Henry Ford Hospital Medical Journal

Volume 35 Number 1 Article 13

3-1987

Mechanical Ventilation: New Modes, Old Modes

John Popovich Jr.

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Recommended Citation Popovich, John Jr. (1987) "Mechanical Ventilation: New Modes, Old Modes," Henry Ford Hospital Medical Journal : Vol. 35 : No. 1 , 63-66. Available at: https://scholarlycommons.henryford.com/hfhmedjournal/vol35/iss1/13

This Article is brought to you for free and open access by Henry Ford Health System Scholarly Commons. It has been accepted for inclusion in Henry Ford Hospital Medical Journal by an authorized editor of Henry Ford Health System Scholarly Commons. Mechanical Ventilation: New Modes, Old Modes

John Popovich, Jr, MD*

echanical ventilation refers to the application of a mechan­ ically delivered breath at preset intervals synchronous with the Mical device to either partially or fully provide ventilation patient's inspiratory efforts. As in assist-control mode ventila­ for a patient. The principal goal of mechanical ventilation is to tion, if the patient does not breathe within a certain time interval, maintain an alveolar ventilation appropriate for the metabolic re­ the tidal volume will be delivered at the set intermittent man­ quirements ofthe patient, thereby maintaining or enhancing car­ datory ventilation rate. bon dioxide excretion. Mechanical ventilation relieves the work Many putative advantages and disadvantages of each of the load that precipitates or accompanies ventilatory failure until the traditional volume-cycled positive-pressure ventilation modes balance betweeil ventilatory demand and breathing capability have been offered. Theoretically, the lowest work of breathing can be reestablished. provided by the patient is offered by the control mode. The res­ The term ventilator mode usually refers to the functioning ar­ piratory muscles are rested effectively when a controlled breath rangement ofthe ventilator, that is, the method by which the is delivered without patient effort. Unfortunately, the respira­ ventilator provides ventilation. Inherently, mode often refers to tory muscle rest provided by the mode may, if used for long peri­ the capabilities of the machine and its circuitry and the interac­ ods, lead to respiratory muscle disuse atrophy (3). Both assist- tion of the ventilator with the patient's spontaneous breathing. control mode ventilation and intermittent mandatory ventilation Discussion of the modes of ventilatory support conventionally should offer theoretical advantage over control mode ventilation deals with volume-cycled positive-pressure ventilation. For the purpose of this article, a broader definition will be assumed and, thus, newer modes of positive-pressure and negative-pressure ventilation will be discussed.

I ! E III E 11 Positive-Pressure Ventilation A. Conventional ventilatory modes Three modes of volume-cycled positive-pressure mechanical ventilation are commonly described: control mode, assist-con­ trol mode, and intermittent mandatory ventilation. Intermittent mandatory ventilation is further divided into synchronized and nonsynchronized intermittent mandatory ventilation. These I : E I I I E I I conventional modes of volume-cycled positive-pressure ventila­ B. tion have been well described in the literature (1,2). Changes in airway pressure for spontaneous breathing and the conventional mode of mechanical ventilation are depicted in the Figure. / AA\ Control mode ventilation describes the delivery of a pre­ A\AA. 1; E 1 E ; selected tidal volume at a selected frequency, inespective of pa­ I II 1 tient effort. In essence, the ventilator and circuitry are totally unresponsive to patient effort or response, Assist-control mode ventilation provides the delivery of a se­ Figure—(A) Spontaneous breathing, (B) controlled mechanical lected tidal volume in response to the patient's respiratory effort, ventilation, (C) assist-control ventilation, (D) nonsynchronized usually sensed by a sub-baseline pressure in the inspiratory limb intermittent mandatory ventilation (continuous flow circuit), of the ventilator circuit. If such efforts are absent or if the pa­ and (E) synchronized intermittent mandatory ventilation tient's respiratory rate falls below a preset "control" rate, the (demand circuit). ventilator will deliver the preselected volume. Intermittent mandatory ventilation provides mechanical tidal volumes at set intervals, but allows the patient to spontaneously Submitted for publication: March 26, 1987. breathe gas of similar oxygenation and humidification as that Accepted for publication: April 20, 1987. •Department of Intemal Medicine, Division of Pulmonary and Critical Care Medicine, delivered by the ventilator between the ventilator-delivered Henry Ford Hospital. breaths. Synchronized intermittent mandatory ventilation com­ Address correspondence to Dr. Popovich, Department of Intemal Medicine, Division of Pulmonary and Critical Care Medicine, Henry Ford Hospital, 2799 W Grand Blvd, Detroit, bines spontaneous and assisted ventilation, providing a mechan­ Ml 48202.

