From Mechanical Breathing With Intermittent Mandatory Ventilation
Weaning E. F. Klein, Jr., MD
Intermittent mandatory ventilation allows a gradual transition from mechanical ventilation to spontaneous breathing. This is accomplished by providing a continuous source of fresh gas from which the patient can breathe as ventilator rates are decreased from 1 to 2 per minute. Monitoring of arterial blood gas tension values documents the safety of further withdrawal of ventilatory support. Many patients are more easily and safely weaned by this method than by conventional techniques.
ventilation is important in prevent¬ associated with respiratory failure in critically ill patients. Although there is no precise defini¬ tion of respiratory failure, the common practice of moni¬ toring arterial blood gas tension values aids the clinician in deciding when ventilatory assistance should be insti¬ tuted. However, there are few, if any, simple determina¬ tions that signify when mechanical ventilatory support should be terminated. Once gas tension levels in arterial blood indicate reasonable cardiopulmonary function, there are few tests that accurately predict success or failure of weaning and discontinuance in ventilatory support. As¬ sessment of two simple ventilatory indexes, inspiratory force (greater than a -25 cm H.,0) and vital capacity (greater than 15 eu cm/kg), will, in many cases, correlate with the ability of the patient to sustain satisfactory spontaneous ventilation.1 Often, however, these objective criteria fail to differentiate patients who can or cannot be weaned successfully. A new type of ventilation was introduced for infants in 197 and for adults in 1973' that eliminated much of the guesswork and tedium of weaning patients from their
Mechanical ing mortality
Accepted
for publication Aug 15, 1974. From the University of Florida College of Medicine, J. Hillis Miller Health Center, Gainesville, Fla. Reprint requests to Department of Anesthesiology, University of Florida College of Medicine, Box 721, J. Hillis Miller Health Center, Gainesville, FL 32610 (Dr. Klein).
ventilators and decreased ventilation time. It was termed intermittent mandatory ventilation (IMV) and essentially combined a constant rate mechanical ventilator with con¬ tinuous oxygen-enriched gas flow to allow spontaneous ventilation during the time intervals between mechanical (mandatory) breaths (Fig 1). Initially, all alveolar ventilation is supplied by the ventilator (mandatory breaths) and the patient is "con¬ trolled" in the conventional sense. As respiratory stability is achieved, the ventilator rate is decreased progressively and the patient is allowed to assume some of the work of breathing. As respiratory indexes continue to improve, spontaneous ventilation will progressively assume a greater and greater proportion of alveolar ventilation and mandatory or machine breaths are simultaneously with¬ drawn by gradual decreases in their frequency (Fig 2). Monitoring of arterial blood gas tension levels following each rate decrease is essential and will verify the safety of further ventilator withdrawal. Once the ventilator rate has been lowered to one or two breaths per minute, most patients can be extubated safely; however, a few will re¬ quire even slower weaning and machine rates of one breath per minute or less prior to complete and final venti¬ lator discontinuance. As stated, satisfactory progression of weaning with IMV is monitored with periodic arterial blood gas tension values. This is similar to conventional weaning except that the arterial carbon dioxide tension (Paco..,) and arte¬ rial oxygen tension (Pao.) values reflect different vari¬ ables and are treated individually. Unacceptable elevation in Paco, during the weaning process indicates an inade¬ quate alveolar ventilation and is corrected by increasing the IMV (machine) rate. Although this may be due to many causes, eg, residual muscle weakness, decreased lung compliance, or excessive respiratory depressants, the treatment is the same (increase in the ventilator compo-
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
Controlled Ventilation
reflect combination of spontaneous and machine breaths elevated as desired.
Fig 1.—Airway pressures
at ambient level
|2?
or
varying
above and below
given base line, which
14
70
12
60
10
iPaco
may be
50
40 o o
30 > C
to
o.
20
4"
10
io
Time, hr
Fig 2.—Total ventilation is comprised of breaths controlled by ventilator as well as breaths supplied spontaneously by patient. Amount of ventilator or mandatory ventilation is withdrawn progressively as patient is able to increase spontaneous ventilation. Total weaning time in this example was approximately 20 hours. nent of alveolar ventilation) and suggests that weaning must either stop or proceed more slowly. Determining a "normal" Paco., value is for each patient is often difficult; however, the patient receiving IMV is allowed to set his own Paco._, value as long as the respiratory compo-
what
nent of acid-base balance does not lower the arterial (pHa) below approximately 7.33.
pH
Deterioration in oxygénation during weaning is treated similarly by altering two other variables, oxygen concen¬ tration and/or changes in levels of positive end-expiratory
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
Fig 3.—In conventional weaning, patient generally is connected to Briggs adaptor for intermittent periods to breathe oxygen-enriched atmosphere. Essentially all or none of respiratory work is supplied mechanically. When successful, weaning time is progressively lengthened until total unsupported ventilation can be maintained.
