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Mechanical Ventilation |
CLASSIFICATIONS OF POSITIVE-PRESSURE
VENTILATORS
Modern
ventilators are classified by the method of cycling from the inspiratory phase
to the expiratory phase (ie, named after that parameter that signals the
termination of the machine's positive-pressure inspiration cycle and changeover
to the passive expiratory phase). The signal to terminate the machine's
inspiratory activity can be a preset volume (volume-cycled ventilator), a preset
pressure limit (pressure-cycled ventilator), or a preset time factor
(time-cycled ventilator).
Volume-cycled ventilation is the most common form of ventilator cycling used
in adult medicine because it provides a consistent breath-to-breath tidal
volume. Termination of the delivered breath is achieved when a set volume has
been delivered. Many
factors are considered in the decision to institute mechanical ventilation.
Since no mode of mechanical ventilation can or will cure a disease process, the
patient should have a correctable underlying problem that can be resolved while
on mechanical ventilation. Mechanical ventilation should not be initiated
without thoughtful consideration because intubation and positive-pressure
ventilation could have potentially harmful effects.
Mechanical ventilation is indicated when the patient's spontaneous
ventilation is not adequate to sustain life. In addition, it is indicated in
critically ill patients to gain control of the patient's ventilation and as a
prophylaxis for impending collapse of other physiologic functions. Physiologic
indications are respiratory or mechanical insufficiency and gas exchange
ineffectiveness. Some common indications for employing mechanical ventilation
include the following:
Clinical judgment and the trend of these values should be used. A patient
with an increasing illness severity should always alert a clinician to consider
instituting mechanical ventilation. Mode of
ventilation
After deciding to institute positive-pressure ventilation with a
volume-cycled ventilator, the next decision is to select the most optimal mode
of initial machine operation.
The initial mode of ventilation should be the assist control (A/C) mode. In
this mode, a tidal volume and rate are preset, but whenever the patient makes an
inspiratory effort, the ventilator senses the effort and delivers the preset
tidal volume. In the A/C mode, the patient can initiate inspiration and control
the breathing frequency; therefore, the respiratory pattern may have irregular
intervals, depending on the timing of patient efforts. If the patient fails to
initiate inspiration, the ventilator automatically goes into a back-up mode and
delivers the preset rate and tidal volume, until it again senses an inspiratory
effort. This back-up rate ensures minimum minute ventilation in the event of
apnea. In the A/C mode, the work of breathing is reduced to only that amount of
inspiration needed to trigger the machine's inspiratory cycle.
This mode is the closest to resting the respiratory muscles since the only
work of breathing is the initial negative-pressure generation to trigger the
machine. This mode has the advantage that the patient can increase minute
ventilation by simply triggering additional breaths above the set back-up rate.
In most cases, patients use their chemoreceptors and stretch receptors to
achieve a minute ventilation that provides a reasonable pH. The respiratory
center in the central nervous system receives input from the chemical receptors
(arterial blood gas tensions) and neural pathways that sense the mechanical work
of breathing (mechanoreceptors). The respiratory rate, respiratory pattern, and
tidal volume are the result of input from these chemoreceptors and mechanical
receptors. From this input, the respiratory center can regulate gas exchange
with the minimum amount of work.
A second possible advantage is that cycling the ventilator into the
inspiratory phase maintains normal ventilatory activity and, therefore, prevents
atrophy of the respiratory muscles.
The potential disadvantages of the A/C mode include respiratory alkalosis.
Because the ventilator delivers the set tidal volume on demand, a potential for
alveolar hyperventilation exists, resulting in hypocapnia (observed most
frequently in patients with end-stage liver disease, hyperventilatory stage of
sepsis, and head trauma).
A second possible disadvantage is that every breath is a full, preset,
positive-pressure breath that influences venous return to the right heart and,
possibly, to global cardiac output. Nevertheless, the A/C mode is the best
initial mode of ventilation and may be switched to another option if hypotension
occurs or hypocarbia is evident on the first arterial blood gas analysis.
