Airway 4: Ventilator Management Portal
Introduction
The
basic physics and fundamental principles of mechanically ventilating a
patient haven’t changed in the recent past; however, the technology and
its application continue to undergo significant evolution the more we
learn about managing specific disease states. This portal focuses on
mechanical ventilation in conjunction with the intubated patient. This
portal will enable the CALS team to understand terminology, initiate
mechanical ventilation, and begin to manage the patient’s
respiratory/ventilatory status. Consult experts shortly after
instituting mechanical ventilation.
For noninvasive ventilation techniques, see Vol III—AIR5 Noninvasive Ventilator Support.
The Basics
The primary purposes of mechanical ventilation are to provide the patient with adequate oxygenation, decrease the work of breathing, and reverse progressive respiratory acidosis. The ventilator accomplishes this by rhythmically providing positive pressure air flow to the patient’s respiratory system in a controlled and reproducible fashion via precise waveforms. These characteristics make mechanical ventilation superior to bag-valve-mask ventilation. Switch the patient to mechanical ventilation from bag-valve-mask as soon as reasonable.
Interrelationships exist between volume, flow, time, pressures, pulmonary compliance, and airway resistance.1 Mechanical ventilators attempt to therapeutically apply these relationships in the clinical context by repeating four basis phases: inspiration, the switch from inspiration to expiration, expiration, and the switch from expiration back to inspiration.
- Inspiration flow and volume may be delivered by several different
delivery strategies. The two most common types of ventilators are
volume-cycled and pressure cycled. Older ventilators are one or the
other; newer models may incorporate features of both.
-
Volume-cycled ventilators: a set tidal volume is chosen, and
inspiration proceeds until this volume of gas is delivered via a
constant flow pattern.
The constant volume delivered by a volume-cycle ventilator ensures constant minute ventilation. For this reason, this is the preferred method for critical care patients. However, with the volume held constant, dynamic changes in pulmonary mechanics due to disease (such as changes in pulmonary compliance and airway resistance) result in varying airway pressures. High pressures may cause barotrauma. - Pressure-cycled ventilators: a peak inspiratory pressure is chosen, and inspiration proceeds in a decelerating flow pattern until this pressure is attained. Flow tapers off as the lungs inflate, which helps limit pressure-related problems and helps ensure a homogenous distribution of gas (in a normal pulmonary system). However, with the pressure held constant, varying (smaller) tidal volumes may result due to the dynamic changes of various diseases. Insufficient gas exchange may result.
-
Volume-cycled ventilators: a set tidal volume is chosen, and
inspiration proceeds until this volume of gas is delivered via a
constant flow pattern.
- The switch from inspiration to expiration can be governed by reaching a set volume, reaching a preset pressure, reaching a set time, or by reacting to a flow that has fallen to a preset value.
- Expiration occurs when the lungs are passively allowed to deflate against atmospheric pressure (or against a set PEEP).
- The switch from expiration to inspiration is governed by reaching a
set time.
Terminology
By manipulating ventilator parameters, one can attempt to match the ventilator’s capability to the needs of the patient. Another basic distinction is whether or not the patient can adequately initiate breaths independently. If he or she cannot, use the vent in the assist/control mode. If the patient has an adequate respiratory drive, then make a decision about whether or not to assist the patient’s own efforts. For this, use the vent in the support mode, where it detects patient inspiratory efforts and supplies a preset assist pressure during inspiration.
Current common choices include:
Assist
Control (AC) Mode of Respiration. Preset minimum respiratory rate and
tidal volume. The patient triggers the ventilator with an inspiration
that creates a sub-baseline pressure, whereupon the ventilator delivers
a full preset tidal volume that provides most of the work of breathing.
If no breaths are initiated by the patient in a specified interval, the
ventilator delivers a breath. Spontaneous, independent breathing
(unassisted breaths) is not allowed. This is the mode most often used
subsequent to an emergency intubation.
Continuous Mandatory Ventilation (CMV). Preset minimum respiratory rate and tidal volume delivered without provision for patient assisted/initiated breaths. CMV has given way to the AC mode. (See previous.)
