Oxygen Administration Devices

Noninvasive Ventilatory Support

Many patients require more support than a passive O ₂ delivery device. Several noninvasive ventilatory interventions can support oxygenation and ventilation, and possibly obviate the need for endotracheal intubation and mechanical ventilation. Intermittent positive pressure breathing aids in clearance of secretions but is labor intensive and, because it is not continuously applied, does not permanently recruit alveoli. Continuous positive airway pressure (CPAP) applied by a tight-fitting mask can maintain and restore functional residual capacity and, therefore, provides a temporary salutary effect on oxygenation as the underlying cause of hypoxia is treated. This intervention has little if any effect on ventilation and requires a nasogastric tube because of associated aerophagia. Also, a decreased level of consciousness is a relative contraindication to the tight-fitting mask because the patient may vomit and may not be able to remove the mask from his or her face, resulting in aspiration. Bilevel positive airway pressure (BiPAP) also uses a tight-fitting mask, but requires a ventilator to deliver a high airway pressure during spontaneous patient-initiated breaths and a lower baseline pressure during exhalation (like PEEP). It may provide enough assistance to prevent fatigue and stave off endotracheal intubation. Similar to CPAP, BiPAP should be considered a short-term therapy that allows for the identification and treatment of the underlying derangement. Continued close monitoring is necessary for patients on CPAP and BiPAP because their condition may deteriorate precipitously. A cautionary note must be sounded regarding the use of noninvasive ventilation to treat postextubation respiratory failure because this may be associated with a higher mortality than standard therapy.

Mechanical Ventilation

There are four primary indications for endotracheal intubation and mechanical ventilation, as given by the mnemonic SOAP:

• Suction

  • Yanker suction

  • Second suction tubing with no tip attached

• Oxygen

  • High-flow oxygen device (e.g. non-rebreather mask with reservoir)

  • Consider CPAP or BIPAP for preoxygenation

  • Second oxygen source with nasal cannula (up to 15 L/min) for apneic oxygenation

• Airway equipment

  • Direct laryngoscope

  • Video laryngoscope

  • Endotracheal tube

  • Elastic bougie

• Pharmacologic agents: paralysis, sedation

  • See Rapid Sequence Intubation

  • Induction agent (e.g. etomidate, ketamine)

  • Paralytic Agent (e.g. succinylcholine, rocuronium)

  • Post-intubation analgesia and sedation (e.g. fentanyl and propofol)

  • Monitoring equipment

  • Telemetry

  • Oxygen saturation

  • Capnography

The first variable to set is the trigger – that is, the variable that will initiate inspiration. The trigger may be a time interval or a threshold rate of air flow. The second variable to set is an inspiratory limit, which may be a volume, pressure, or maximum air flow rate. The third variable to set is the cycle, which may be a volume, pressure, or time. Based on these variables, the ventilator will deliver one of three types of breaths, mandatory, assisted, or spontaneous. A mandatory breath is triggered, limited, and cycled by the machine. An assisted breath is triggered by the patient, but is limited and cycled by the ventilator. A spontaneous breath is triggered, limited, and cycled by the patient.

Volume-Cycled Ventilation

This type of ventilation delivers a preset V t with each breath. Advantages include delivery of a reliable minute volume and ease of use. The major disadvantage is potential for high airway pressures and resulting lung injury. The different modes of volume-cycled ventilation include controlled mandatory ventilation (CMV), assist control ventilation (AC), and intermittent mandatory ventilation (IMV). With CMV, the patient receives a set number of fixed-volume breaths, but is unable to increase minute ventilation by triggering additional breaths. CMV is typically only used in patients in the operating room under general anesthesia. AC differs from CMV in that the patient can trigger additional breaths. Every triggered breath will be a full machine-cycled breath. AC is used when full ventilatory support is required but is not suitable for the agitated patient who is tachypneic because it may lead to severe respiratory alkalosis. IMV allows spontaneous breathing. It delivers intermittent fixed-volume breaths and allows the patient to breathe spontaneously between mechanical breaths. Synchronized IMV (SIMV) allows the mechanical breaths to be triggered by the patient's own respiratory effort and avoids stacking of breaths. Varying degrees of pressure support may be added to the spontaneous breaths to assist the patient. SIMV is a useful mode of ventilation when attempting to wean the patient or when there is patient-ventilator asynchrony. In general, volume-cycled ventilation is the most uncomfortable for the patient and may result in significant patient-ventilator dyssynchrony, requiring significant amounts of sedatives.

