Acta anaesth. scand. 1977, 21, 252-256
Weaning from Mechanical Ventilation by means of Intermittent Assisted Ventilation I.A.V. Case Reports
SVEIN HARBOE Department of Anaesthesia, Rikshospitalet, University Hospital, Oslo, Norway
A new ventilator is described which is capable of interposing controlled breaths synchronized with the patient’s own breathing rhythm. This ventilation pattern is called “intermittent assisted ventilation” (IAV). I t differs from intermittent mandatory ventilation (IMV) in that each ventilator cycle is triggered by the patient. IAV constitutes a new approach to the problems during the critical period of weaning from mechanical ventilation. Further, this new ventilator provides means for continuous display and recording of airway gas flow and pressures and expired minute volume (EMV) during different types of ventilation, e.g. controlled ventilation, intermittent assisted, and spontaneous ventilation. Received I0 October, accepted for publication 14 December 1976
Mechanical ventilation is a well-established breathing in adults. This will be referred to treatment of respiratory failure, but the prob- as “intermittent assisted ventilation” (IAV). lem of how and when to discontinue the I n addition, airway flow and pressures may treatment remains a controversial one be continuously recorded. This new method (SLADEN1973, GILBERT,et al. 1974). Con- is discussed in the present paper. tinuous positive airway pressure (CPAP), introduced by GREGORY et al. (1971), has been successfully used for weaning children TECHNICAL DESCRIPTION from a ventilator. Adult patients appear to The Servo Ventilator 900 R is a further benefit less from such treatment (GRENVIK development of Servo Ventilator 900, which 1973). Intermittent mandatory ventilation has been described in detail by INGELSTEDT (IMV) has also been used with apparent et a]. (1972). A new electronic servo-system success (DOWNS et al. 1973, 1974a, 1974b, has been added for IAV function, and this 1 9 7 4 ~ )One . shortcoming of I M V is that its permits a mixture of controlled and sponpressure characteristics may be out of phase taneous ventilation without extra tubes and with the patient’s own ventilation. The gas supply. expired minute volume (EMV) is usually not During spontaneous breathing, informamonitored. tion from the pressure transducer is transRecently Siemens-Elema AB (Solna, mitted to the electronic servo unit, which in Sweden) have modified their Servo Ventila- turn controls flow through the inspiratory tor 900 in such a way that it can produce valve and maintains the airway pressure at patient-triggered sighs during spontaneous a preset level. As long as the pressure remains * Part of this paper has previously been presented at below this value, gas is fed to the patient’s The Twelfth Congress of the Scandinavian Society of circuit through the inspiratory valve to cornpensate for the pressure difference caused by Anesthesiologists in Oulu, Finland, 7-12 July, 1975.
INTERMITTENT ASSISTED VENTILATION
the patient’s own inhalation. When the pressure rises above the preset level during exhalation, the expiratory valve opens and the inspiratory valve shuts. The EMV, as calculated from the expiratory flow, is displayed continuously on the panel of the ventilator during spontaneous breathing, as well as during controlled ventilation. An alarm is triggered if the EMV exceeds preset lower and upper limits. In the new ventilator, a control has been added for the IAV function. The control has four positions: f/2, f/5, f/10 and 0. If the number of breaths per minute (f) has been set at 20, the three first
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positions of the IAV knob mean 10, 4 and 2 assisted ventilations per minute, respectively. The tidal volume of these forced breaths are those set on the ventilator, i.e. the minute volume divided by f. In zero position, the IAV control cancels all assistance from the ventilator, but the EMV is still monitored. During spontaneous ventilation, CPAP can be provided by setting the lower pressure limit at the desired inspiratory pressure. The expiratory positive pressure is then maintained by an adjustable spring-loaded valve. Electrical signals indicating the airway pres-
Fig. 1 . Tracings from a 57-year-old male with mitral valve replacement, who was studied during three different ventilation patterns on the day of weaning from the ventilator: (A) Controlled ventilation; (B) intermittent assisted ventilation; and (C) spontaneous breathing (before extubation).
