Ventilator-Induced Lung Injury (VILI): This umbrella term encompasses several
lung injuries that can occur due to mechanical ventilation [7]. Two key contributors are volutrauma and barotrauma. Volutrauma refers to injury caused by
delivering excessive tidal volumes, overstretching lung tissue and causing alveolar injury. Barotrauma, on the other hand, arises from high airway pressures during ventilation, potentially leading to alveolar rupture and air leaks (including
pneumothorax and pneumomediastinum). Of note, barotrauma occurs due to
excessive transpulmonary pressure (or distending pressure, the pressure exerted
94
outward on the lung tissue relative to the pressure in the pleural space); high
airway pressure itself is not necessarily injurious if balanced against an opposing
pressure exerting an inward force on the lung, as in obesity or scuba diving.
Additionally, atelectrauma, damage to the lung tissue due to mechanical shearing
forces with repeated opening and closing (recruitment and derecruitment) of
alveoli, can also occur during mechanical ventilation, due to insufcient use of
PEEP to maintain lung recruitment.
• Infections: The presence of an endotracheal tube or tracheostomy disrupts the
natural airway defenses, increasing the risk of ventilator-associated pneumonia
(VAP). Pharmacists can play a crucial role in optimizing antibiotic selection and
minimizing the emergence of antibiotic resistance in mechanically ventilated
patients.
• Airway Complications: Mechanical ventilation can also lead to complications
directly affecting the airway. Airway stenosis, a narrowing of the airway due to
inammation or scarring, can develop after prolonged endotracheal tube placement or as a result of tracheostomy placement. Airway bleeding can occur during–after tube placement (e.g., tracheoinnominate stula) or as a consequence of
airway suctioning through articial airways.
• Respiratory Muscle Atrophy: When the ventilator takes over the work of breathing, respiratory muscles can weaken over time. This deconditioning, termed
respiratory muscle atrophy, can make it challenging for patients to breathe independently upon attempts at liberation from the ventilator.
T. Peck and R. M. Schwartzstein
By understanding these potential complications, healthcare professionals,
including critical care pharmacists, can strive to minimize their occurrence. Careful
selection of ventilator settings, implementation of lung-protective ventilation strategies, judicious use of analgesics and sedation, meticulous infection control practices, and early initiation of weaning protocols are all crucial aspects of mitigating
the risks associated with mechanical ventilation.
4.8 Pathways Forward Once Initiated
onMechanical Ventilation
While initiating mechanical ventilation provides vital support to critically ill
patients, the ultimate goal is to transition them back to spontaneous breathing whenever possible. This section explores key pathways forward once mechanical ventilation has been initiated:
• Spontaneous Awakening Trials (SATs) and Spontaneous Breathing Trials (SBTs):
As the patient’s condition improves, healthcare professionals can assess their
readiness to breathe independently. A spontaneous awakening trial (SAT) evaluates the patient’s level of consciousness and ability to follow simple commands
after minimizing or discontinuing pharmacologic sedation. SBT can be performed in parallel with or separate from an SAT. During an SBT, the ventilator
4 The Basics ofMechanical Ventilation
support is reduced or withdrawn for a predetermined period, typically to minimal
ventilator support on pressure support ventilation, allowing the patient to breathe
spontaneously. Pharmacists can play a role by ensuring that appropriate medication adjustments are made before and during the SBT to optimize respiratory
drive and minimize the risk of complications. Successful completion of an SAT
and SBT paves the way for extubation, the removal of the endotracheal tube.
• Extubation: Extubation signies a major milestone in the recovery process.
However, careful planning and meticulous attention to detail are crucial to ensure
a smooth transition. Once a patient passes an SAT and SBT, healthcare providers
should consider whether the patient has had adequate resolution of the initial
cause of their respiratory failure leading to intubation, whether the patient may
require additional procedures or diagnostic tests for which the patient should
remain intubated (usually to be able to tolerate deep sedation), and whether the
patient will be able to maintain adequate ventilation and gas exchange once
removed from mechanical ventilation (e.g., will respiratory secretions or neuromuscular weakness prevent the patient from maintaining adequate respiratory
function without mechanical ventilation). In some cases, patients may be extubated but subsequently placed on other respiratory support devices like NIPPV
or high-ow nasal cannula (HFNC) to support respiratory function postextubation and reduce the risk of post-extubation respiratory failure. Pharmacists
contribute to the extubation process by reviewing medications that might affect
airway reactivity or coughing, potentially causing difculties after extubation.
Additionally, they can recommend medications to manage pain and secretions,
maximizing the likelihood of successful liberation from mechanical ventilation.
