Mechanical Ventilation

Chapter 76 Mechanical Ventilation



Rashna F. Ginwalla, MD, Leah Lande, MD, John Mullarkey, RRT, Thomas A. Santora, MD




Cases Considered



1. 80-year-old male admitted for an elective laparoscopic cholecystectomy is found to be breathing at a rate of 35 breaths/min.

2. 25-year-old male status-post gunshot wound to the abdomen, transferred to ICU postoperation, needs proper vent settings.

3. 77-year-old female status-post subtotal colectomy for lower GI bleeding is ready for weaning.

4. 42-year-old male status-post motor vehicle collision (MVC) develops distress on the ventilator 4 days after admission.

5. 57-year-old male with pancreatitis and adult respiratory distress syndrome (ARDS).



Speaking Intelligently


I am frequently asked to render pulmonary critical care consultation in the following settings: when the patient requires (1) urgent intubation and mechanical ventilation, (2) adjustment of mechanical ventilation already instituted, (3) weaning from mechanical ventilation, (4) intervention for an acute problem that has developed related to mechanical ventilation, or (5) the development of ARDS. Accordingly, we have set forth consideration of the five patients above to consider how to speak intelligently and how to proceed in each of these contexts.



Clinical Thinking


When encountering the patient with a respiratory problem, consider:


1. The ABCs—airway, breathing, and circulation.

2. Does this patient have a problem that is life threatening?

3. Is the pulmonary dysfunction related to ventilation or oxygenation?

4. What are the underlying problems that have led to these respiratory abnormalities?



Case 1


An 80-year-old male admitted for an elective laparoscopic cholecystectomy is found to be breathing at a rate of 35 breaths/min; he is diaphoretic with nasal alar flaring, he holds onto the side rails of his stretcher; he has rales up to his midchest on auscultation. You find out that he has not taken his furosemide (“water pill”) during the past week.



Working Dx


Respiratory distress from pulmonary congestion/volume overload.


You decide to place him on 100% O2 by nonrebreather mask while you prepare for urgent intubation to reduce his work of breathing. Once he is intubated, you move him to the ICU to complete the evaluation, which will include CXR, EKG, and cardiac enzyme evaluation to assess for possible acute myocardial infarction as a cause of the pulmonary congestion. In addition, you administer furosemide empirically to reduce the pulmonary congestion.




Medical Knowledge


Satisfactory spontaneous breathing requires a patient to:


1. Oxygenate adequately

2. Remove carbon dioxide sufficiently

3. Protect his/her airway

If these three criteria cannot be met, tracheal intubation and mechanical ventilatory support may be required. In the setting of respiratory distress, careful judgment is needed to determine if and when to initiate mechanical ventilation.


Increased work of breathing, also known as respiratory distress, manifests with the following:


1. Tachypnea

2. Shallow respirations

3. Use of accessory muscles of respiration

4. Nasal alar flaring

5. Subjective dyspnea

Blood gases, while able to provide objective evidence of distress, are the last parameters to deteriorate in the evolution of pulmonary dysfunction. The decision to intubate and initiate mechanical ventilation should be made on clinical appreciation of respiratory distress. See Sabiston 24; Becker 11, 12.




Case 2


A 25-year-old male presents with a gunshot wound to his abdomen. He is intubated and taken to the OR, where his abdominal injuries are addressed definitively by a single resection of his small bowel. He is brought to the ICU on a ventilator at the following settings: assist-control (A/C) mode, RR 12, tidal volume (Vt) 500 mL, fraction of inspired O2 (FIO2) 100%, positive end-expiratory pressure (PEEP) 5. He is sedated and paralyzed. Your baseline ABG returns: pH 7.24, PaCO2 55 mmHg, PaO2 240 mmHg, HCO3 16 mEq/L.



Working Dx


Acidosis with hypoventilation; acceptable oxygenation.




(PaO2 : FIO2 ratio 240; see discussion of ARDS under Case 5.)


To correct the acidosis, you increase the RR to 20, which will increase the minute ventilation from 6 to 10 L/min. You also decrease the FIO2 to 50% to minimize the risk of oxygen toxicity.




Medical Knowledge



Modes of Ventilation/Settings


Editor’s note: Understanding mechanical ventilation first requires familiarization with the vernacular used in the respiratory care field. A glossary of the different types of ventilatory approaches follows. Note that there are two basic modes of ventilation: volume limited (gas flow continues until a predetermined inspired volume is achieved) and pressure limited (gas flow continues until a predetermined airway pressure is reached). Following a description of the ventilation modes, primary and secondary ventilatory controls are outlined.



Volume-limited Ventilation


Volume-limited ventilation involves the delivery of gas until a preset tidal volume (VT) is reached; airway pressures can vary depending on compliance or stiffness of the lungs. Expiration is passive. The following settings vary by how the ventilator responds to spontaneous inspiratory effort.


a. CMV (controlled mechanical ventilation): Provides a preset number of breaths per minute at a preset VT regardless of patient effort. The ventilator is time-cycled, so it is triggered by passage of a preset amount of time. This mode of ventilation is commonly used in the operating room due to a blunted respiratory effort from general anesthetic agents.

b. A/C (assist-control): Provides a preset VT whenever a patient spontaneously initiates a breath. In addition, the “control” portion provides a preset number of breaths as a backup in case the patient’s spontaneous rate is less than optimal. For each type of breath delivered, whether spontaneously or machine initiated, the tidal volume delivered is constant.

c. SIMV (synchronized intermittent mandatory ventilation): The machine delivers a preset number of breaths at a preset tidal volume. If the patient takes a spontaneous breath between machine-controlled breaths, the machine assists with pressure support at whatever preset pressure is programmed into the ventilator (see PSV below). Tidal volumes for these spontaneous breaths are completely determined by how much gas the patient can pull from the ventilator with a given amount of pressure support.


Pressure-limited Ventilation


This mode involves the delivery of gas until a preset peak airway pressure is reached, so the actual volume of gas delivered can vary per breath based on lung compliance.


a. PCV (pressure control ventilation): Gas flow is limited by a predetermined airway pressure and terminated after a predetermined duration of time (inspiratory time, Ti). The breathing cycle can be initiated by a patient’s inspiratory effort and is also mandated by the physician-determined respiratory rate. In either case once the breath is initiated, the inspiratory flow is rapid to establish the physician-determined pressure (known as the PCV level).

b. PSV (pressure support ventilation): In contrast to PCV, there are no mandatory breaths—PSV only supports spontaneous breaths and therefore mandates that the respiratory drive be intact.

c. CPAP (continuous positive pressure ventilation): This mode is used as a support for patients who are spontaneously breathing but cannot maintain adequate mechanics and/or oxygenation on their own. Each breath, and its tidal volume, is patient generated. The ventilator provides flow to pressurize the airway in an attempt to recruit and/or maintain open alveoli. CPAP is usually combined with PSV as a specific ventilation strategy or used to wean from other modes of mechanical ventilation. See Sabiston 24, Becker 12.

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Mar 20, 2017 | Posted by in GENERAL SURGERY | Comments Off on Mechanical Ventilation

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