Care of Patients Requiring Mechanical Ventilation
Tracheal intubation for mechanical ventilation is most commonly undertaken in ICU patients to manage pulmonary failure. Both nasotracheal and orotracheal intubation appear to be relatively safe for at least 2–3 weeks. When compared with orotracheal intubation, nasotracheal intubation may be more comfortable for the patient and more secure (fewer instances of accidental extuba-tion). Nasal intubation, however, has significant adverse events associated with its use, including nasal bleeding, transient bacteremia, submucosal dissection of the nasopharynx or oropharynx, and sinusitis or otitis media (from obstruction of sinus outflow or of the auditory tubes). Nasal intubation will also generally necessitate use of a smaller diameter tube than orotracheal intubation, and this can make it more difficult to clear secretions and can limit fiberoptic bronchoscopy to use of smaller devices.
Intubation usually can be carried out without the use of sedation or muscle paralysis in agonal and unconscious patients. However, topical anesthesia of the airway and sedation are helpful in patients who still have active airway reflexes. More vigorous and uncooperative patients require varying degrees of sedation; administration of a paralytic agent also greatly facilitates orotracheal intubation. Small doses of relatively short-acting agents are generally used; popular agents include midazolam, etomidate, dexmedetomidine, and propofol. Succinylcholine or a nondepolarizing neuromuscular blocker can be used for paralysis after a hypnotic is given.
The time of tracheal intubation and initiation of mechanical ventilation can be a period of great hemo-dynamic instability. Hypertension or hypotension and bradycardia or tachycardia may be encountered. Responsible factors include activation of autonomic reflexes from stimulation of the airway, myocardial depression and vasodilation from sedative-hypnotic agents, straining by the patient, withdrawal of intense sympathetic activity, and reduced venous return due to positive pressure in the airways. Careful monitor-ing is required during and immediately following intubation.
When left in place for more than 2–3 weeks, both orotracheal and nasotracheal tubes predispose patients to subglottic stenosis. If longer periods of mechanical ventilation are necessary, the tracheal tube should generally be replaced by a cuffed tracheostomy tube. If it is anticipated that a tracheal tube will be required for more than 2 weeks, a tracheostomy may be performed soon after intuba-tion. There is a trend to earlier tracheostomy in vic-tims of trauma, particularly those with major head injuries. While earlier tracheostomy does not reduce mortality, it does tend to reduce the incidence of pneumonia, the duration of mechanical ventilation, and the length of stay.
Depending on the type of pulmonary failure, mechanical ventilation is used to provide either par-tial or full ventilatory support. For full ventilatory support, CMV, AC, or PCV is generally employed with a respiratory rate of 10–12 breaths/min and a VT of 8–10 mL/kg; lower VT (6–8 mL/kg) may be necessary to avoid high peak inflation pressures (>35–40 cm H2O) and pulmonary barotrauma and volutrauma. High airway pressures that overdistend alveoli (transalveolar pressure >35 cm H2O) have been shown experimentally to promote lung injury. Likewise, compared with a VT of 12 mL/kg, a VT of 6 mL/kg and plateau pressure (Pplt) less than 30 cm H2O have been associated with reduced mortality in patients with ARDS. Partial ventilatory support is usually provided by low SIMV settings (<8 breaths/ min), either with or without pressure support. Lower Pplt (<20–30 cm H2O) can help preserve cardiac output, may be less likely to alter normal ventilation/perfusion relationships, and is the cur-rent recommendation.
Patients breathing spontaneously on SIMV must overcome the additional resistances of the tracheal tube, demand valves, and breathing circuit of the ventilator. These imposed resistances increase the work of breathing. Smaller tubes (<7.0 mm i.d. in adults) increase resistance and should be avoided whenever possible. The simultaneous use of pressure support of 5–15 cm H 2O during SIMV can compen-sate for tube and circuit resistance.
The addition of 5–8 cm H2O of PEEP during positive-pressure ventilation preserves FRC and gas exchange. This “physiological” PEEP is purported to compensate for the loss of a similar amount of intrinsic PEEP (and decrease in FRC) in patients following tracheal intubation. Periodic sigh breaths (large VT) are not necessary when a PEEP of 5–8 cm H2O accompanies VT of appropriate volumes.
Sedation and paralysis may be necessary in patients who become agitated and “fight” the ventilator. Repetitive coughing (“bucking”) and straining can have adverse hemodynamic effects, can interfere with gas exchange, and may predispose to pulmo-nary barotrauma and self-inflicted injury. Sedation with or without paralysis may also be desirable when patients continue to be tachypneic despite high mechanical respiratory rates (>16–18 breaths/min).
Commonly used sedatives include opioids (morphine or fentanyl), benzodiazepines (usu-ally midazolam), propofol, and dexmedetomidine. These agents may be used alone or in combination and are often administered by continuous infusion. Nondepolarizing paralytic agents are used in com-bination with sedation when sedation alone and all other means to ventilate the patient have failed.
Patients on mechanical ventilation require continu-ous monitoring for adverse hemodynamic and pul-monary effects from positive pressure in the airways. Continuous electrocardiography and pulse oximetry are useful. Direct intraarterial pressure monitoring also allows frequent sampling of arterial blood for respiratory gas analysis (both a convenience and a disadvantage, given the large number of unneces-sary laboratory tests that are often performed on patients with critical illness). Accurate recording of fluid intake and output is necessary to assess fluid balance. An indwelling urinary catheter will lead to an increased risk of urinary tract infections and should be avoided when possible, but it is help-ful for monitoring urinary output. Central venous (and rarely pulmonary artery) pressure monitor-ing are used in hemodynamically unstable patients. Frequent chest radiographs are commonly obtained to confirm tracheal tube and central venous cath-eter positions, evaluate for evidence of pulmonary barotrauma or pulmonary disease, and determine whether there are signs of pulmonary edema.
Airway pressures (baseline, peak, plateau, and mean), inhaled and exhaled VT (mechanical and spontaneous), and fractional concentration of oxy-gen should be closely monitored. Monitoring these parameters not only allows optimal adjustment of ventilator settings but helps detect problems with the tracheal tube, breathing circuit, and ventilator.
For example, an increasing Pplt for a set VT can indi-cate worsening compliance. A declining blood pressure and increasing Pplt from dynamic hyperinflation (autoPEEP) can be quickly diagnosed by discon-necting the patient from the ventilator. Inadequate suctioning of airway secretions and the presence of large mucus plugs are often manifested as increas-ing peak inflation pressures (a sign of increased resistance to gas flow) and decreasing exhaled VT. An abrupt increase in peak inflation pressure together with sudden hypotension strongly suggests a pneumothorax.
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