• Tracheostomy Placement and Chronic Mechanical Ventilation: In some cases,
prolonged mechanical ventilation may be necessary. When long-term ventilation support is anticipated, placement of a tracheostomy, a surgical opening in
the trachea, might be preferred over an endotracheal tube. This allows for
improved patient comfort (and minimization of sedation) and can facilitate liberation from the ventilator (it is easier with a tracheostomy for the patient to
attempt trials without ventilatory support). Pharmacists can play a role in managing medications specic to tracheostomy care, such as medications to promote secretion clearance. However, chronic mechanical ventilation requires
ongoing monitoring and management by a specialized team to ensure optimal
patient outcomes.
95
4.9 Mechanical Ventilation Management inExamples
ofRespiratory Failure
Although mechanical ventilation is used in a wide range of challenging situations
and for many different lung diseases, two specic conditions can serve as models
for its safe and effective application in critical care—acute respiratory distress syndrome (ARDS) and severe asthma exacerbation.
96
T. Peck and R. M. Schwartzstein
4.9.1 Acute Respiratory Distress Syndrome
ARDS is dened as acute onset of hypoxemia within a week of an insult known to
cause ARDS with the presence of bilateral lung opacities on imaging (not explained
by cardiogenic pulmonary edema, lung nodules, pleural effusions, or atelectasis as
the primary cause of hypoxemia), necessitating the use of mechanical ventilation
with PEEP of at least 5cm H2O (or NIPPV with expiratory pressure of at least 5cm
H
O) or high-ow O2 nasal cannula with a ow of at least 30 L/minute [8, 9].
2
Hypoxemia with a ratio of partial pressure of oxygen (PaO2) to FiO2 less than or
equal to 300mmHg qualies as ARDS. The pathological hallmark of ARDS is the
presence of alveolar injury and dysfunction, initially with signicant interstitial and
alveolar edema associated with marked inammation, progressing later to proliferative and brotic phases of disease with resolution and recovery highly variable
between patients.
The mainstay of mechanical ventilation strategy in ARDS is to support adequate
oxygenation in the face of signicant lung injury while safely ventilating the patient
to avoid further lung injury. The term “lung-protective ventilation” encompasses
this strategy and includes the important concept of low tidal volume ventilation,
which calls for targeting tidal volume to 6–8mL/kg corrected for ideal body weight
(IBW) (and in some cases, even lower tidal volumes are used, down to 4mL/kg
IBW) to avoid volutrauma and limit driving pressure [10]. Additionally, plateau
pressure of 30cm H2O or less is a common goal, as well as driving pressure of
15cm H2O or less, with the goal of limiting excessive inspiratory pressures to avoid
the risk of barotrauma and excessive mechanical stress on the lung [11]. The team
also strives to reduce the FiO2 to 0.6 or less to avoid oxygen toxicity to the lung.
These targets, however, are balanced against PEEP to promote effective lung
recruitment and prevent atelectasis, enhancing oxygenation and permitting reduced
FiO2. Applied PEEP should be carefully adjusted to achieve this goal while avoiding
excessive PEEP that can cause overdistension of alveoli, thereby impairing gas
exchange, impacting hemodynamics, and causing lung injury. There are various
methods of titrating PEEP to the clinical scenario including increasing the PEEP
systematically in response to increasing FiO2 requirement, performing a decremental PEEP trial to assess for optimal PEEP using measurements of lung compliance
and oxygenation, and employing esophageal manometry to estimate the transpulmonary pressure at end expiration [12].
Other advanced strategies used in parallel with mechanical ventilation are
employed for more severe ARDS including prone positioning, pharmacologic neuromuscular blockade, use of inhaled pulmonary vasodilators, and use of extracorporeal membrane oxygenation. Prone positioning can remove the weight of the heart
from compressing the lungs; this allows for better lung recruitment, which facilitates redistribution of pulmonary edema and tidal volume, and improved ventilationperfusion matching [13]. Neuromuscular blockade can be used in patients with
refractory hypoxemia and respiratory effort/patient-ventilator dyssynchrony that
may be worsening their hypoxemia [14, 15].
4 The Basics ofMechanical Ventilation
97
4.9.2 Severe Asthma Exacerbation
Mechanical ventilation can also be difcult to manage in severe obstructive lung
disease. A particularly striking example of this difculty occurs in severe acute
exacerbation of asthma requiring mechanical ventilation. Because of the remarkable ability of patients with asthma to compensate for impaired respiratory physiology, the reversible nature of asthma exacerbations with medical therapy, and the
potential difculty of mechanically ventilating them, healthcare providers typically
make every effort to maximize the treatment of asthma to attempt rescue before
proceeding with intubation and mechanical ventilation only if deemed necessary.