Tidal volume and rate
For a patient without preexisting lung disease, the tidal volume and rate
traditionally are selected by the "12-12 rule". In the A/C mode, a tidal volume
of 12 cc/kg of lean body weight is preset to be 12 times per minute.
In patients with chronic obstructive pulmonary disease (COPD), the tidal
volume and rate are slightly reduced to the "10-10 rule" to avoid overinflation
and hyperventilation. In the A/C mode, a tidal volume of 10 cc/kg of lean body
weight is delivered 10 times per minute.
In acute respiratory distress syndrome (ARDS), new information suggests that
the lungs function better and volutrauma is minimized with low tidal volumes of
6-8 cc/kg. In the A/C mode, tidal volumes are preset at 6 cc/kg of lean body
weight at a rate of 8-10 breaths per minute. These lower volumes may lead to a
slight degree of hypercarbia, which typically is recognized and accepted without
correction, thus the term permissive hypercapnia.
Double checking the selected tidal volume
Once a tidal volume is selected, the peak airway pressure necessary to
deliver a single breath should be recorded. As tidal volume increases, the
pressure required to force that volume into the lung also increases. During
mechanical ventilation, a persistent breath-to-breath peak pressure higher than
45 cm H2O is a risk factor for barotrauma. The tidal volume suggested
by the above rules may need to be decreased in some patients to keep the peak
airway pressure, if possible, below 45 cm H2O.
Sighs
Since a spontaneously breathing individual typically sighs 6-8 times per hour
to avoid microatelectasis, giving periodic machine breaths of 1.5-2 times the
preset tidal volume 6-8 times per hour once was recommended. Often the peak
pressure needed to deliver such a volume would predispose to barotrauma.
Today, sighs usually are not used if the patient is receiving tidal volumes
of 10-12 cc/kg or requires the use of positive end-expiratory pressure (PEEP).
When using low tidal volumes, sighs are preset at 1.5-2 times the tidal volume
and delivered 6-8 times per hour, provided the peak pressures are safe.
Initial FIO2
When initiating mechanical ventilation, the highest priority is to provide
effective oxygenation. After intubation, 100% FIO2 always should be
used until adequate oxygenation has been documented by postintubation and
postmechanical ventilation arterial blood gases. A short period on
FIO2 of 100% is not dangerous to the patient on mechanical
ventilation and offers the clinician several advantages.
First, an FIO2 of 100% protects the patient against hypoxemia if
unrecognized problems develop from the intubation procedure. With the
PaO2 measured on an FIO2 of 100%, the clinician can easily
calculate the next desired FIO2 and quickly estimate the shunt
fraction.
The degree of shunt on 100% FIO2 can be estimated by a rough rule
of thumb. The measured PaO2 is subtracted from 700 mm Hg. For each
100 mm Hg difference, a 5% shunt exists. A shunt of approximately 25% requires
the use of PEEP.
Inadequate oxygenation despite the administration of 100% oxygen should
prompt a search for complications of endotracheal intubation or
positive-pressure breathing (mainstem intubation or pneumothorax). If such
complications are not present, then PEEP is needed to treat intrapulmonary shunt
pathology.
Since only a few disease processes can create an intrapulmonary shunt, the
presence of a significant shunt narrows the patient's potential source of
hypoxemia to alveolar collapse (major atelectasis) and alveolar filling with
something other than gas (lobar pneumonia, ARDS, congestive heart failure [CHF],
hemorrhage).
Positive end-expiratory pressure
PEEP is a mode of therapy used in conjunction with mechanical ventilation. At
the end of exhalation (either mechanical or spontaneous), patient airway
pressure is maintained above atmospheric pressure by exerting a pressure that
opposes complete passive emptying of the lung. This pressure typically is
achieved by maintaining a positive pressure flow at the end of exhalation. This
pressure typically is measured in centimeters of H2O.
PEEP therapy can be effective when used in patients with a diffuse lung
disease that results in an acute decrease in functional residual capacity (FRC),
which is the volume of gas that remains in the lung at the end of a normal
expiration. FRC primarily is determined by the elastic characteristics of the
lung and chest wall. In many pulmonary diseases, FRC is reduced because of the
collapse of the unstable alveoli. This reduction in lung volume decreases the
surface area available for gas exchange and results in intrapulmonary shunting
(ie, unoxygenated blood returning to the left side of the heart). If FRC is not
restored, a high concentration of inspired oxygen may be required to maintain
the arterial oxygen content of the blood in an acceptable range.