Intermittent Mandatory Ventilation (IMV). Preset minimum respiratory rate and tidal volume. While spontaneous, independent breathing is allowed, the ventilator does not coordinate breaths with the patient. (Excessive pressures generated from this dyscoordination may cause barotraumas.) This has given way to SIMV. (See SIMV.)
Pressure Support Ventilation (PSV). Pressure support is preset (not tidal volume) to assist the spontaneously breathing patient’s every effort. Some machines provide for a backup IMV rate if the patient quits breathing on her own. This is becoming more popular in selected patients with intact ventilatory drive because it produces less barotraumas and cardiovascular effects.
Synchronized Intermittent Mandatory Ventilation (SIMV). Preset minimum respiratory rate and tidal volume that is coordinated with the patient’s attempts. Independent (unassisted) breaths are allowed. This mode replaces IMV in most newer ventilator models.
Other Useful Terminology:
Minute Ventilation: The product of tidal volume (TV) and Respiratory
Rate (RR)
Positive End Expiratory Pressure (PEEP). This mode of therapy consists of a constant positive airway pressure maintained during expiration that opposes the complete passive emptying of the lungs. PEEP helps to improve gas exchange by helping to keep alveoli open in conditions that close or fill alveoli. PEEP helps to reduce high FIO2 levels (fraction of inspired O2 or the percentage of O2 delivered). The tradeoffs of PEEP can be over-distention of normally aerated alveoli, increased intrathoracic pressure effects, and increased intracranial pressure (ICP). Auto-PEEP or intrinsic PEEP describes the increased end expiratory pressure caused by gas trapping due to incomplete exhalation of previous breath(s).
Plateau pressure: The pressure measured at the end of inspiration following flow cessation; a surrogate measurement of alveolar pressure (and end inspiratory volume). 30 cm H2O is upper limit.
Peak airway pressure: The pressure necessary to deliver single breath (over airway resistance and lung stiffness). Keeping it below 40 to 45 cm H2O is a commonly cited value to avoid barotrauma.
Tidal Volume: The volume inspired and expired with normal breath.
Contraindications and Adverse Consequences of Mechanical Ventilation
There are no absolute contraindications to mechanical ventilation. Many complications of intubation and mechanical ventilation exist, with more to consider the longer the patient remains on the vent. Adverse consequences of applying controlled positive pressure to the pulmonary system include:
Volutrauma. Over-distention of alveoli can result in parenchymal damage to them, perpetuating the lung injury cycle.
Barotrauma. High peak inflation pressures (> 45 cm H2O) can result in disrupted alveoli and entry of air into surrounding tissues. Interstitial emphysema, pneumothorax (simple and tension), pneumomediastinum, and pneumoperitoneum can result.
Decreased surfactant, which leads to atelectasis, which in turn leads to a need for greater pressures to open up the collapsed alveoli.
Cardiovascular effects. Increased
intrathoracic pressures lead to decreased venous return and cardiac
dysfunction, both of which can reduce cardiac output.
Decreased
cardiac output and/or altered vascular pressures and resistances can
adversely affect the brain, kidneys, liver, and stomach.
Prolonged exposure to high concentrations of O2 can lead to:
Free–radical formation with subsequent cellular damage (oxygen toxicity) and
Nitrogen washout with subsequent atelectasis.
These adverse consequences of mechanical ventilation greatly influence ventilator settings and management.
Ventilator Settings
The following values are useful as starting points in the emergency
setting:
Mode: AC or SIMV are versatile modes (see above).
Tidal
Volume (TV): 10 to 15 mL/kg of ideal body weight has been used as the
initial starting range; a lower range (5 to 10 mL/kg) is being
recommended by some due to less baro/volutrauma effects, especially in
COPD and ARDS patients.2 The tradeoff of lower volumes is
shunting/worse oxygenation and hypercapnia.
Respiratory Rate (RR,
breaths per minute): adult: 10 to 15; PEDS: 12 to 20; infants 20 to 30.
