The Intensive Care Manual - part 3 potx

SUMMARY Our understanding of shock and SIRS response has evolved to one that is physio- logically based. Resuscitation is now based on close monitoring and hemodynamic support and replacement of intravascular volume. REFERENCES 1. Davies MD, Hagen PO. Systemic inflammatory response syndrome. Brit J Surg 1997;84:920–935. 2. Von Rueden TK, Dunham MC. Evaluation and management of oxygen delivery and consumption in multiple organ dysfunction syndrome in multiple organ dysfunction and failure, 2nd ed. In Secor VH, ed. Mosby Yearbook. St. Louis, MO: 1996:384–401. 3. Bone RC, et al. Definitions for sepsis and organ failure and guidelines for the use of innovative therapies in sepsis. Crit Care Med 1992;20:864–874. 4. Reddy PS, Curtiss EL, O’Toole JD, et al. Cardiac tamponade: Hemodynamic observa- tions in man. Circulation 1978;8:265–269. 5. Eisenberg MJ, Schiller NB. Bayes theorem and the echocardiographic diagnosis of cardiac tamponade. Am J Cardiol 1991;68:1242–1250. 6. Iberti TJ, Leibowitz AB, Papadakos PJ, et al. Low sensitivity of the anion gap as a screen to detect hyperlactemia in critically ill patients. Crit Care Med 1990; 18:275–277. 7. Rose S, Illerhaus M, Wiercinski A, et al. Altered calcium regulation and function of human neutrophils during multiple trauma. Shock 2000;13:92–99. 8. Muller-Berghaus G. Pathophysiologic and biochemical events in disseminated intravascular coagulation: dysregulation of procoagulant and anticoagulant pathways. Seminar Thromb Hemost 1989;15:58–70. 9. Rasmussen HH, Ibel LS. Acute renal failure: Multivariate analysis of causes and risk factors. Am J Med 1982;733:211–218. 10. Cariou A, Mondi M, Luc-Marle J, et al. Noninvasive cardiac output monitoring by aortic blood flow determination: Evaluation of the Sometec Dynemo-3000 system. Crit Care Med 1988;12:2066–2072. 11. Packman MI, Rackow EC. Optimum left heart filling pressure during fluid resuscitation of patients with hypovolemic and septic shock. Crit Care Med 1983;11:165–169. 12. Cochran Injuries Group, Albumin Reviewers. Human albumin administration in critically ill patients: Systemic review of randomized controlled trials. Brit Med J 1998;317:235–40. 13. Treib J, Haass A, Pindur G, et al. All medium starches are not the same: Influence of the degree of hydroxyethyl substitution of hydroxyethyl starch on plasma volume, hemorrheologic conditions, and coagulation transfusion. Transfusion 1996;36:450–455. 14. Mattox KL, Maninagas PA, Moore EE, et al. Prehospital hypertonic saline/dextran infusion for post–traumatic hypotension: The USA multicenter trial. Ann Surg 1991; 213:482–491. 15. Drobin D. Volume kinetics of Ringer’s solution in hypovolemic volunteers. Anesthesi- ology 1999;90:81–91. 3 / Shock 69 ch03.qxd 11/7/01 4:09 PM Page 69 16. Funk W, Balinger V. Microcirculatory perfusion using crystalloid or colloid in awake animals. Anesthesiology 1995;82:975–982. 17. Britt LD, Weireter LJ, Riblet JL, et al. Complex and challenging problems in trauma surgery. Surg Clin N Am 1996;76:645–660. 18. Lund N, DeAsla RJ, Guccione AL, et al. The effect of dopamine and dobutamine on skeletal muscle oxygenation in normoxemic rats. Cir Shock 1991;33:164–170. 19. Zeni F, Freeman B, Natanson C. Anti-inflammatory therapies to treat sepsis and septic shock: A reassessment. Crit Care Med 1997;25:1095–1100. 70 The Intensive Care Manual ch03.qxd 11/7/01 4:09 PM Page 70 INTRODUCTION INVASIVE MECHANICAL VENTILATION Indications Objectives Modes Settings Mechanical Ventilation for Specific Conditions Complications Discontinuation of Mechanical Ventilation NONINVASIVE MECHANICAL VENTILATION Indications and Objectives Modes 71 CHAPTER 4 Approach to Mechanical Ventilation ANTHONY P. P IETROPAOLI Ventilator Settings Complications Discontinuation of Noninvasive Mechanical Ventilation CONCLUSION ch04.qxd 11/7/01 4:12 PM Page 71 Copyright 2001 The McGraw-Hill Companies. Click Here for Terms of Use. INTRODUCTION Mechanical ventilation is defined as the use of a mechanical device to assist the respiratory muscles in the work of breathing and to improve gas exchange. In this chapter, mechanical ventilation is divided into two techniques: one requiring a tube in the trachea to deliver ventilation (invasive) and another applied with a mask (noninvasive). The indications, objectives, modes, settings, complications, and discontinuation strategies are reviewed for both invasive and noninvasive mechanical ventilation and some disease-specific strategies for invasive mechanical ventilation. INVASIVE MECHANICAL VENTILATION Indications Mechanical ventilation is indicated to support the patient with respiratory failure when adequate gas exchange cannot otherwise be maintained. As reviewed in chapter 1, there are two major categories of acute respiratory failure: hypoxemic (type 1) and hypercapneic (type 2). Patients with either of these often need mechanical ventilation. Many patients present with a mixture of the two types of respiratory failure, and of course, these patients also respond to mechanical ventilation. Invasive mechanical ventilation is often chosen over noninvasive methods when altered mental status or hemodynamic instability accompany acute respiratory failure. The timing of intubation and initiation of mechanical ventilation is a source of controversy, and the decision is often more a matter of art and experience than science. Tracheal intubation is indicated for situations other than provision of mechanical ventilation, such as to provide airway protection and relieve upper airway obstruction. 1 Table 4–1 lists some commonly accepted indications for endotracheal intubation and mechanical ventilation. Objectives Mechanical ventilation is supportive and meant to reverse abnormalities in respiratory function, while specific therapies are used to treat the underlying cause of respiratory failure. The physiologic goals of mechanical ventilation are reversal of gas exchange abnormalities, alteration of pressure-volume relationships in the respiratory system, and reduction in the work of breathing. 2 These physiologic goals are interrelated and attain specific clinical results, as shown in Figure 4–1. Other goals in specialized circumstances include allowing use of heavy sedation or neuromuscular blockade and stabilization of the chest wall when injury has disrupted its mechanical function. 2 72 The Intensive Care Manual ch04.qxd 11/7/01 4:12 PM Page 72 4 / Mechanical Ventilation 73 TABLE 4–1 Indications for Intubation and Invasive Mechanical Ventilation • Cardiac arrest • Respiratory arrest • Refractory hypoxemia (unresponsive to maximal supplemental oxygen administration and noninvasive ventilatory support) • Progressive respiratory acidosis (unresponsive to medical therapy, oxygen administration, and noninvasive ventilatory support) • Symptoms of progressive respiratory fatigue (unresponsive to medical therapy, oxygen administration, and noninvasive ventilatory support) • Clinical signs of respiratory failure (unresponsive to medical therapy, oxygen administration, and noninvasive ventilatory support) • Tachypnea • Use of accessory muscles (e.g., sternocleidomastoid, scalene, intercostal, abdominal) • Paradoxical inward abdominal movement during inspiration • Progressive alteration of mental status • Inability to speak in full sentences • Airway protection (in a patient with an extremely impaired level of consciousness) • Relief of upper airway obstruction (often manifested by stridor on physical examina- tion) FIGURE 4–1 Objectives of mechanical ventilation. Interrelationship between physiologic objectives of mechanical ventilation is shown. By accomplishing each of these physiologic objectives, specific clinical goals are met. (Adapted with permission from Slutsky AS. ACCP consensus conference: Mechanical ventilation. Chest 1993; 104(6):1833–1859. ch04.qxd 11/7/01 4:12 PM Page 73 Modes Mechanical ventilators were popularized during the polio epidemics of the 1950s. The initial ventilators were primarily negative pressure ventilators, or “iron lungs.” Later, positive pressure ventilators gained popularity and today are used almost exclusively. As ventilator technology has progressed, the ways of delivering positive pressure mechanical ventilation have proliferated. In daily practice, however, four basic modes of positive pressure ventilation are most commonly used. These modes can be classified on the basis of how they are triggered to deliver a breath, whether