Mechanical ventilation

 Mechanical ventilation: (1) Parameters: (a) Rate: Number of mechanical breaths delivered per minute (b) FiO 2 : Fraction of oxygen in inspired gas (c) PIP: Peak inspiratory pressure attained during respiratory cycle (d) Positive end-expiratory pressure (PEEP): Distending pressure that increases functional residual capacity (FRC), or volume of gas at the end of exhalation (e) Mean airway pressure (P aw ): Average airway pressure over entire respiratory cycle, which correlates to mean alveolar volume (f) Tidal volume (V T ): Volume of gas delivered during inspiration (g) Time: May indicate a function of time spent in inspiration (T in high pressure (T high ), or in low pressure (T low ) i (2) Modes of ventilation: (a) Controlled ventilation: Ventilation is completely mechanical with no spontaneous ventilation efforts expected from the patient.

(i) Pressure-controlled ventilation (PCV): A preset respiratory rate and T i deliver a pressure-limited breath (the set pressure is maintained during inspiration). V T is determined by the preset pressure as well as lung compliance and resistance. (ii) Volume-controlled ventilation (VCV): A preset respiratory rate and T i deliver a preset V T (b) Intermittent mandatory ventilation (IMV): Allows the patient to breathe spontaneously between a preset number of (mandatory) mechanical breaths (i) Synchronized IMV (SIMV): If patient initiates spontaneous breath, mandatory breath is synchronized with patient effort rather than spaced evenly over each minute. (ii) If spontaneous breathing rate is less than mandatory rate, some mandatory breaths will be delivered in the absence of patient effort. (iii) Delivered breaths may be volume regulated or pressure limited. (c) Airway-pressure-release ventilation (APRV): Most of the respiratory cycle is spent at a high distending pressure (a functionally high CPAP phase) with intermittent, short release to a low pressure for a brief ventilation phase. Spontaneous breathing can be superimposed at any point in the cycle. (d) Support ventilation: Mechanical breaths support patientinitiated breaths, but no mandatory breaths are provided. (i) Pressure support (PS): Delivers a preset amount of pressure to assist spontaneous respiratory effort (ii) Often used in combination with other modes of ventilation to support spontaneous breaths greater than preset respiratory rates (e) High-frequency oscillatory ventilation (HFOV): Gas flow pressurizes the system to the preset P aw while a piston moves backward and forward to force and withdraw a small V T (that approximates anatomic dead space) into the lungs at rates exceeding normal respiratory rates. (3) Management: The three subdivisions of mechanical ventilatory support are the acute (lung recruitment), maintenance (lung recovery), and weaning phases. (a) Acute: See Table 1.3 for ventilation parameter initial settings and titration effects. (b) Maintenance: To avoid volutrauma, barotrauma, or oxygen toxicity, maintain V T FiO 2 ≤60%.

MECHANICAL VENTILATION PARAMETER SETTINGS AND EFFECTS Parameter Initial Setting 13 16 19 Effect of [ on PaCO 2 15 Effect of [ on PaO 2 PIP PEEP V T Rate I:E ratio FiO 2 ≤28 cmH 2 O or ≤29e32 cmH wall compliance 3e5 cmH 2 O 2 O for reduced chest YY [ 5e8 mL/kg or 3e6 mL/kg for poorly compliant lungs YY Normal rate for age (33%) 1:2 (67%) <50% and/or to maintain PaO 100 mmHg and SpO HIGH-FREQUENCY VENTILATION PARAMETERS Amplitude (DP) aw Set to produce a visible wiggling motion to the level of the lower abdomen YY No change 2 2 ≥95% between 80 and No change Y Frequency (Hz) Range from 3e20 Hz (180e1200 breaths per min) [[ P 5 cmH FiO 2 2 O > P aw of previous conventional ventilation Minimal Y , Fraction of inspired oxygen; I:E, inspiratory to expiratory; Hz, hertz; P aw of carbon dioxide (arterial); PaO 2 , mean airway pressure; PaCO [ [[ [ Minimal [ [ [[ No change Y [ 2 , partial pressure , partial pressure of oxygen (arterial); PEEP, positive end-expiratory pressure; PIP, peak inspiratory pressure; V T , tidal volume.

Categorized into four basic types: 1. Hypovolemic: Inadequate fluid intake, increased fluid losses (hemorrhage, gastroenteritis, burns). Assess for tachycardia, narrow pulse pressure, delayed capillary refill, cool extremities. 2. Cardiogenic: Congenital heart disease, myocarditis, cardiomyopathy, arrhythmia. Assess for increased respiratory effort from pulmonary edema, hepatomegaly, jugular venous distention, and cyanosis. 3. Distributive: Sepsis, anaphylaxis, neurogenic (e.g., high cervical spine injury) a. Assess for tachycardia, fever, and petechial, purpuric, or urticarial rash. b. Warm septic shock is characterized by bounding peripheral pulses, f lash capillary refill, and wide pulse pressure. c. Cold septic shock is characterized by decreased peripheral pulses, delayed capillary refill, and narrow pulse pressure. d. Neurogenic shock is characterized by hypotension with a wide pulse pressure, normal HR or bradycardia, and hypothermia. 4. Obstructive: Tension pneumothorax, cardiac tamponade, pulmonary embolism, ductal-dependent congenital cardiac abnormalities a. Early clinical presentation is indistinguishable from hypovolemic shock. Progression of shock leads to signs and symptoms similar to cardiogenic shock. b. Cardiac tamponade is characterized by muffled heart sounds and pulsus paradoxus. c. Ductal-dependent lesions may be characterized by higher preductal versus postductal BP or arterial oxygen saturation. C. Management 1. Administer 100% supplemental oxygen initially regardless of oxygen saturation to optimize oxygen delivery. Once perfusion restored, titrate as able to avoid adverse effects from hyperoxia. 2. See Table 1.4 for type- and etiology-specific pathophysiology and management of shock. 3. See Table 1.5 for vasoactive agents to support cardiac output. Vasoactive agents affect SVR (vasodilators and vasoconstrictors), cardiac contractility (inotropes), or HR (chronotropes). Some agents increase blood flow via contractility and vasodilation (inodilators) or increase perfusion pressure via contractility and vasoconstriction (inoconstrictors).