(PEEP). The usual practice of weaning patients who have been receiving PEEP completely back to am¬ bient expiratory pressures appears to be unnecessary, and extubation frequently can be effected safely from as much as 5 to 6 cm H.,0 PEEP. The salutary effects of increased mobility and spontaneous coughing and expectoration ap¬ parently compensate for the loss of the previously ele¬ vated expiratory pressures in maintaining adequate pul¬ monary function. Two questions logically may be asked regarding this mode of ventilation in weaning: When the patient is al¬ ready at peak inspiration and the mandatory breath oc¬ curs, does this raise pressures within the chest excessively and cause an increased incidence of barotrauma and pneu¬ mothorax? Is this mode really any better or safer than the traditional method where ventilation progresses from controlled to assisted to spontaneous ventilation? The first question appears to be of minimal conse¬ quence. Patients' respiratory rates tend to fall into cy¬ cle with the machine rate and synchronous breathing is ef¬ fected. It is not uncommon in this mode of ventilation to see a patient who has just finished a spontaneous breath to pause and wait for his mandatory breath to occur. In the rare occasion that dissynchronous breathing does oc¬ cur, pressure relief valves within a ventilator system will prevent buildup of excessive airway pressure. Further, it should be remembered that most patients using ventila¬ tors are breathing at functional residual capacities sub¬ stantially lower than normal, so the volume combination of a spontaneous and mandatory breath still places the pressure
Fig 4.—Breathing with IMV initially may be 100% controlled, but generally withdrawn as work of breathing is assumed by patient.
control is
lung volume well below the total lung capacity. When comparing IMV and assisted modes of ventila¬ tion, it should be pointed out that even in the assisted modes, the ventilator is doing virtually all of the work of breathing, and triggering of the ventilator cycle is, by de¬ sign, a minimal work expenditure. The progression from assisted to spontaneous ventilation, then, is essentially an all-or-none phenomenon; the patient either does all of the work of breathing or none (Fig 3 and 4). Commercially available IMV ventilators currently are somewhat scarce, but simple modifications of existing ventilators have been reported.4
The IMV system has been utilized in our institution ex¬ clusively for the past 24 months with great acceptance and success. Other advantages of this technique of ventilation and weaning have been reported, including a paradoxical decrease in oxygen consumption compared to standard modes of controlled ventilation and a simplification in the management of the patient with chronic obstructive pul¬ monary disease and acute respiratory failure who does not respond to traditional conservative management.-7 Not only does IMV facilitate and simplify weaning of the pa¬ tient who has an "adequate" inspiratory force and vital capacity, but also patients who have not passed the "min¬ imal" criteria for conventional weaning trials, on occasion, have been weaned successfully with IMV.S
Figure 2 was provided by John B. Downs, by Joseph M. Civetta, MD.
vided
MD.
Figures 3 and 4
were
pro¬
References 1. Browne AGR, Pontoppidan H, Chiang H, et al: Physiological criteria for weaning patients from prolonged artificial ventilation, in Abstracts of Scientific Papers: Annual Meeting of American Society of Anesthesiologists, pp 69-70, 1972. 2. Kirby RR, Robison EJ, Schulz J, et al: A new pediatric volume ventilator. Anesth Analg 50:533-537, 1971. 3. Downs JB, Klein EF Jr, Desautels D, et al: Intermittent mandatory ventilation: A new approach to weaning patients from mechanical ventilators. Chest 64:331-335, 1973. 4. Desautels DA, Bartlett JL: Methods of administering intermittent mandatory ventilation (IMV). Respir Care 19:187-191,
1974. 5. Downs
JB, Block AJ, Vennum KB: Intermittent mandatory ventilation in the treatment of patients with chronic obstructive pulmonary disease. Anesth Analg 53:437-443, 1974. 6. Downs JB, Mitchell LA: Intermittent mandatory ventilation following cardiopulmonary bypass. Crit Care Med 2:39-40, 1974. 7. Downs JB, Perkins HM, Modell JH: Intermittent mandatory ventilation: An evaluation. Arch Surg 109:519-523, 1974. 8. Downs JB, Perkins HM, Sutton WW: Successful weaning after five years of mechanical ventilation. Anesthesiology 40:602-603, 1974.