these breaths are targeted to a set volume or pressure, and how the ventilator cycles from inspiration to expiration (Table 4–2). CONTROLLED MECHANICAL VENTILATION Controlled mechanical ventilation (CMV) is included here only for the purposes of instruction. CMV, or volume control (VC), was the first volume-targeted mode (Figure 4–2a). As its name suggests, it is a pure “control” mode; that is, the minute ventilation (VE,) is completely governed by the machine (VE = VT × respiratory rate). The physician sets the respiratory rate, tidal volume, inspiratory flow rate, ratio of inspiratory to expiratory time (I:E) fraction of inspired oxygen (FIO 2 ), and positive end- expiratory pressure (PEEP). In VC, the patient is unable to trigger the ventilator to deliver additional breaths. This mode works well for patients who are unresponsive or heavily sedated, but not for conscious patients, whose respiratory efforts are not sensed by the ventilator, which leads to patient discomfort and increased work of breathing. As a result, this mode has largely been abandoned. ASSIST-CONTROL VENTILATION This mode is similar to VC mode except that the ventilator senses respiratory efforts by the patient (Figure 4–2b). As in VC, the physician sets a respiratory rate, tidal volume, flow rate, I:E, F IO 2 , and 74 The Intensive Care Manual TABLE 4–2 Basic Modes of Mechanical Ventilation Mode Trigger Target Cycle Volume control a Ventilator VT Time and VT Assist-control a Ventilator ± patient VT Time and VT SIMV a Ventilator ± patient VT/VI (SIMV Time and VT/VT breaths only) (SIMV breaths only) Pressure-control b Ventilator ± patient Inspiratory pressure Time Pressure-support c Patient Inspiratory pressure Flow ABBREVIATIONS: SIMV, synchronized intermittent mandatory ventilation; VT, tidal volume; ± = with or without. a All volume-targeted modes cycle from inspiration to expiration at the end of inspiratory time, which corresponds to the instant that the VT is reached. The target VT is achieved by setting a fixed inspiratory flow for a fixed inspiratory time interval. b In pressure-control mode, the desired pressure is achieved almost immediately after the onset of inspiration. Target pressure is maintained for the duration of set inspiratory time. c In pressure-support mode, the pressure target is maintained until inspiratory flow falls to about 20% of peak flow. Inspiratory time varies from breath to breath. ch04.qxd 11/7/01 4:12 PM Page 74 4 / Mechanical Ventilation 75 FIGURE 4–2 Airway opening pressure (PaO), lung volume (V), and inspiratory (I), and expiratory (E) flow rate (V) versus time during mechanical ventilation. a. Volume control (VC), also known as controlled mechanical ventilation (CMV). During both breaths shown, defined tidal volume (V T) and inspiratory flow rate are delivered, resulting in Pa O 2 shown. In this mode, ventilator does not detect patient efforts. A reduction in airway pressure from patient effort (arrow) does not result in significant V T or inspiratory flow. b. Assist-control (AC) ventilation. Notice that ventilator senses decrease in airway pressure induced by patient effort (indicated by arrow) and delivers same V T and flow in response. a. b. ch04.qxd 11/7/01 4:12 PM Page 75 76 The Intensive Care Manual FIGURE 4–2 (continued) c. Synchronized intermittent mandatory ventilation (SIMV). First breath is ventilator- delivered in absence of patient effort. Next, patient effort causes decrease in Pa O during synchronization period (boxes), so fully supported breath is delivered. Next effort occurs outside of synchronization period, and patient breathes spontaneously. Resulting volume and pressure are completely patient-generated. Last breath is identical to first, delivered according to set respiratory rate. End of synchronization period coincides with onset of the back-up SIMV breath. d. Pressure-control (PC) ventilation. Airway pressure is set, and V T and flow rate that result are variable and depend on inspiratory time, airway resistance, respiratory system compliance, and patient effort. In example shown, patient is relaxed. First breath is delivered automatically by ventilator, based on fixed back-up respiratory rate. Second breath is delivered early, when patient lowers