Applying continuous positive pressure at the end of exhalation (PEEP,
continuous positive airway pressure [CPAP]) causes an increase in alveolar
pressure and increases alveolar volume. This increase in lung volume increases
the surface area by reopening or stabilizing collapsed or unstable alveoli. This
“splinting” or “propping open” of the alveoli with positive pressure may provide
a better matching of ventilation to perfusion, thereby reducing the shunt
effect.
Once a true shunt is changed to a ventilation/perfusion (V/Q) mismatch, lower
concentrations of oxygen can be used to maintain an adequate PaO2.
PEEP therapy also has been reported to be effective in improving lung
compliance. When FRC and lung compliance are decreased, more energy and volume
are needed to inflate the lung. By applying PEEP, the lung volume at the end of
exhalation is increased, which decreases the work of breathing because the lung
is already partially inflated; therefore, less volume and energy are needed to
inflate the lung.
In summary, when used to treat patients with a diffuse lung disease, PEEP
should improve compliance, decrease dead space, and decrease the intrapulmonary
shunt effect. The most significant benefit of PEEP is that the patient can
maintain an adequate PaO2 at a lower, safer concentration of oxygen
(<60%), thereby reducing the risk of oxygen toxicity.
Because PEEP is not a benign mode of therapy and can lead to serious
consequences, the ventilator operator should have a definite indication to use
it. Typically, the addition of external PEEP is justified when a PaO2
of 60 mm Hg cannot be achieved with a FIO2 of 60% or if the initial
shunt fraction is greater than 25%. No recommendations exist for adding external
PEEP during initial ventilator setup to satisfy misguided attempts to supply
prophylactic PEEP or physiologic PEEP. Most clinicians use the lowest amount of
positive pressure that provides an adequate PaO2 with a safe
FIO2.
Summary of initial ventilator set up
ADJUSTING THE VENTILATOR AND AVOIDING
COMPLICATIONS
After
initiating ventilator support, the goal during the maintenance phase is to
achieve adequate gas exchange without toxicity or complications, while allowing
the primary cause of the respiratory failure to be treated or resolve. The first
arterial blood gas typically is obtained 15-20 minutes after the patient is set
up on the ventilator and dictates how the first changes in ventilation and
oxygenation should be made.
Oxygenation decisions
The target PaO2 in respiratory failure is 60 mm Hg and/or 90%
saturation of the hemoglobin. A PaO2 of 60 mm Hg usually is on the
shoulder of the oxygen-hemoglobin saturation curve and typically is sufficient
to ensure adequate oxygen delivery to the tissues.
When the clinician wishes to change the PaO2, 2 variables might be
adjusted, the FIO2 and the amount of PEEP. When using supplemental
FIO2, improvement in oxygenation cannot reliably be achieved with
changes in rate or tidal volume.
With the target PaO2 identified, the FIO2 can be adjusted using the formula:
New FIO2=(old FIO2 X desired PaO2)/measured
PaO2.
PEEP principally is used to lower the risks of oxygen toxicity and is
employed when a safe PaO2 cannot be achieved at 60% FIO2.
Initiated at 5 cm, PEEP usually is increased in 3 cm H2O increments,
while evaluating the effect on oxygenation every 15-20 minutes.
Ventilation decisions
In the early stages of mechanical ventilation, the target PaCO2 is
based on the resultant pH. Errors in acid base rarely are clinically serious if
mechanical ventilation is set up initially in the A/C mode because the pH is the
primary stimulus to breathe. The PaCO2 is the value that results in a
normalized pH. This is the correct PaCO2 because it results in a
reasonable pH.
