The Science Journal of the American Association for Respiratory Care

1997 OPEN FORUM Abstracts

Prolonged Weakness Following Neuromuscular Blockade: Etiology and Prevention

John W Hoyt MD, Sunday, December 7, 1997.

Mechanical ventilation is a frequent critical care intervention in intensive care units. Fifty years of development have resulted in a wide variety of ventilation modes and devices to meet varying patient needs. Often new modes are developed simply because they are possible. The appropriate clinical application of these innovative ventilation modes occurs after they have been used extensively in patients. While respiratory therapists are quite familiar with the technical aspects of providing many modes of mechanical ventilation in the ICU, mechanical ventilation is also a part of anesthesia practice employing different types of mechanical ventilators.

The demands of an operating room environment are different than those of an ICU. Neuromuscular blockers are frequently employed in the operating room for brief periods of time and controlled mechanical ventilation is the mode most often employed. Manual ventilation using an anesthesia bag allows the anesthesiologist physical contact with the patient. Changes in respiratory system compliance relate to amount of neuromuscular blockade, presence of chronic or acute bronchospastic lung disease, and airway obstruction from secretions, endotracheal tube occlusion, or right mainstem intubation. Changes in compliance may be due to the surgical procedure such as abdominal distention with carbon dioxide to facilitate laparoscopic surgery.

Probably the most significant difference in OR ventilation is the use of a semi-closed system. This is chosen to reduce the amount of volatile anesthetic agent used, reducing cost and decreasing environmental pollution. CO_{2} is removed with an in-line absorber. Other benefits of a semi-closed system are heat and moisture conservation. Patient temperature falls during general anesthesia due the cold environment, body cavity exposure, and anesthetic effects on temperature regulation and cutaneous blood flow. Hypothermia is associated with increased cardiac morbidity and wound infections.

Modern anesthesia ventilators tend to mimic bag-ventilation using high pressured gas to pressurize a box surrounding a bellows. Tidal volume is controlled and by the bag (bellows) size, and inspiratory flow is controlled by varying the driving pressure. Electrical monitors of exhaled tidal volume as well as visual monitors of exhaled volume provide the anesthesiologist with clues as to compliance and changes in airway dynamics. Display of tidal volume, minute ventilation, respiratory rate and airway pressure are features of modern anesthesia ventilators. These devices are usually pressure-driven, time-cycled, volume-limited ventilators. There is no capacity for patient triggering ventilation and no way to apply positive end expiratory pressure directly from the ventilator. The ventilator simply replaces the hand squeezing the bag. In order to apply PEEP, additional devices must be added to the breathing circuit. To safely use the low gas flow, gas analysis of anesthetic agents, oxygen, nitrous oxide and nitrogen must be provided. An in-line oxygen analyzer is the minimum acceptable monitor if closed circuit anesthesia is considered.

One quirk about semi-closed system anesthesia and mechanical ventilation is that the airway pressure gauge is isolated from the patient circuit by two one-way valves. Extrinsic PEEP is applied on the exhalation circuit on the patient side of the one-way valve. This means that PEEP valves must be calibrated and accurate because the added PEEP will not be reflected in the measured airway pressures. Mechanical ventilation in critically ill patients requiring either very high airway pressures or non-conventional modes of ventilation is generally provided using an intensive care ventilator and the anesthesia machine is not used. Total intravenous anesthesia must be used. Some ICU ventilators such as those of the Semen series can be adapted to use anesthetic gas inflow into the delivered gas system. The interface of the anesthesiologist and the critical care ventilator is a stressful one that requires respiratory therapy cooperation and support.

AARC 50th Anniversary, December 6 - 9, 1997, New Orleans, Louisiana.

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