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
Weaning E. F. Klein, Jr., MD
Intermittent mandatory ventilation allows a gradual transition from mechanical ventilation to spontaneous breathing. This is accomplished by providing a continuous source of fresh gas from which the patient can breathe as ventilator rates are decreased from 1 to 2 per minute. Monitoring of arterial blood gas tension values documents the safety of further withdrawal of ventilatory support. Many patients are more easily and safely weaned by this method than by conventional techniques.
ventilation is important in prevent¬ associated with respiratory failure in critically ill patients. Although there is no precise defini¬ tion of respiratory failure, the common practice of moni¬ toring arterial blood gas tension values aids the clinician in deciding when ventilatory assistance should be insti¬ tuted. However, there are few, if any, simple determina¬ tions that signify when mechanical ventilatory support should be terminated. Once gas tension levels in arterial blood indicate reasonable cardiopulmonary function, there are few tests that accurately predict success or failure of weaning and discontinuance in ventilatory support. As¬ sessment of two simple ventilatory indexes, inspiratory force (greater than a -25 cm H.,0) and vital capacity (greater than 15 eu cm/kg), will, in many cases, correlate with the ability of the patient to sustain satisfactory spontaneous ventilation.1 Often, however, these objective criteria fail to differentiate patients who can or cannot be weaned successfully. A new type of ventilation was introduced for infants in 197 and for adults in 1973' that eliminated much of the guesswork and tedium of weaning patients from their
Mechanical ing mortality
Accepted
for publication Aug 15, 1974. From the University of Florida College of Medicine, J. Hillis Miller Health Center, Gainesville, Fla. Reprint requests to Department of Anesthesiology, University of Florida College of Medicine, Box 721, J. Hillis Miller Health Center, Gainesville, FL 32610 (Dr. Klein).
ventilators and decreased ventilation time. It was termed intermittent mandatory ventilation (IMV) and essentially combined a constant rate mechanical ventilator with con¬ tinuous oxygen-enriched gas flow to allow spontaneous ventilation during the time intervals between mechanical (mandatory) breaths (Fig 1). Initially, all alveolar ventilation is supplied by the ventilator (mandatory breaths) and the patient is "con¬ trolled" in the conventional sense. As respiratory stability is achieved, the ventilator rate is decreased progressively and the patient is allowed to assume some of the work of breathing. As respiratory indexes continue to improve, spontaneous ventilation will progressively assume a greater and greater proportion of alveolar ventilation and mandatory or machine breaths are simultaneously with¬ drawn by gradual decreases in their frequency (Fig 2). Monitoring of arterial blood gas tension levels following each rate decrease is essential and will verify the safety of further ventilator withdrawal. Once the ventilator rate has been lowered to one or two breaths per minute, most patients can be extubated safely; however, a few will re¬ quire even slower weaning and machine rates of one breath per minute or less prior to complete and final venti¬ lator discontinuance. As stated, satisfactory progression of weaning with IMV is monitored with periodic arterial blood gas tension values. This is similar to conventional weaning except that the arterial carbon dioxide tension (Paco..,) and arte¬ rial oxygen tension (Pao.) values reflect different vari¬ ables and are treated individually. Unacceptable elevation in Paco, during the weaning process indicates an inade¬ quate alveolar ventilation and is corrected by increasing the IMV (machine) rate. Although this may be due to many causes, eg, residual muscle weakness, decreased lung compliance, or excessive respiratory depressants, the treatment is the same (increase in the ventilator compo-
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
Controlled Ventilation
reflect combination of spontaneous and machine breaths elevated as desired.
Fig 1.—Airway pressures
at ambient level
|2?
or
varying
above and below
given base line, which
14
70
12
60
10
iPaco
may be
50
40 o o
30 > C
to
o.
20
4"
10
io
Time, hr
Fig 2.—Total ventilation is comprised of breaths controlled by ventilator as well as breaths supplied spontaneously by patient. Amount of ventilator or mandatory ventilation is withdrawn progressively as patient is able to increase spontaneous ventilation. Total weaning time in this example was approximately 20 hours. nent of alveolar ventilation) and suggests that weaning must either stop or proceed more slowly. Determining a "normal" Paco., value is for each patient is often difficult; however, the patient receiving IMV is allowed to set his own Paco._, value as long as the respiratory compo-
what
nent of acid-base balance does not lower the arterial (pHa) below approximately 7.33.
pH
Deterioration in oxygénation during weaning is treated similarly by altering two other variables, oxygen concen¬ tration and/or changes in levels of positive end-expiratory
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
Fig 3.—In conventional weaning, patient generally is connected to Briggs adaptor for intermittent periods to breathe oxygen-enriched atmosphere. Essentially all or none of respiratory work is supplied mechanically. When successful, weaning time is progressively lengthened until total unsupported ventilation can be maintained.