Lower adult rates (8 to 12) are being recommended by some in order to
allow more time for exhalation in order to decrease air trapping and
airway pressure.2 The tradeoff of lower rates is potential hypercapnia.
Inspiration/Expiration
Ratio: a common starting value is 1:2. The inspiratory phase (when
pressure and volume are transmitted) is kept as short as practical to
avoid complications; this ratio may be reduced to 1:3 or 1:4 to promote
venous return or to increase time available for expiration in order to
avoid air ‘trapping’ and auto PEEP in COPD patients. The tradeoff of
shortened inspiratory time is ventilation-perfusion inequality due to
gas mal-distribution.
Inspiratory Flow Rate: This important
setting is dependent on other setting choices (RR, I/E, tidal volume),
and therefore often implicitly set by the ventilator. Otherwise, it can
be explicitly set at 60 L/min for most clinical circumstances. Higher
rates, such as 100 L/min, can be used to deliver the tidal volume more
quickly, which allows more time for expiration and decreases the work
of breathing. The tradeoff of a faster rate can be
ventilation-perfusion inequalities.
Fraction of Inspired O2 (FiO2):
Start at 100% O2 in the initial emergency setting. Use ABGs to guide
the subsequent reduction of oxygen levels, with the general goal of
keeping O2 saturation above 90% and/or PaO2 above 60 mm Hg.
PEEP:
There is controversy over using a physiologic PEEP of 3 to 5 cm H2O as
an initial setting. Indications for therapeutic use include a pulmonary
shunt fraction >25%. (See ABGs below.) To reduce high FIO2
levels
that support PaO2 above 60 mm Hg, start at 5 cm.
Sensitivity:
Patients who can breathe on their own need to generate a negative
pressure in order to trigger the ventilator for assistance. Setting the
sensitivity at –1 to –2 cm H2O allows the ventilator to sense such
efforts. As it may be difficult for patients to generate a negative
pressure, newer ventilators that sense inspiratory flow instead help to
reduce this work of triggering the ventilator.
Alarm Settings (if not preset):
High pressure alarm: 10 cm H2O above
Peak Airway Pressure (PAP)
PEEP: 20 cm H2O (unless underlying
disease process requires high PEEP)
Low tidal volume/expiratory volume: 100
cc below set tidal volume
Apneic time: 15 seconds
Low pressure alarm: usually preset
Basics of Managing the Mechanically Ventilated Patient
Mechanical ventilation is a significant therapeutic intervention that greatly increases the need for assessment and subsequent interventions. With interlocking relationships between volume, flow, time, pressures, pulmonary compliance, and airway resistance, it is obvious that changing one parameter affects others, with consequences to patient care. Both over-treatment and under-treatment pose problems for the patient. Tailor ventilator settings to the needs of the patient: change one setting at a time, and check impact on the patient.
Attend to patient pain and anxiety; intravenous morphine and benzodiazepines are commonly used. Assess for physiologic conditions and more optimal ventilator settings before sedating a patient struggling against the ventilator.
Keep equipment nearby to address potential complications. Patient monitoring includes pulse oximetry, cardiac monitor, capnometry, blood pressure, and pulmonary assessments.
Hypotension while on the ventilator can have multiple causes. Pneumothorax and dynamic hyperinflation are two ventilator-related causes that need rapid differentiation. In both cases, air needs to be released, but the methods are quite different. If the patient has no obvious signs of tension pneumothorax, which needs immediate intervention (see Vol I—Pathway 6, Adult Respiratory Emergencies), check the patient for dynamic hyperinflation (while continuing to investigate pneumothorax and other causes). Disconnect the patient from the ventilator for one minute in order to let the presumed hyperinflation reverse itself. If this is the problem, blood will return to the heart, and the blood pressure will normalize. Connect the patient back to the ventilator and adjust settings to mitigate against this recurring. Lower pressure settings and higher inspired O2 levels may be necessary
ABGs play an important role in assessment and management. Consider placing an arterial line for frequent access. (Vol II—CIRC SKILLS 1 Arterial and Venous Catheter Insertion) Obtain initial ABGs 15 minutes after starting mechanical ventilation and 15 to 30 minutes after all subsequent changes in ventilator settings
PaO2 Levels Guide O2 Interventions:
Correlate O2 sats on ABGs with pulse oximetry in order to use pulse oximetry with confidence.