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sure, the flow rates and the EMV may be displayed on an oscilloscope or recorded continuously, For the recordings shown in Figure 1, a Mingograf ink jet recorder (SiemensElema AB, Solna, Sweden) was used.
information on changes occurring in the patient’s respiratory condition. Two case protocols of patients are presented which show the usefulness of IAV for weaning. Case I. T.G., a 50-year-old female, had a n uneventful
CLINICAL USE O F T H E VENTILATOR During 1974/75, the ventilator was used on more than 50 adult patients. Recordings obtained from a patient during controlled ventilation, during IAV, and during spontaneous breathing are shown in Figure 1. If the ventilator, as in Figure IA, is set for a n inspiratory pause of 10% of the respiratory time cycle, an end-inspiratory pressure level is obtained during a phase of no-flow. This may be used to calculate static compliance. In Figure IB, the ventilator has been set to deliver an IAV of 5 breaths per minute, each with a tidal volume of 750 ml. IAV is triggered by the patient’s own attempt to inhale spontaneously. The IAV mechanism ensures that the ventilator will never force air into the patient when his lungs are already filled by his own inspiratory efforts. Note that the airway pressure rises more slowly during IAV inspiration than during controlled ventilation, because the patient inspires actively at the same time. When the patient breathes spontaneously (Fig. lC), EMV, airway pressure and flow are continuously recorded, since he is still connected to the ventilator. The tracings show that the patient breathes regularly 14 times per minute with a peak inspiratory flow of only 0.5 l!s. The EMV remains steady a t 6-7 1. These values are consistent with normal quiet breathing in the adult. A most important parameter to follow during IAV, as well as during controlled ventilation in general, is the EMV. In our experience it reduces the need for frequent blood gas determinations. Continuous monitoring of static compliance, airway pressure and flow patterns during IAV and controlled ventilation may give valuable additional
mitral valve replacement in July 1974. In January 1975, she was admitted as an emergency with severe heart failure. Valve dysfunction was diagnosed. The patient was operated on promptly and a large thrombus was removed from her left atrium. I n spite of large doses of adrenalin, cardiac function was inadequate after extra-corporeal circulation had been discontinued. Intra-aortic balloon pumping was required for 24 hours. The patient was ventilated on the Servo Ventilator 900 in the post-operative period. Weaning started on a T-piece on the 4th postoperative day, with extubation on the 7th post-operative day. During the next 24 hours her respiratory rate rose from 35 to 60/min. Although blood gases remained acceptable, she soon became exhausted and very anxious, and was re-intubated and ventilated. Chest X-ray indicated congestive failure. Tracheotomy was carried out on the 9th post-operative day. Weaning was attempted for a second time on the 12th post-operative day. T h e patient became distressed after only 5 min of spontaneous ventilation. When IAV was then instituted with the Servo Ventilator 900B in position f/2, she could tolerate this for 10-15 min, and felt more confident. Weaning was continued on IAV, but she remained apprehensive of the treatment, due to the first unsuccessful attcmpt. Progress slowed down gradually. O n the 18th postoperative day, after a period during which her anxiety during every attempt at weaning had been particularly severe, she was returned from controlled ventilation to IAV while asleep. When she woke up a n hour later, she was informed about the change. Her confidence grew, and further progress was satisfactory. Weaning was completed on the 24th post-operative day. She was decannulated 2 days later. One year later she was still a t home and able to manage her housework without help.
ZI. A 54-year-old male underwent emergency surgery due to a large aortic aneurism. During induction of anesthesia the patient went into a condition of shock with a fall in systolic pressure to 30-50 mmHg, which lasted for about 15 min before the bleeding could be controlled surgically. The patient receil-ed 30 units of whole blood. Post-operatively he developed a prolonged paralytic ileus with gross abdominal distension, renal failure, and disseminated intravasal coagulation. During the post-operative period he was ventilated using the Servo Ventilator 900B. During the first 2 weeks he was unable to breathe spontaneously on a T-piece for more than 5 min. IAV allowed much
Case
INTERMITTENT ASSISTED VENTILATION
longer periods off controlled ventilation. During the last 2 weeks at the hospital, he stayed on IAV for longer periods. On the 25th day after the operation his abdomen was almost normal, and the patient was encouraged to stay on IAV for increased periods of time, during which rapid progress was made towards normal breathing. He was decannulated 28 days after the operation.