(PEEP). The usual practice of weaning patients who have been receiving PEEP completely back to am¬ bient expiratory pressures appears to be unnecessary, and extubation frequently can be effected safely from as much as 5 to 6 cm H.,0 PEEP. The salutary effects of increased mobility and spontaneous coughing and expectoration ap¬ parently compensate for the loss of the previously ele¬ vated expiratory pressures in maintaining adequate pul¬ monary function. Two questions logically may be asked regarding this mode of ventilation in weaning: When the patient is al¬ ready at peak inspiration and the mandatory breath oc¬ curs, does this raise pressures within the chest excessively and cause an increased incidence of barotrauma and pneu¬ mothorax? Is this mode really any better or safer than the traditional method where ventilation progresses from controlled to assisted to spontaneous ventilation? The first question appears to be of minimal conse¬ quence. Patients' respiratory rates tend to fall into cy¬ cle with the machine rate and synchronous breathing is ef¬ fected. It is not uncommon in this mode of ventilation to see a patient who has just finished a spontaneous breath to pause and wait for his mandatory breath to occur. In the rare occasion that dissynchronous breathing does oc¬ cur, pressure relief valves within a ventilator system will prevent buildup of excessive airway pressure. Further, it should be remembered that most patients using ventila¬ tors are breathing at functional residual capacities sub¬ stantially lower than normal, so the volume combination of a spontaneous and mandatory breath still places the pressure
Fig 4.—Breathing with IMV initially may be 100% controlled, but generally withdrawn as work of breathing is assumed by patient.
control is
lung volume well below the total lung capacity. When comparing IMV and assisted modes of ventila¬ tion, it should be pointed out that even in the assisted modes, the ventilator is doing virtually all of the work of breathing, and triggering of the ventilator cycle is, by de¬ sign, a minimal work expenditure. The progression from assisted to spontaneous ventilation, then, is essentially an all-or-none phenomenon; the patient either does all of the work of breathing or none (Fig 3 and 4). Commercially available IMV ventilators currently are somewhat scarce, but simple modifications of existing ventilators have been reported.4
The IMV system has been utilized in our institution ex¬ clusively for the past 24 months with great acceptance and success. Other advantages of this technique of ventilation and weaning have been reported, including a paradoxical decrease in oxygen consumption compared to standard modes of controlled ventilation and a simplification in the management of the patient with chronic obstructive pul¬ monary disease and acute respiratory failure who does not respond to traditional conservative management.-7 Not only does IMV facilitate and simplify weaning of the pa¬ tient who has an "adequate" inspiratory force and vital capacity, but also patients who have not passed the "min¬ imal" criteria for conventional weaning trials, on occasion, have been weaned successfully with IMV.S
Figure 2 was provided by John B. Downs, by Joseph M. Civetta, MD.
vided
MD.
Figures 3 and 4
were
pro¬
References 1. Browne AGR, Pontoppidan H, Chiang H, et al: Physiological criteria for weaning patients from prolonged artificial ventilation, in Abstracts of Scientific Papers: Annual Meeting of American Society of Anesthesiologists, pp 69-70, 1972. 2. Kirby RR, Robison EJ, Schulz J, et al: A new pediatric volume ventilator. Anesth Analg 50:533-537, 1971. 3. Downs JB, Klein EF Jr, Desautels D, et al: Intermittent mandatory ventilation: A new approach to weaning patients from mechanical ventilators. Chest 64:331-335, 1973. 4. Desautels DA, Bartlett JL: Methods of administering intermittent mandatory ventilation (IMV). Respir Care 19:187-191,
1974. 5. Downs
JB, Block AJ, Vennum KB: Intermittent mandatory ventilation in the treatment of patients with chronic obstructive pulmonary disease. Anesth Analg 53:437-443, 1974. 6. Downs JB, Mitchell LA: Intermittent mandatory ventilation following cardiopulmonary bypass. Crit Care Med 2:39-40, 1974. 7. Downs JB, Perkins HM, Modell JH: Intermittent mandatory ventilation: An evaluation. Arch Surg 109:519-523, 1974. 8. Downs JB, Perkins HM, Sutton WW: Successful weaning after five years of mechanical ventilation. Anesthesiology 40:602-603, 1974.
Downloaded From: http://archsurg.jamanetwork.com/ by a University of Arizona Health Sciences Library User on 06/02/2015