Pressure-Cycled Ventilation

Pressure-controlled ventilation is designed to protect the lung from alveolar overdistention and epithelial injury. A set pressure is applied to the ventilatory circuit during each breath, allowing the lungs to expand based on thoracic compliance. The major advantages are lower mean and peak airway pressures and an exponential decelerating flow pattern, which tends to be more comfortable for the patient. The major disadvantage is fluctuating minute ventilation in the presence of changing lung compliance. Pressure-cycled breaths can be delivered in an analogous fashion to volume-cycled breaths in an AC or SIMV mode. Pressure-support ventilation (PSV) is a spontaneous ventilatory mode. A negative inspiratory force created by the patient will trigger the ventilator to apply a certain pressure to the ventilator circuit. PSV is the most comfortable mode of ventilation because the patient can control all the elements of inspiration and expiration; accordingly, PSV has become the mode of choice for weaning patients off mechanical ventilation. The major disadvantage of PSV is that minute ventilation cannot be ensured and hypoventilation and apnea can occur; thus, patients must have an intact respiratory drive and be carefully monitored.

Difficult to Ventilate Patients

Patients with severe lung disease can be a challenge to oxygenate and ventilate. On volume-cycled ventilator modes, airway pressures may climb; on pressure-cycled modes, the delivered V t may decrease. The goals include maintaining airway pressures less than 35 to 40 cm H ₂ O, and an Sa o ₂ of 90% or higher. Definitive recommendations for optimal ventilator strategies are not available, but there are a number of maneuvers that may be attempted. Prone positioning, inhaled nitric oxide, and permissive hypercapnia have been discussed earlier. Inverse ratio ventilation involves lengthening inspiratory time to more than 50% of the respiratory cycle, which increases the mean airway pressure and recruits air spaces by auto-PEEP in a manner similar to that of applied PEEP. Inverse ratio ventilation should be used with caution in patients with diminished dynamic compliance, such as chronic obstructive pulmonary disease (COPD), asthma, and ARDS, given their propensity for air trapping. Air trapping should be suspected in a patient whenever an increased minute volume results in an increased Pa co ₂ (hypercapnia). In severe cases, pharmacologic paralysis, which relaxes the chest wall musculature and allows for synchronization of ventilator and patient while decreasing VO ₂ and CO ₂ production, may be required. Tracheal gas insufflation provides 2 to 10 liters/min of 100% O ₂ delivered 1 cm above the carina. It decreases Pa co ₂ by washing out the proximal anatomic dead space and can be useful when permissive hypercapnia is being used to attenuate respiratory acidosis.

A tracheostomy may be necessary to facilitate weaning and discontinuation of mechanical ventilation in some patients because it can decrease dead space ventilation and the work of breathing and improve patient comfort, thus decreasing sedation requirements and improving pulmonary toileting and clearance of secretions. The timing of tracheostomy for respiratory failure is a controversial topic. Older studies suggested that the procedure be performed on patients who have remained on mechanical ventilation longer than 14 to 20 days; however, more recent data have shown decreases in ICU length of stay and duration of mechanical ventilation without increased complication rates when tracheostomy is performed within 7 days of the occurrence of respiratory failure. This has been further supported by studies showing the same benefits in a mixed population of medical, surgical, and trauma patients who underwent tracheostomy within 3 days of the initiation of mechanical ventilation. Percutaneous tracheostomy is an attractive modality because it is more convenient than traditional tracheostomy done in the operating room and may be associated with reduced costs, transport complications, delays, and postoperative hemorrhage and infection than open techniques.