Use the PaO2 on 100% O2 to calculate the amount of shunting by rule of thumb: subtract PaO2 from 700 mm Hg. For every 100 mm Hg difference, a 5% shunt exists. A 25% shunt probably indicates either a significant alveolar collapse or filling problem, and a need for PEEP.
Increase or decrease in FIO2 should generally influence PaO2 and O2 sats in the same direction. Adjustments can be made by formula (new FIO2 = [old FIO2 X desired PaO2]/measured PaO2) or by increments of 5% or so.
Increase in PEEP: PaO2 should rise (but if it causes cardiac output to decrease, PaO2 could fall). Titrate upward in 2 to 3 cm increments.
Decrease in PEEP: If the patient is dependent on PEEP, PaO2 will decrease. Do not abruptly stop PEEP; titrate downward incrementally only if patient has satisfactory saturation on 40% FIO2. If saturation drops unacceptably with decreasing levels, reinstitute previous level of satisfactory PEEP support.
PCO2 and pH Levels Guide Minute Ventilation Adjustments:
Generally speaking, a decrease in minute ventilation (a decrease in tidal volume and/or RR) results in PaCO2 rising; an increase in minute ventilation (an increase in tidal volume and/or RR) results in PaCO2 lowering.
If the patient is paralyzed, ventilation is controlled by the RR and tidal volume settings. If the tidal volume is judged adequate, the RR may be changed by the formula: New RR = (old RR X measured PCO2)/desired PCO2. Otherwise, increments of 2 breaths/min or 50 mL to 100 mL may be used.
In assisted modes, the patient’s pH is one of the drivers of ventilation and will influence the resultant PCO2.
End-tidal CO2 monitoring (capnography) is a highly recommended adjunct for knowledge of tube placement and management of CO2 levels of head-injured patients and COPD patients.
Ventilated patients lose the ability to humidify their airways and handle secretions. Frequent, sterile suctioning (with 5 to 10 cc saline) is indicated and managed by standing protocol.
ET tube cuff pressure needs to be monitored to prevent long-term complications.
Ventilator monitoring: expiratory volume, auto-PEEP, plateau pressure, peak airway pressure (PAP), alarms:
Check PAP after setting/changing tidal volume. If >45 cm H2O, may need to decrease tidal volume to avoid risk of barotrauma.
High pressure alarm (Increased PAP):
Is the plateau pressure unchanged? Think of airway obstruction and assess for increased airway resistance: kinked ET tube, bronchospasm, aspiration, secretions, etc.
Is the plateau pressure increased? Think of decreased compliance and assess for chest wall/pulmonary compliance problems (pneumothorax, pulmonary edema, pneumonia, muscle relaxant wearing off), auto PEEP, atelectasis, abdominal distension, asynchronous breathing…
Mechanical failure of the vent?
PEEP alarm: assess for evidence of autoPEEP: dynamic hyperinflation, tachpynea. If present, lowering the I:E ratio by various means should help.
Low Tidal Volume/expiratory volume alarm: Assess patient, vent, and settings.
Apneic alarm: assess patient and vent if using assist mode; assess ventilator function if using control mode.
Low pressure alarm: usually means a major leak or vent disconnection.
