Surgical Critical Care
0039-6109/91 $0.00
+
.20
Weaning from Mechanical Ventilation
Nathan E. Coates, MD, * and John A. Weigelt, MDt
Weaning from mechanical ventilation and removal of artifical airways seem to occupy a substantial amount of patient care activities in the surgical intensive care unit (l'Ol.l). However, it must be realized that the majority of patients require short periods of mechanical ventilatory support and are rapidly weaned and extubated. It is estimated that 75% of surgical patients are extubated immediately postoperatively. 35, 75, 104 An additional 15% of patients require ventilation for less than a week and are extubated with minimal difficulty. 75 The remaining 10% require 7 or more days of mechanical ventilation. Various methods and criteria are used clinically to assess a patient's ability to be weaned from mechanical ventilation and extubated. A successful weaning trial is an important goal because a failed weaning attempt increases morbidity, although a clear relation to an increased mortality rate is not present.": 35, 50, 86 Specific weaning criteria are really not needed in the first and largest group of patients. These patients can be extubated once they have recovered from anesthesia. Very simple tests are used, including the ability of the patients to respond appropriately to questions and hold their heads off the pillow. Other criteria that attempt to evaluate oxygenation and ventilation capacity are used for patients in the other two groups, although the utility and accuracy of these criteria continue to be challenged. At the present time, no one set of criteria nor one weaning modality is clearly the best. This article will review the current weaning criteria felt to be useful. The discussion will emphasize what the criteria are trying to measure and why they might relate to weaning success. The current commonly used From the Section of Surgical Critical Care and Trauma Surgery, Department of Surgery, The University of Texas Southwestern Medical Center, Dallas, Texas
*Assistant
Instructor tChief and Professor
Surgical Clinics of North America-Vol. 71, No.4, August 1991
859
860
NATHAN
E.
COATES AND JOHN
A.
WEIGELT
weaning processes will also be discussed, and one method of weaning will be outlined.
WEANING CRITERIA It is important to remember that withdrawal of mechanical support is not considered until the primary problem that led to ventilatory failure is improving or, better yet, resolved. Various criteria are used to help evaluate a patient as a weaning candidate. The importance of making a correct assessment of weaning capability is emphasized by Demling and associates, who showed that premature weaning and extubation were associated with a deterioration of clinical status.:" The measurements used as weaning criteria can be conveniently divided into oxygenation and ventilation categories. These are listed in Table 1. Oxygenation criteria are fairly simple to measure in most patients. Pulse oximetry can provide a noninvasive means of evaluating arterial oxygenation (Pa0 2); however, the most accurate assessment still relies on an arterial blood gas measurement. A value of 55 to 60 mm Hg (torr) on an FI0 2 of 21 % is usually considered acceptable for oxygenation. A Pa0 2 of 100 torr on an FI0 2 of 40% has been used as an oxygenation criterion for weaning from positive end-expiratory pressure (PEEP).24 An oxygen saturation greater than 90% on any FI0 2 is usually adequate for weaning. Alveolar arterial oxygen differences and shunt fraction (QS/QT) are other criteria used to measure oxygenation. The alveolar arterial oxygen tension on 100% FI0 2 should be greater than 350 mm Hg. 51, 109 The QSI QT should be less than 15% before ventilator support is discontinued. 51 Accurate measurement of these parameters requires a pulmonary artery Table 1. Conventional Weaning Criteria Measurements of oxygenation Pa0 2 >70 mm Hg (torr) on FI0 2 40% Arterial saturation >90% Alveolar-arterial oxygen tension (AaD0 2)
0039-6109/91 $0.00
+
.20
Weaning from Mechanical Ventilation
Nathan E. Coates, MD, * and John A. Weigelt, MDt
Weaning from mechanical ventilation and removal of artifical airways seem to occupy a substantial amount of patient care activities in the surgical intensive care unit (l'Ol.l). However, it must be realized that the majority of patients require short periods of mechanical ventilatory support and are rapidly weaned and extubated. It is estimated that 75% of surgical patients are extubated immediately postoperatively. 35, 75, 104 An additional 15% of patients require ventilation for less than a week and are extubated with minimal difficulty. 75 The remaining 10% require 7 or more days of mechanical ventilation. Various methods and criteria are used clinically to assess a patient's ability to be weaned from mechanical ventilation and extubated. A successful weaning trial is an important goal because a failed weaning attempt increases morbidity, although a clear relation to an increased mortality rate is not present.": 35, 50, 86 Specific weaning criteria are really not needed in the first and largest group of patients. These patients can be extubated once they have recovered from anesthesia. Very simple tests are used, including the ability of the patients to respond appropriately to questions and hold their heads off the pillow. Other criteria that attempt to evaluate oxygenation and ventilation capacity are used for patients in the other two groups, although the utility and accuracy of these criteria continue to be challenged. At the present time, no one set of criteria nor one weaning modality is clearly the best. This article will review the current weaning criteria felt to be useful. The discussion will emphasize what the criteria are trying to measure and why they might relate to weaning success. The current commonly used From the Section of Surgical Critical Care and Trauma Surgery, Department of Surgery, The University of Texas Southwestern Medical Center, Dallas, Texas
*Assistant
Instructor tChief and Professor
Surgical Clinics of North America-Vol. 71, No.4, August 1991
859
860
NATHAN
E.
COATES AND JOHN
A.
WEIGELT
weaning processes will also be discussed, and one method of weaning will be outlined.
WEANING CRITERIA It is important to remember that withdrawal of mechanical support is not considered until the primary problem that led to ventilatory failure is improving or, better yet, resolved. Various criteria are used to help evaluate a patient as a weaning candidate. The importance of making a correct assessment of weaning capability is emphasized by Demling and associates, who showed that premature weaning and extubation were associated with a deterioration of clinical status.:" The measurements used as weaning criteria can be conveniently divided into oxygenation and ventilation categories. These are listed in Table 1. Oxygenation criteria are fairly simple to measure in most patients. Pulse oximetry can provide a noninvasive means of evaluating arterial oxygenation (Pa0 2); however, the most accurate assessment still relies on an arterial blood gas measurement. A value of 55 to 60 mm Hg (torr) on an FI0 2 of 21 % is usually considered acceptable for oxygenation. A Pa0 2 of 100 torr on an FI0 2 of 40% has been used as an oxygenation criterion for weaning from positive end-expiratory pressure (PEEP).24 An oxygen saturation greater than 90% on any FI0 2 is usually adequate for weaning. Alveolar arterial oxygen differences and shunt fraction (QS/QT) are other criteria used to measure oxygenation. The alveolar arterial oxygen tension on 100% FI0 2 should be greater than 350 mm Hg. 51, 109 The QSI QT should be less than 15% before ventilator support is discontinued. 51 Accurate measurement of these parameters requires a pulmonary artery Table 1. Conventional Weaning Criteria Measurements of oxygenation Pa0 2 >70 mm Hg (torr) on FI0 2 40% Arterial saturation >90% Alveolar-arterial oxygen tension (AaD0 2)