In an acute are of asthma, airway inammation, bronchospasm, and mucus
plugging can lead to increasing airow limitation. The muscle work associated with
breathing increases as increased airway resistance causes both inhalation and exhalation to be effortful and difcult, with patients experiencing signicant air hunger.
Additionally, as expiratory ow is limited, full exhalation requires a longer time and
can be truncated by the next inspiratory effort, leading to gas trapping, a phenomenon in which the lung becomes hyperinated because an extra volume of air remains
in the lungs at the end of exhalation beyond the normal relaxed volume or functional
residual capacity. This process can occur cyclically such that the trapped volume of
air at end exhalation continues to increase, leading to an increasing degree of hyperination, which shortens inspiratory muscles, further increasing the effort associated with breathing. Eventually, this can cause increased intrathoracic pressure,
which impairs venous return, decreasing cardiac lling and causing hypotension or
even cardiac arrest.
Once mechanical ventilation is initiated, it is imperative to monitor asthma
patients for evidence of gas trapping and dynamic hyperination [16]. If increasing
amounts of trapped air accumulate within the chest, there is additional pressure at
end expiration above the applied PEEP; this observed PEEP in gas trapping is called
“intrinsic PEEP” or “autoPEEP” [17]. On the ventilator, this phenomenon can be
observed by performing an end-expiratory breath hold to assess for end-expiratory
airway pressure and comparing this to the applied PEEP. Lung-protective ventilation with low tidal volumes targeted at 6–8cc/kg of ideal body weight helps minimize the risk of volutrauma and limits the volume of inhaled gas that must be
exhaled, decreasing the risk of dynamic hyperination. Inspiratory time can be
shortened to allow for a longer expiratory phase, though a shorter inspiratory time
with a xed volume (as in volume control ventilation) means that inspiratory ow
will be increased, which can lead to increased peak airway pressure.
Managing mechanically ventilated patients with severe asthma exacerbation also
requires various pharmacologic measures. Standard therapies for acute asthma exacerbation include systemic steroids and inhaled bronchodilators (inhaled betaand muscarinic antagonists) with the goals of reducing airway inammation and
reducing airway resistance. Intravenous magnesium can be used as an adjunct therapy for bronchodilation, though the evidence for this is less clear. For patients requiring sedation, ketamine is an adjunctive sedating agent that provides both sedating/
agonists
98
analgesic and bronchodilating properties. Another supplementary treatment is the use
of neuromuscular blockade for patients in whom safe mechanical ventilation is challenging due to signicant respiratory effort by the patient; the use of neuromuscular
blockade allows clinicians to take full control of a patient’s ventilation temporarily,
providing interventions by mechanical ventilation to address the patient’s impaired
ventilation, which may be poorly tolerated without sedation and neuromuscular
blockade. Heliox (a gas mixture of helium and oxygen) can be used instead of a standard mixture of ambient air and supplemental oxygen, as the addition of low-density
helium gas allows for decreased overall gas density and decreased airway resistance.
T. Peck and R. M. Schwartzstein
4.10 The Role ofthePharmacist inMechanical Ventilation
Pharmacists play a pivotal role in ensuring safe and effective medication management for patients on mechanical ventilation. These patients often have complex
medication regimens treating the pathologies underlying their critical illness and
facilitating care through sedation, and even minor drug interactions or adverse
effects can have signicant consequences.
Mechanically ventilated patients often require sedation to tolerate the ventilator
and prevent patient-ventilator dyssynchrony. Recent data suggest that up to 25% of
patients who survive acute respiratory failure and a period of mechanical ventilation
will suffer mental health issues post-extubation [1–3]. This is thought to be due to
unrecognized dyspnea due to the underlying disease and/or the manner in which the
ventilation is being provided and may occur despite relieving the work of breathing
with mechanical ventilation. Thus, appropriate analgesia must be provided along
with sedation. Pharmacists play a crucial role in selecting appropriate sedatives and
analgesia, monitoring their effects on respiratory drive, and ensuring adequate pain/
dyspnea control to minimize the need for excessive sedation. They can also recommend alternative routes of administration for medications, such as enteral or intravenous, if the usual routes are inaccessible.
Pharmacists actively collaborate with the interdisciplinary team, including inten-
sivists, nurses, and respiratory therapists. They advocate for medication adjustments
based on monitoring data and patient response, ensuring optimal synergy between
medication therapy and ventilator management. Additionally, pharmacists can contribute to the development and implementation of evidence-based institutional protocols for sedation management and medication administration in mechanically
ventilated patients.
4.11 Summary
Mechanical ventilation with positive-pressure support of ventilation is a mainstay of
treatment in the ICU for patients with acute respiratory failure. Optimal management of these patients requires a solid understanding of the basic principles of the