ECMO or CO ₂ removal may offer enough lung protection to salvage critically ill patients, but expertise and availability are often limited. Their use is best restricted to rescue patients with severe respiratory failure unresponsive to other modalities of advanced ventilatory support in the hope that the patients' lungs will recover while avoiding further exposure to the potentially injurious aspects of mechanical ventilation. The influenza A (H1N1) pandemic led to the investigation of ECMO as a means to treat the many patients who suffered from H1N1-associated ARDS. Although the initial results in terms of survival and recovery of pulmonary function are promising, only observational data exist at this time and it must be remembered that prior investigations into ECMO for ARDS have uniformly failed to show any survival benefit. The exception to this is a recent randomized trial (CESAR 2009) in the UK of 180 patients with severe, acute respiratory failure; some survival benefit without significant disability was noted in patients treated with ECMO when compared with conventional ventilation.

Weaning from Mechanical Ventilation

Patients who are intubated for pulmonary failure usually require a period of weaning to regain strength and to prove their ability to ventilate and oxygenate themselves. When considering removing a patient from the ventilator, it is important first to ensure that the underlying problem leading to intubation has been rectified and the patient is otherwise stable. Then, one may make the same SOAP assessment as when determining the need for intubation:

• 1.

  Are the s ecretions too much for the patient to handle?

• 2.

  Is the patient o xygenating adequately (i.e., Pa o ₂ /F i o ₂ >200, which requires that F i o ₂ ≤0.40 to 0.50 and PEEP <5 to 8 cm H ₂ O)?

• 3.

  Can the patient protect his or her a irway?

• 4.

  Is p ulmonary function adequate?

Ideally, the patient is assessed while breathing spontaneously and a number of parameters may be obtained to assess pulmonary function. Negative inspiratory force (>−20 to −30 cm H ₂ O), minute ventilation (<10 to 15 liters/min), V t (>5 mL/kg), and respiratory rate (<30 breaths/min) are all useful indicators. Perhaps the most reliable single test is the frequency (f)/V t ratio, the Tobin or Rapid Shallow Breathing Index. A value higher than 105 predicts failure of extubation with a 95% likelihood, whereas a value lower than 80 predicts success in 95% of patients. There are four primary methods of weaning. Multiple daily T piece trials may be performed with extubation once the patient can tolerate several hours. This is labor-intensive and may cause the patient undue stress, particularly if she or he is intubated with a small-diameter endotracheal tube. A single T piece trial may be performed daily with extubation if it is successful. If this trial is unsuccessful, the patient is rested for 24 hours and the test is repeated the following day. IMV and PSV weaning are popular, without a clear-cut advantage of one over the other. It is clear, however, that trials of spontaneous breathing shorten weaning time, so daily sedation holidays and spontaneous breathing trials are mandatory.

Prior to extubating a patient, the bedside clinician should systematically review the patient's overall condition in addition to the previously mentioned SOAP assessment, focusing on factors other than respiratory mechanics. Upper airway edema and obstruction should be ruled out by checking for an endotracheal tube cuff leak. An unambiguous and objective method of doing this requires the patient to cough around the endotracheal tube with the cuff down and a finger occluding the tube's lumen; however, care must be taken to prevent aspiration of secretions pooled above the balloon prior to its deflation. The chart and anesthesia record should be reviewed to ensure that the initial intubation was straightforward in case the patient needs to be reintubated. Patients intubated after multiple attempts, bronchoscopic assistance, or via a retrograde intubation are best extubated under controlled circumstances rather than in the middle of the night. Finally, factors necessitating increased ventilatory demand should be corrected, if possible, such as acid-base disturbances, hepatic or renal failure, high fever, sepsis, pronounced anxiety, and agitation. Patients who are difficult to sedate and alternate between agitation and oversedation may benefit from the α ₂ -adrenergic agonist dexmedetomidine which exerts only minimal effects on hemodynamic stability or respiratory drive.