TABLE 1-139 Contraindications for Noninvasive Ventilation

Cardiac or respiratory arrest

Severe encephalopathy

Severe gastrointestinal bleed

Hemodynamic instability

Unstable angina and myocardial infarction

Facial surgery or trauma

Upper airway obstruction

High-risk aspiration or inability to protect airways

Inability to clear secretions

1-139. The answer is A.(Chap. 269) Modes of ventilation differ in how breaths are triggered, cycled, and limited. All modes allow determination of either the pressure or volume limit. Assist control and synchronized intermittent mandatory ventilation (SIMV) are volume cycled, in which a fixed volume is delivered to the patient by the machine using the necessary inspiratory pressure. Pressure control and

pressure support are pressure cycled, in which a known pressure limit is imposed and volume delivered by the machine may vary. Continuous positive airway pressure does not alter pressure or deliver a fixed volume to the patient. Assist control and SIMV differ by the response to patient initiated breaths. Both will deliver a fixed volume when the patient does not initiate a breath. However, with SIMV, if the patient is breathing at a rate greater than set on the machine, each spontaneous breath is dependent completely on patient effort. On assist control, each patient initiated breath above the set rate is supported by the machine by delivering the set rate. In patients with a high respiratory rate, this can result in hyperventilation, and intrinsic PEEP because of inadequate time for exhalation of the full tidal volume. In the patient described, because each breath that is either initiated by the patient or the machine is at a set rate and a fixed volume, this is most consistent with the assist control mode of mechanical ventilation.

1-140. The answer is D. (Chap. 269) Determining when an individual is an appropriate candidate for a spontaneous breathing trial is important for the care of mechanically ventilated patients. An important initial step in determining if a patient is likely to be successfully extubated is to evaluate the mental status of the patient. This can be difficult if the patient is receiving sedation, and it is recommended that sedation be interrupted on a daily basis for a short period to allow assessment of mental status. Daily interruption of sedation has been shown to decrease the duration of mechanical ventilation. If the patient is unable to respond to any commands or is completely obtunded, the individual is at high risk for aspiration and unlikely to be successfully extubated. In addition, the patient should be hemodynamically stable and the lung injury stable or improving. If these conditions are met, the patient should be on minimal ventilatory support. This includes the ability to maintain the pH between 7.35 and an Sa0₂

greater than 90% while receiving an FI0₂ of 0.5 or less and a PEEP of 5 cmH₂) or less. The presence

of rapid shallow breathing during a spontaneous breathing trial identifies patients who are less likely to be extubated successfully.

1-141. The answer is B. (Chap. 269) Patients initiated on mechanical ventilation require a variety of supportive measures. Sedation and analgesia with a combination of benzodiazepines and narcotics are commonly used to maintain patient comfort and safely while mechanically ventilated. Recent studies have shown the utility of minimizing sedation in critically ill patients. However, adequate pain control is an essential component of patient comfort. In addition, patients are immobilized and are thus at high risk for development of deep venous thrombosis and pulmonary embolus. Prophylaxis with unfractionated heparin or low-molecular-weight heparin should be administered subcutaneously. Prophylaxis against diffuse gastrointestinal mucosal injury is also indicated, particularly in individuals with neurologic insult and those with severe respiratory failure and adult respiratory distress syndrome. Gastric acid suppression can be managed with H₂-receptor antagonists, proton pump inhibitors, and

sucralfate. It is also recommended that individuals who are expected to be intubated for more than 72 hours receive nutritional support. Prokinetic agents are often required. Frequent positional changes and close surveillance for skin breakdown should be instituted in all intensive care units to minimize development of decubitus ulcers. In the past, frequent ventilator circuit changes had been studied as a measure for prevention of ventilator-associated pneumonia, but they were ineffective and may even have increased the risk of ventilator-associated pneumonia.

