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									                           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

               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.
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  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) •
                                                           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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