airway pressure and triggers ventilator (arrow). c. d. ch04.qxd 11/7/01 4:12 PM Page 76 PEEP. Breaths are delivered automatically, regardless of patient effort (“control”). In assist-control (AC) mode, however, the ventilator detects patient effort and responds by delivering a breath identical to the controlled one (“assist”). The patient can therefore breathe faster than the back-up control rate, but all breaths have the same tidal volume, flow rate, and inspiratory time. So AC mode allows better synchrony between patient and ventilator than VC mode, while still pro- viding a baseline minute ventilation. A more descriptive and accurate name for this mode is “volume-targeted assist-control ventilation.” However, the term “AC” is well entrenched and likely will not be replaced by this more cumbersome name. Like all modes of mechanical ventilation, AC has disadvantages. If the back-up respiratory rate is set too far below the patient’s spontaneous rate, exhalation time progressively decreases, since inspiratory time is fixed by the back-up respi- 4 / Mechanical Ventilation 77 FIGURE 4–2 (continued) e. Pressure-support (PS) ventilation. Inspiratory pressure is fixed in this mode, as in pressure- control mode. However, this mode is flow-cycled instead of time-cycled. Inspiratory pressure ceases when inspiratory flow rate decreases to about 20% of its peak. V T and flow are determined by inspiratory pressure, airway resistance, respiratory system compliance, and patient effort. First breath shows moderate inspiratory effort. In second example, patient makes a prolonged inspiratory effort, resulting in more prolonged delivery of inspiratory pressure and a larger V T. Third example shows rapid deep breath, resulting in very high peak inspiratory flow rate but short duration of inspiratory pressure. The resulting V T is midway between other two examples. (Modified with permission, from Schmidt GA, Hall JB. Management of the ventilated patient. In Hall JB, Schmidt GA, Wood LDH, eds. Principles of critical care, 2nd ed. New York: McGraw-Hill, 1998:517–535.) e. ch04.qxd 11/7/01 4:12 PM Page 77 ratory rate and flow rate. In the extreme, this may result in inadequate time for exhalation (Figure 4–3). As a result, lung volume remains above functional residual capacity (FRC) when the next breath is delivered, a process called dynamic hyperinflation. 2 This increased lung volume is associated with elevation in the alveolar pressure at end-exhalation, or “auto-PEEP” (Figure 4–3). The adverse consequences of these events are discussed later. Another problem occurs when patients with high minute ventilation requirements make persistent inspiratory efforts while a breath is being delivered. If this effort is strong enough, the patient 78 The Intensive Care Manual FIGURE 4–3 Dynamic hyperinflation and auto-PEEP (positive end-expiratory pressure) result from inadequate exhalation time. Simplified schematic shows two lung units, consisting of alveolus and airway, both at end exhalation. In a, there is adequate time for complete exhalation to resting lung volume, or functional residual capacity (FRC). The alveolar pressure is zero, or equal to level of externally applied PEEP. In b, there is inadequate time for exhalation. This occurs when exhalation time is too short and/ or time required to exhale to FRC is pathologically prolonged. Former occurs during mechanical ventilation when inspiratory time is too long or respiratory rate is too high; latter occurs in obstructive lung diseases, like chronic obstructive pulmonary disease (COPD) and asthma. In either case, lung volume remains above FRC at end exhalation (dynamic hyperinflation), resulting in abnormally elevated P A (auto-PEEP). ch04.qxd 11/7/01 4:12 PM Page 78 [...]