Occasionally, a patient may hyperventilate on the A/C mode and may develop a
respiratory alkalosis (sepsis, liver disease, head trauma). Respiratory
alkalosis with a pH greater than 7.5 may lead to cardiac arrhythmia when
hypokalemia develops or may lead to seizure activity when cerebral blood flow is
reduced by the vasoconstricting effects of hypocarbia. When this occurs, the
clinician should first confirm that anxiety and pain are adequately controlled
with pharmacotherapy. If hyperventilation persists, consider changing the mode
of ventilation from assist control (where every breath is a positive-pressure
breath of preset volume) to the synchronized intermittent mandatory ventilation
(SIMV) or continuous positive airway pressure (CPAP) mode.
Synchronous intermittent mandatory ventilation
Intermittent mandatory ventilation (IMV) originally was developed for weaning
patients from the ventilator. Later, this mode was found effective as a primary
means of ventilating adult patients. Like the control mode, the ventilator
delivers the mandatory breaths at a preset tidal volume and rate. When the
patient breaths above the set rate on SIMV, the additional breaths are
spontaneous breaths in tidal volumes that the patient is strong enough to pull.
For the spontaneous breathing, the ventilator provides a gas source of oxygen of
the same concentration, temperature, and humidity as is available to the patient
for mandatory breathing. During the spontaneous phase of ventilation, the
patient determines the respiratory rate and tidal volume. SIMV was developed to
coordinate the mandatory breath delivered from the ventilator with the beginning
of a patient's spontaneous inspirations when possible. Three types of breaths
are possible on SIMV, as follows:
For the hyperventilating patient, the spontaneous breaths will be of smaller
tidal volume than the preset machine breath, resulting in less alveolar wash out
and less hypocarbia.
Continuous positive airway pressure
In extreme cases of hyperventilation, the clinician may default to the CPAP
mode. With this mode, the ventilator relies on the patient to breathe
spontaneously. The ventilator does not cycle or give any preset
positive-pressure breaths; however, it can allow the clinician to maintain a
higher than atmospheric end-exhalation pressure. When dialed in, this is termed
CPAP and has the same physiologic characteristics as PEEP. When placed on the
CPAP mode, the patient does all the work of breathing without the aid of a
mechanical back-up rate from the ventilator. The ventilator circuit provides a
gas source of oxygen at a prescribed concentration, temperature, and humidity.
This mode of therapy primarily is used to wean the patient from mechanical
ventilation and rarely is used as a stand-alone mode because it relies solely on
the patient's respiratory drive. In a patient who is hyperventilating, a rapid
spontaneous respiratory rate ensures that the spontaneous tidal volume will be
small. Alveolar washout may beless and hypocarbia will resolve.
Paralysis
Occasionally, hyperventilation with significant respiratory alkalosis does
not resolve on either of the above ventilator modes. In this situation, the
patient is placed back on the A/C mode, with preset tidal volume and rate, and
then paralyzed with a neuromuscular blocking agent. When performed, the
clinician must realize that the patient's ventilation (PaCO2 and pH)
is now completely controlled by the preselected rate and tidal volume. If
adjustment in the PaCO2 and, thus, the pH is desired, the clinician
must now limit attention to 1 of these 2 variables. In adult medicine, tidal
volume usually is preserved and rate is adjusted using the formula: New
rate=(old rate X measured PCO2)/desired PCO2.
Sedation decisions
Attention to the patient's anxiety and pain are imperative for successful
mechanical ventilation. Benzodiazepines can be administered as bolus doses (as
needed) but largely have been supplanted by the use of a continuous intravenous
benzodiazepine drip. Pain usually is responsive to small intravenous boluses of
narcotics (eg, morphine); however, an important caveat to remember is that if
the patient is struggling against the ventilator, then the ventilator almost
always can be set up in a better fashion. Checking and optimizing airway
pressures and triggering sensitivity, inspiratory flow rate, and
inspiratory-to-expiratory (I:E) ratio are wise before increasing sedation.
Peak and plateau pressures
In the ventilated patient, examining the peak and plateau pressures on the
ventilator manometer often can identify the cause of acute respiratory distress.
The peak pressure is the amount of pressure the machine requires to deliver the
set tidal volume over the airway resistance and lung stiffness. To avoid
barotrauma, this value should be kept below 40-45 cm H2O.
