According to a large international study, the most common reasons for initiating mechanical ventilation are acute respiratory failure, coma, complications of chronic obstructive pulmonary disease, and neuromuscular disorders.3 Airway protection, airway obstruction, the use of general anesthesia, hemodynamic instability, hypoxemia, metabolic acidosis, and the need for pulmonary toilet are also cited as general indications for mechanical ventilation.4 Many of these clinical indications are determined by the discovery of abnormal ABG values. If a patient's VT is inadequate, the pressure can be increased to augment his or her effort in order to achieve larger volumes.\n6 Although recent research by Blackwood and colleagues found wide differences with regard to weaning protocols in modern ICUs,6 four objectives are generally agreed upon as essential for successfully weaning and liberating a patient from mechanical ventilation: adequate oxygenation and ventilation, hemodynamic stability, adequate mental status, and resolution of correctable conditions.5 Weaning involves a stepwise reduction in the amount of ventilator support and evaluation of the patient's response, most often by serial ABG measurements.
The Surgical Patient Mechanical ventilation in adults who need respiratory assistance Richard G. Winters, MSPAS, PA-C; Donald A. Reiff, MD (HCO3). Advanced age and chronic respiratory conditions may alter these parameters, but for most patients, arterial T he ﬁrst mention of artiﬁcial respiration was blood gas (ABG) values consistent with the aforementioned made in 1555 by Andreas Vesalius, who de- criteria are indications for respiratory assistance, most scribed inserting a reed through an opening often requiring endotracheal intubation and mechanical made in the trachea and blowing into it, caus- ventilation. ing the lung to rise again. Not until 1928 did Functionally, mechanical ventilation takes over the pro- Vesalius’ vision become a reality, when the ﬁrst iron lung, cess of respiration for the patient and provides support to developed by Philip Drinker, was used to assist patients help correct the acute respiratory abnormalities that cause who were paralyzed as a result of poliomyelitis. Advances hypoxemia and hypercapnia. Correction of hypoxemia in science and technology since the days of the iron lung is accomplished through the delivery of air with an O2 have led to the modern era of mechanical ventilation and, content greater than normal and the application of posi- with it, the era of ICUs and critical care medicine. tive end-expiratory pressure (PEEP). Correction of acute More than 5 million people are admitted annually to the respiratory acidosis is achieved by controlling the patient’s nearly 6,000 ICUs in the United States, and admissions respiratory rate and tidal volume (VT) to achieve adequate are expected to rise as the population ages.1 This large and minute ventilation (MV). MV is the volume of air expired growing number of patients, coupled with the increasing per minute and is calculated by multiplying the VT of each use of PAs to take care of them, means that PAs need to breath by the number of respirations per minute. Normal understand the objectives of mechanical ventilation, the MV values range from 5 to 8 L/min and vary in response indications for initiating it, and the protocols for weaning to the production of CO2. In postoperative, severely ill, or patients from it. Clinicians should also be familiar with the injured patients, increased metabolic demand often leads most common ventilation modes available. to CO2 production in excess of the body’s ability to regu- late, resulting in hypercapnia, acute respiratory acidosis, PHYSIOLOGY and respiratory failure. Mechanical ventilation is designed to assist patients with the Continued on page 44 most fundamental function of the lungs: exchanging oxygen (O2) and carbon dioxide (CO2) with the external environ- ment. Several factors inﬂuence the ability of the lungs to perform this function, including airway resistance, mechani- cal resistance of the chest wall and abdomen, and pulmonary compliance.2 All of these can be greatly affected in the post- operative, acutely ill, or traumatically injured patient, result- ing in impaired gas exchange and acute respiratory failure. In turn, acute respiratory failure can quickly lead to hypoxemia and/or acute respiratory acidosis, both of which are life- threatening conditions that demand immediate intervention. © Fairman Studios LLC Clinically, hypoxemia is deﬁned as PaO2 less than 60 mm Hg and acute respiratory acidosis as arterial blood pH less than 7.25 in a patient breathing room air. Acute respiratory acidosis typically occurs as the PaCO2 rises higher than 50 mm Hg without a compensatory rise in arterial bicarbonate 42 JAAPA • MAY 2010 • 23(5) • www.jaapa.com The Surgical Patient INITIATING MECHANICAL VENTILATION Once the decision has been made to initiate mechanical The objective of mechanical ventilation is to reduce the pa- ventilation, the patient is preferentially intubated via the tient’s work of breathing and reverse life-threatening hypox- orotracheal route, and the initial mode and settings for the emia and/or acute respiratory acidosis. Although mechanical ventilator are determined. Several different protocols for ventilation is the most common intervention used in the treat- initial settings have been established; the most commonly ment of critically ill and postoperative patients with impaired used settings are listed in Table 1. gas exchange, the indications for initiation remain the subject of some disagreement. According to a large international MODES OF MECHANICAL VENTILATION study, the most common reasons for initiating mechanical Assist-control ventilation (ACV) is the most common ventilation are acute respiratory failure, coma, complications
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