Some Specific Disease Management Considerations
The most common reason for instituting ventilation is to decrease the work of breathing due to respiratory failure.3 These critically ill patients often have pulmonary compliance, airway resistance, and ventilatory-perfusion mismatch patterns that can change from moment to moment, obviating a standard approach to a general condition. Furthermore, management complexity increases with the patient’s co-morbidities.*
Asthma: With airflow obstruction comes air trapping, increased residual volumes, auto-PEEP, and increased risk of barotrauma. If an asthmatic arrests on the vent, immediately assess and treat for tension pneumothorax. A technique called permissive hypercapnia has been used for preemptive management of the risk of barotrauma, using decreased RR, increased flow rate, and a prolonged expiratory phase.4 Recommended initial settings2 are respiratory rate of 8 to 10, 80 to 100L/min, tidal volume of 6 to 8 cc/kg, and no PEEP, with the goal of keeping plateau pressure less than 30 cm H2O. However, lowered minute ventilation leads to hypercapnia/respiratory acidosis, which patients don’t tolerate well. Sedation and paralysis are needed. Do not use this technique in severe hypoxemia/metabolic acidosis/hypertension.
Chronic Obstructive Pulmonary Disease
(COPD): The optimal settings/strategy for this disease are unknown.
Numerous experts recommend a short inspiratory time, a prolonged
expiratory time, and a decreased minute ventilation.5 PEEP is
controversial: PEEP may decrease the work of breathing, but it exposes
damaged lungs to increased pressure with increased risk of barotrauma
and cardiovascular effects.6,7 In patients who are chronic retainers of
CO2, do not try to normalize PCO2, as this will put them into sudden
metabolic alkalosis. (Their chronically elevated bicarbonate that was
compensating for their retained CO2 is now unopposed.) Sudden metabolic
alkalosis can cause seizures and cardiovascular instability.
Pulmonary
Edema (cardiogenic): PEEP is an integral component of management; start
at 5 cm H2O and carefully titrate upward as needed.8
ARDS
(Acute Respiratory Distress Syndrome): This refers to a form of
noncardiogenic pulmonary edema that can arise from severe acute
alveolar injury due to many causes, with the common characteristic of
hypoxemia due to shunting that is resistant to supplemental oxygen.
PEEP is an integral component of management; start at 5 cm H2O and
carefully titrate upward as needed.8 Note the evolving nature of
ventilator management of these patients as noted above.
Considerations and Caveats
Instituting mechanical ventilation is ultimately based on clinical judgment.
Ventilators come in many different models and offer a sophisticated array of modes, flow patterns, alarms, and trend capabilities. Become familiar with your model’s features before you need to use it.
Mechanical equipment can fail. Institute a process of testing your ventilators on a regular basis/before use. See operator’s manual.
All modes of volume-cycled ventilators with the same settings—AC, CMV, IMV, SIMV—provide the same minute ventilation and airway pressures when used in the paralyzed (RSI) patient.
Patients with intact ventilatory drives may require interventions to decrease their tendency to breathe asynchronously with the vent. Consider sedation and muscle relaxants. (Never paralyze without sedation.)
Coordinating efforts when changing to transport ventilators is an essential task, as is ensuring proper vent management during transport.
Nebulizer units can be placed inline with t-piece connector. This allows the medication to be nebulized instead of just putting it down the ETT.
References
- Principles and Practices of Mechanical Ventilation. New York, New York: McGraw Hill. 1994.
- Jain S, Hanania NA, Guntupalli KK. Ventilation of patients with asthma and obstructive lung disease. Crit Care Clin. 1998;14:685-705.
- Tobin MJ. Medical progress: advances in mechanical ventilation. N Engl J. Med. 2001;344:1986-1996.
- Jain S, Hanania NA, Guntupalli KK. Ventilation of patients with asthma and obstructive lung disease. Crit Care Clin. 1998;14:685-705.
- Corbridge TC, Hall JB. Techniques for ventilating patients with obstructive pulmonary disease. J Crit Illness. 1994;9:1027-1036.
- Tuxen DV. Detrimental effects of positive end-expiratory pressure during controlled mechanical ventilation of patients with severe airflow obstruction. Am Rev Respir Dis. 1989;140:5-9.
- Sydow M, et al. Effect of low-level PEEP on inspiratory work of breathing in intubated patients, both with healthy lungs and with COPD. Intensive Care Med. 1995;21:887-895.
- Emergency Medicine. Howell, JM, ed. vol 11. Phil, Pa: WB Saunders. 1998. p 9.