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et a]. (1970). Moreover, a regular sighing which tends to prevent formation of atelectases in anaesthetized patients (BENDIXEN et al. 1964) is introduced in IMV/IAV. I n severe cases, particularly those treated with mechanical ventilation for several weeks, our experience with the IAV method is very encouraging. However, additional studies DISCUSSION must be carried out before we can select with Weaning from prolonged mechanical ventila- any certainty the patients who will benefit tion may present difficult problems (SLADEN most from the use of this new ventilation 1973, GILBERT et al. 1974). Several measure- technique. able variables have been used to predict whether or not a patient can come off the REFERENCES ventilator (BENDIXENet al. 1965, PONTOPH. H . , BULLWINKEL, B., HEDLEY-WHYTE, J. PIDAN et al. 1970, GILBERT et al. 1974). Still, BENDIXEN, & LAVER,M . B. (1964) Atelectasis and shunting it is often necessary to resort to trial and error during spontaneous ventilation in anesthetized with the inherent dangers of hypoventilation, patients. Anesthesiolopy 25, 297. hypoxia and undue stress. Gradual weaning BENDIXEN, H. H., EGBERT, L. D., HEADLEY-WHYTE, J., LAVER,M. B. & PONTOPPIDAN, H . (1965) procedures, therefore, have been described Respiratory Care. C. V. Mosby Co., St. Louis. and are widely applied (KIRBYet al. 1972, P., MODELL, J. H., KIRBY,R. R., KLEIN, DOWNSet al. 1973, 1974a, 1974b, 1974~). CULLEN, E. F. & LONG,W. (1975) Treatment of flail chest: With the Servo Ventilator 900 B, we are now Use of intermittent mandatory ventilation and able to monitor the EMV throughout the positive end expiratory pressure. Arch. Surg. 110, 1099. weaning procedure. If the EMV should at any DOWNS, J. B., KLEIN,E., DESAUTELS, D., MODELL, J. H . time fall below a preset value, an audible and & KIRBY,R. R. (1973) Intermittent mandatory visible alarm signal is given. Both controlled ventilation: A new approach to weaning patients and assisted ventilation can easily be changed from mechanical ventilators. Chest 64, 331. to IAV, with or without PEEP/CPAP. No DOWNS, J. B., BLOCK,^. A. & VENNUM, K. B. (1974a) Intermittent mandatory ventilation in the treatment extra tubing, valves or reservoir bag are of patients with chronic obstructive pulmonary necessary. The ratios between the IAV tidal disease. Anesth. Analg. 53,437. volumes and the spontaneous tidal volumes DOWNS, J . B., PERKINS, H . M. & MODELL, J. H. (1974b) are easy to adjust: if too large, it is uncomIntermittent mandatory ventilation. An evaluation. fortable for the patient; if too small, there is Arch. Surg. 109, 519. J.B., PERKINS, H.M. & SUTTON,W. W. ( 1 9 7 4 ~ ) no sighing effect. Several authors have DOWNS, Successful weaning after five years of mechanical reported that the use of IMV reduces the ventilation. Anesthesiolopy 40, 602. time on the ventilator and makes the weaning GILBERT,R., AUCHINLOSS, J. H . , PEPPI,B. S. & more comfortable (DOWNS et al. 1973, 1974a, ASHUTOSH, J , H. (1974) The first few hours off 1974b, CULLENet al. 1975). However, in a respirator. Chest 65, 162. G. A., KITTERMAN, J. A., PHIBBS, R. H., acute respiratory insufficiency, the combina- GREGORY, TOOLEY, W. H . & HAMILTON, W. K. (1971) Treattion of IMV and high level of PEEP requires ment of the idiopathic respiratory-distress syndrome close monitoring of the patient (KIRBYet al. with continuous positive airway pressure. New 1975). One of the greatest benefits of I h W / Eng1.J. Med. 284, 1333. IAV seems to be that the patient quickly GRENVIK, A. (1973) Acute respiratory failure. Current Therapy, ed. Conn, H. F. Saunders, Philadelphia, starts to train his own respiratory muscles p. 103. (DOWNS et al. 1973), and IAV may further INGELSTEDT,s.,JONSON, B., NORDSTRBM, L. & OLSSON, prevent the irregular breathing pattern which S. G. (1972) A Servo-controlled ventilator measuring frequently occurs after prolonged mechanical expired minute volume, airway flow and pressure. ventilation, as demonstrated by PONTOPPIDAN Acta anaesth. scand. 16, Suppl 47, 7.
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KIRBY,R. R., ROBINSON, E., SCHULZ, J. & DE HEMOS (1972) Continuous flow as a n alternative to assisted or controlled ventilation in infants. Anesth. Analg. 51, 87 I . KIRBY,R. R., DOWNS, J. B., CIVETTA,J. M. DANNEMILLER, F. J., KLEIN,E. F. & HODGES, H. (1975) High level positive end expiratory pressure (PEEP) in acute respiratory insufficiency. Chest 67, 156. PONTOPPIDAN, H., LAVER,M. B. & GEFFIN,B. (1970) Acute respiratory failure in the surgical patient. A h . Surg. 4, 163. SLADEN, A. (1973) Weaningfrom Ventilation. 20th Annual
Anesthesiology Review Course, June 1973, 1,ecture no. 118. Society of Air Force Anesthesiologists, Lecture Notes, Convention Center, San Antonio, Texas, U.S.A. Address : Svein Harboe, M.D. Department of Anaesthesia Rikshospitalet Oslo Norway