1-142. The answer is D. (Chap. 269) Mechanical ventilation is frequently used to support ventilation in individuals with both hypoxemic and hypercarbic respiratory failure. Mechanical ventilators provide

warm, humidified gas to the airways in accordance with preset ventilator settings. The ventilator serves as the energy source for inspiration, but expiration is a passive process, driven by the elastic recoil of the lungs and chest wall. PEEP may be used to prevent alveolar collapse on expiration. The physiologic consequences of PEEP include decreased preload and decreased afterload. Decreased preload occurs because PEEP decreases venous return to the right atrium and may manifest as hypotension, especially in an individual who is volume depleted. In addition, PEEP is transmitted to the heart and great vessels. This complicated interaction leads to a decrease in afterload and may be beneficial to individuals with depressed cardiac function. When using mechanical ventilation, the physician should also be cognizant of other potential physiologic consequences of the ventilator settings. Initial settings chosen by the physician include mode of ventilation, respiratory rate, fraction of inspired oxygen, and tidal volume if volume-cycled ventilation is used or maximum pressure if pressure-cycled ventilation is chosen. The respiratory therapist also has the ability to alter the inspiratory flow rate and waveform for delivery of the chosen mode of ventilation. These choices can have important physiologic consequences for the patient. In individuals with obstructive lung disease, it is important to maximize the time for exhalation. This can be done by decreasing the respiratory rate or decreasing the inspiratory time (decrease the inspiration-to-expiration ratio, prolong expiration), which is accomplished by increasing the inspiratory flow rate. Care must also be taken in choosing the inspired tidal volume in volume-cycled ventilatory modes because high inspired tidal volumes can contribute to development of acute lung injury caused by overdistention of alveoli.

1-143. The answer is B.(Chap. 269) Patients intubated for respiratory failure because of obstructive lung disease (asthma or chronic obstructive pulmonary disease) are at risk for the development of intrinsic positive end-expiratory pressure (auto-PEEP). Because these conditions are characterized by expiratory flow limitation, a long expiratory time is required to allow a full exhalation. If the patient is unable to exhale fully, auto-PEEP develops. With repeated breaths, the pressure generated from auto-PEEP continues to rise and impedes venous return to the right ventricle. This results in hypotension and increases the risk for pneumothorax. Both of these conditions should be considered when evaluating this patient. However, because breath sounds are heard bilaterally, pneumothorax is less likely, and tube thoracostomy is not indicated at this time. Development of auto-PEEP has most likely occurred in this patient because the patient is currently agitated and hyperventilating as the effects of the paralytic agent wear off. In AC mode ventilation, each respiratory effort delivers the full tidal volume of 550 mL, and there is a decreased time for exhalation, allowing auto-PEEP to occur. Immediate management of this patient should include disconnecting the patient from the ventilator to allow the patient to fully exhale and decreasing the auto-PEEP. A fluid bolus may temporarily increase the blood pressure but would not eliminate the underlying cause of the hypotension. After treatment of the auto-PEEP by disconnecting the patient from the ventilator, sedation is important to prevent further occurrence of auto-PEEP by decreasing the respiratory rate to the set rate of the ventilator. Sedation can be accomplished with a combination of benzodiazepines and narcotics or propofol. Initiation of vasopressor support is not indicated unless other measures fail to treat the hypotension and it is suspected that sepsis is the cause of hypotension.

1-144. The answer is A.(Chaps. 5 and 269) To obtain a stable airway for invasive mechanical ventilation, patients must safely undergo endotracheal intubation. In most patients, paralytic agents are used in combination with sedatives to accomplish endotracheal intubation. Succinylcholine is a depolarizing neuromuscular blocking agent with a short half-life and is one of the most commonly used

paralytic agents. However, because it depolarizes the neuromuscular junction, succinylcholine cannot be used in individuals with hyperkalemia because the drug may cause further increases in the potassium level and potentially fatal cardiac arrhythmias. Some conditions in which it is relatively contraindicated to use succinylcholine because of the risk of hyperkalemia include acute renal failure, crush injuries, muscular dystrophy, rhabdomyolysis, and tumor lysis syndrome. Acetaminophen overdose is not a contraindication to the use of succinylcholine unless concomitant renal failure is present.

1-145. The answers are 1-C; 2-B; 3-D; 4-A.(Chaps. 270, 271, and 272) A variety of vasopressor agents are available for hemodynamic support. The effects of these medications depend on their effects on the sympathetic nervous system to produce changes in heart rate, cardiac contractility, and peripheral vascular tone. Stimulation of 04 adrenergic receptors in the peripheral vasculature causes

vasoconstriction and improves MAP by increasing systemic vascular resistance. The Pj receptors are

located primarily in the heart and cause increased cardiac contractility and heart rate. The P₂ receptors

are found in the peripheral circulation and cause vasodilatation and bronchodilation. Phenylephrine acts solely as an a-adrenergic agonist. It is considered a second-line agent in septic shock and is often used in anesthesia to correct hypotension after induction of anesthesia. Phenylephrine is also useful for spinal shock. The action of dopamine depends on the dosage used. At high doses, dopamine has high affinity for the a receptor, but at lower doses (<5 |ig/kg/min), it does not. In addition, dopamine acts at Pj