... of the straps and accepting a small air leak and by placement of a wound dressing over the bridge of the nose .33 Gastric distention is less likely if peak mask pressure is kept below 30 cm H2O .33 Routine placement of nasogastric tubes for gastric decompression are not considered necessary .33 Manipulation of the mask to direct air leakage inferiorly toward the mouth rather than superiorly toward the. .. (as might be expected in the example) causes the patient to “tug” on the ventilator, thus increasing the work of breathing During volume-targeted ventilation, the prescribed flow rate cannot be exceeded If patient demand for inspiratory flow exceeds the set rate, ch04.qxd 11/7/01 86 4:12 PM Page 86 The Intensive Care Manual the patient’s efforts will be ineffective, increasing the likelihood of patient... trigger sensitivity When the ventilator detects patient effort, it delivers a breath identical to the backup-controlled breaths, allowing the patient to breathe faster than the back-up rate Tidal volume is determined by IP, inspiratory time, ∆V airway resistance, respiratory system compliance ( ∆P), and patient effort The de- ch04.qxd 11/7/01 80 4:12 PM Page 80 The Intensive Care Manual livered volume... positive-pressure ventilation in patients with acute respiratory failure Clin Chest Med 1996; 17 (3) : 537 –542 ch04.qxd 11/7/01 100 4:12 PM Page 100 The Intensive Care Manual Complications NIMV is characterized by a lower risk of complications than invasive mechanical ventilation .33 The most common adverse events in patients undergoing NIMV are facial skin necrosis, gastric distention, and conjunctivitis .33 ... is provided by the ventilator While this reduces the likelihood of air-trapping and breath-stacking, it also can increase the work of breathing Interestingly, if the mandatory respiratory rate is less than approximately 80% of the patient’s actual rate, the high level of work expended during the spontaneous breaths will also be expended during the mandatory breaths .3 This occurs because the respiratory... 10 30 cm H2O 0 .3 1.0% 3 20 cm H2O Patient comfort, pH > 7.25, avoid auto-PEEP Plateau pressure ≤ 35 cm H2O IP FIO2 PEEP TS Flow rate I:E Pressure: −1–2 cm H2O Flow −1 3 L/min 40–100 L/min 1:1.5 to 1 :3 Plateau pressure ≤ 35 cm H2O O2 sat ≤ 90%, FIO2 ≤ 0.6% Plateau pressure ≤ 35 cm H2O, O2 sat ≥ 90% Patient triggering ventilator effectively Patient comfort; avoid auto-PEEP Patient comfort; avoid auto-PEEP... LONG-TERM OUTCOME ACID-BASE ABNORMALITIES Basics Acidosis Alkalosis APPROACH TO MANAGEMENT OF HYPONATREMIA AND HYPERNATREMIA Hyponatremia Hypernatremia SUMMARY Use of Diuretics and Dopamine Other Measures 1 03 Copyright 2001 The McGraw-Hill Companies Click Here for Terms of Use ch05.qxd 11/7/01 104 4:12 PM Page 104 The Intensive Care Manual INTRODUCTION Acute renal failure is a common diagnosis in the. .. airway pressure (BiPAP) provides different inspiratory and expiratory pressures The inspiratory assistance can be either time-cycled (pressure-control ventilation) or flow-cycled (pressure-support ventilation) The ventilator is triggered when the patient makes an inspiratory effort The methods of patient triggering (either reduction in airway pressure or baseline airflow) are similar to those used... critical care, 2nd ed New York: McGrawHill, 1998; 517– 535 .) on mechanical ventilation is to separate problems with the patient and endotracheal tube from problems with the ventilator This can be done by simply disconnecting the patient from the machine and ventilating by hand with a bag-valve-mask apparatus.4 However, the patient should not be bagged too vigorously, because it may cause auto-PEEP and... asking the patient to make a maximal inspiratory effort against an occluded airway from resting lung volume and then measuring the pressure generated at the mouth Poor patient cooperation limits the reliability of this test A one-way valve, allowing expiration but not inspiration, permits performance of the test in uncooperative patients b Sensitivity is the likelihood of meeting the threshold if the . If this effort is strong enough, the patient 78 The Intensive Care Manual FIGURE 4 3 Dynamic hyperinflation and auto-PEEP (positive end-expiratory pressure) result from inadequate exhalation. modes is beyond the scope of this chapter, and the reader is referred to in-depth discussions of mechanical ventilation 5 and a re- cent review article. 6 80 The Intensive Care Manual ch04.qxd. except that the ventilator senses respiratory efforts by the patient (Figure 4–2b). As in VC, the physician sets a respiratory rate, tidal volume, flow rate, I:E, F IO 2 , and 74 The Intensive Care Manual TABLE

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