The plateau pressure is the amount of pressure required to keep the lung
expanded after airflow is complete. The difference between peak and plateau
pressure is an indication of the airway resistance. These values, when recorded
daily and compared, often can help the clinician to understand why the
ventilator is taking more pressure to deliver the same tidal volume. If both
peak and plateau pressures increase, the lungs must be stiffer. If the peak
pressure rises but the plateau does not, then the airway resistance has
increased.
Inspiratory-to-expiratory ratio
In both spontaneous-assisted and ventilator-assisted breathing, exhalation is
a passive event, depending on the elastic properties of the thorax. In a healthy
person, the expiratory time is at least twice the inspiratory time, resulting in
an inspiratory-to-expiratory (I:E) ratio of 1:2. In conventional ventilation,
patient comfort is important to allow an I:E of at least 1:2. When ventilating a
patient with chronic obstructive lung disease, an I:E ratio of 1:3 or 1:4 may be
necessary.
If the I:E ratio is not preserved, the inspired gas is not allowed enough
time to exit the lung before the next breath is given. Subsequently, breaths
become stacked and intrathoracic pressure increases, resulting in positive
pressure at end-expiration (ie, intrinsic PEEP).
With mechanical ventilation, the inspiratory time can be adjusted by changing
the peak inspiratory flow rate. If the clinician wishes to change the I:E ratio
and allow more time for exhalation, the peak inspiratory flow rate can be
increased or the overall tidal volume can be decreased. The largest tidal volume
possible without excessive peak inspiratory pressure (PIP), the slowest
respiratory rate consistent with effective CO2 elimination, and
inspiratory flow rates fast enough to decrease the I:E ratio all tend to reduce
intrinsic PEEP.
Adjusting external positive end-expiratory pressure
A PEEP level of 10 cm H2O rarely causes hemodynamic problems in
the absence of intravascular volume depletion. The cardiodepressant effects of
PEEP often can be minimized by judicious intravascular volume support or cardiac
inotropic support. While peak pressure is related to the development of
barotrauma, arterial hypotension is related to the mean airway pressure, causing
a decrease in either venous return to the heart or right ventricular function.
A PEEP level greater than 10 cm H2O generally is an indication to
follow cardiac output (CO) with a Swan-Ganz catheter. The left atrial filling
pressure (LAP) should be corrected for a PEEP greater than 10 cm: LAP=(pulmonary
capillary wedge pressure [PCWP]-[PEEP/2]).
Withdrawal of PEEP from a patient should not be performed until the patient
has satisfactory oxygenation on 40%. A formal PEEP wean, then, is performed by
reducing the PEEP in 3- to 5-cm decrements while watching the hemoglobin-oxygen
saturations. An unacceptable drop in the hemoglobin-oxygen saturation should
prompt the clinician to immediately reinstitute the last PEEP level that
provided good hemoglobin-oxygen saturation. A
better understanding of ventilator-induced lung injury recognizes that the
alveolar epithelium is at risk of both barotrauma and volutrauma. Barotrauma
refers to rupture of an alveolus with subsequent air entry into the pleural
space (pneumothorax) and/or air tracking back along the vascular bundle to the
mediastinum (pneumomediastinum). Volutrauma refers to the lung injury that can
occur when alveoli are overdistended and produce proinflammatory cytokines that
augment or perpetuate the initial lung injury that led to mechanical
ventilation.
With computer feedback, many modern ventilators allow the operator to make
fine adjustments in tidal volume, airway pressures, and timing of the
respiratory cycle in attempts to limit ventilator-induced lung injury. These
methods of mechanical ventilation often are based on attractive physiologic
hypotheses and are interesting to perform. While each method has its proponents,
objective evidence fails to show that any one of the alternative ventilation
methods is more successful than conventional mechanical ventilation with proper
attention to tidal volume.
Currently, most clinicians employ alternative ventilation methods only when
conventional mechanical ventilation strategy fails, which is rare.
INDICATIONS FOR MECHANICAL VENTILATION
INITIAL VENTILATOR SETTINGS
NEWER MODES OF MECHANICAL VENTILATION
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