receptors and dopaminergic receptors. The effect on these receptors is greatest at lower doses. Norepinephrine and epinephrine affect both a and Pj receptors to increase peripheral vascular

resistance, heart rate, and contractility. Norepinephrine has less Pj activity than epinephrine or

dopamine and thus has less associated tachycardia. Norepinephrine and dopamine are the recommended first-line therapies for septic shock. Epinephrine is the drug of choice for anaphylactic shock. Dobutamine is primarily a Pj agonist with lesser effects at the P₂ receptor. Dobutamine increases

cardiac output through improving cardiac contractility and heart rate. Dobutamine may be associated with development of hypotension because of its effects at the P₂ receptor causing vasodilatation and

decreased systemic vascular resistance.

1-146 and 1-147. The answers are В and C, respectively.(Chap. 270) Hypovolemic shock is the most common form of shock and occurs either because of hemorrhage or loss of plasma volume in the form of gastrointestinal, urinary, or insensible losses. Symptoms of hemorrhagic and nonhemorrhagic shock are indistinguishable. Mild hypovolemia is considered to be loss of less than 20% of the blood volume and usually presents with few clinical except save for mild tachycardia. Loss of 20-40% of the blood volume typically induces orthostasis. Loss of more than 40% of the blood volume leads to the classic manifestations of shock, which are marked tachycardia, hypotension, oliguria, and finally obtundation. Central nervous system perfiision is maintained until shock becomes severe. Oliguria is a very important clinical parameter that should help guide volume resuscitation. After assessing for an adequate airway and spontaneous breathing, initial resuscitation aims at reexpanding the intravascular volume and controlling ongoing losses, Volume resuscitation should be initiated with rapid IV infusion of isotonic saline or Ringer's lactate. In head-to-head trials, colloidal solutions have not added any benefit compared with crystalloid and in fact appeared to increase mortality for trauma patients. Hemorrhagic shock with ongoing blood losses and a hemoglobin of 10 g/dL or less should be treated with transfusion of packed red blood cells (PRBCs). After hemorrhage is controlled, transfusion of

PRBCs should be performed only for hemoglobin of 7 g/dL or less. Patients who remain hypotensive after volume resuscitation have a very poor prognosis. Inotropic support and intensive monitoring should be initiated in these patients. An algorithm for the resuscitation of a patient in shock is shown in Figure 1-148.

VS UnslablQ HR >f 20 andV'Qr SBP<00

Genual monrton'ng

CVP<15

* Airway control

* Assure ventHalton

* Augment circulalion (crystefloid +/- Wood)

V£ Normalised

CVP=-15

Achm ini&ier crystalloid +t- blood 'Hct*30 ■CVP?1S

Gor&ifef cardiac dysfunction

иг lampOriade

■ECHO

• Treal appropriately

VS unstable or acidosis worsens —*■ insert РАС ■*

i

Cl<3 5;PCWP<l6

Cl<3&: i$< PCWP*2fj

Cl <3.S; PCWP >20

Administer crysiailoid

+/- blood PCWP>1S,Hct>30

Administer 500 mL crystalloid

bofusas until preload -— maximal СI

(Starling's curve}

• Inotrope as indicated

• Consider ECHO

Monitor Ci deterioration

Mainiain optima] PCWP •Crystalloid * stood (Ha *30)

FIGURE 1-148

1-148. The answer is D.(Chap. 270) The patient is in cardiogenic shock from an ST-elevation myocardial infarction. Shock is a clinical syndrome in which vital organs do not receive adequate perfusion. Understanding the physiology underlying shock is a crucial factor in determining appropriate management. Cardiac output is the major determinant of tissue perfusion and is the product of stroke volume and heart rate. In turn, stroke volume is determined by preload, or ventricular filling, afterload, or resistance to ventricular ejection, and contractility of the myocardium In this patient, the hypoxic and damaged myocardium has suddenly lost much of its contractile function, and stroke volume will therefore decrease rapidly, dropping cardiac output. Systemic vascular resistance will increase in

order to improve return of blood to the heart and increase stroke volume. Central venous pressure is elevated as a consequence of increased vascular resistance, decreased cardiac output and poor forward flow, and neuroendocrine-mediated vasoconstriction. The pathophysiology of other forms of shock is shown in Table 1-149 as a comparison.

Date: 2016-04-22; view: 598