Henry Ford Hosp Med J—Vol 35, No 1. 1987 Mechanical Ventilation—Popovich 63 in preventing disuse atrophy. As respiratory muscle function is ventilation that allows the patient to breathe from a metered, an important determinant of weaning from mechanical ventila­ preselected minute volume of fresh gas, with the ventilator de­ tion, a mode of ventilation that prevents respiratory muscle atro­ livering the volume difference between the preselected minute phy and discoordination would be favored. volume and the patient's spontaneous minute ventilation (12,13). Respiratory muscle function also may be affected during me­ The system assures the patient a constant, predetennined minute chanical ventilation by excessive work of breathing required by ventilation. A potential disadvantage ofthis mode is that patients the patient, if this work of breathing is sufficient to produce or may breathe at rapid rates and low tidal volumes approaching or perpetuate muscle fatigue. Machine settings and equipment dur­ equal to physiological dead space, thereby resulting in ineffec­ ing both assist-control and intermittent mandatory ventilation tive ventilation yet maintaining the predetermined minute may contribute to this excessive work of breathing. Concep­ ventilation. To prevent this disadvantage, mandatory minute tually, assist-control ventilation allows the patient to control the volume ventilators must monitor spontaneous and machine tidal level of ventilation but spares the patient the majority of the volumes and rates, triggering alarms if these values fall from energy expenditure. Recent work has demonstrated that even preselected ranges. Additional technology is required to provide under the most favorable conditions of minute ventilation, trig­ this monitoring, thereby adding expense and the potential of ma­ ger sensitivity, and inspiratory flow rate, normal subjects receiv­ chine failure. The effects of this type of ventilation on patient ing assisted mechanical ventilation expend energy equivalent to inspiratory work load should be considered similar to syn­ 33% to 50% of the work of passive lung inflation (4). Under the chronized intermittent mandatory ventilation, due to the de­ least favorable conditions, inspiratory work of breathing may mand valve flow system during spontaneous breathing. This substantially exceed the energy needed by the ventilator to in­ m.ode has been reported as offering more rapid weaning from flate the passive thorax (4). In an evaluation of 20 critically ill mechanical ventilation, but this has never been proven. patients receiving assisted mechanical ventilation, Marini et al Inspiratory pressure support, or pressure support, ventilation demonstrated that the patient's component of the mechanical provides a predetermined pressure to the patient's circuit during work load during assisted ventilation was an average of 62.6% the inspiratory phase (14). Pressure support is the simultaneous (range 30.3% to 116.3%) of work performed during spon­ pressurization of the ventilator circuit as the demand valve opens taneous breathing 30 seconds after discontinuation of ventilator during spontaneous breathing efforts. Pressure support aug­ support (5). ments spontaneous breaths with a variable, clinician-selected Intermittent mandatory ventilation circuits may also contrib­ pressure, which is provided as long as the patient effort con­ ute to high patient work loads. Intermittent mandatory ventila­ tinues. The patient thus has control of the ventilatory rate and the tion systems using "demand" flow during the spontaneous inspiratory assist time and can interact with the machine-deliv­ breathing phase, which is required for synchronized intermittent ered pressure to detennine the inspiratory flow and the delivered mandatory ventilation, may require the generation of a signifi­ tidal volume. Pressure support is designed to assist each spon­ cant negative pressure to initiate flow (6,7). Such systems may taneous breath, but can be given in conjunction with intermittent also have high maximal expiratory resistance (6). Both of these mandatory ventilation. Reported advantages are improvement in factors may significantly worsen respiratory muscle perfor­ the patient's comfort, reduction in the patient's ventilatory work, mance by "loading" weakened or fatigued respiratory muscles and provision of a more balanced pressure and volume change of patients with minimal reserve. Several ventilators impose dif­ form of muscle work to the patient (14). Pressure support has fering work loads on inspiratory and expiratory muscles (6), also been reported to allow more rapid weaning from mechan­ This disadvantage imposed by the demand valve systems re­ ical ventilation after coronary artery bypass surgery (15). A quired by synchronized intermittent mandatory ventilators is not major potential advantage of this method of support may be to offset by any major advantage over nonsynchronized intermit­ provide additional energy to overcome system-imposed work, tent mandatory ventilators (8). especially the increased work of breathing associated with the Rest of fatigued respiratory muscles is vital for functional activation of demand valves on synchronized intermittent man­ recovery (9,10). As respiratory muscle function is a critical de­ datory ventilators and to assist breathing efforts in difficult-to- terminant of weaning from mechanical ventilation, modes of wean patients. Experience and research is limited with this mechanical ventilation that properly provide adequate rest for mode. Further work is needed to detennine the role, if any, of reversal of fatigue without producing disuse functional losses pressure support ventilation in the armamentarium of the respi­ would be favored. Both assist-control and intermittent man­ ratory intensive care unit team. datory ventilation have putative advantages, but neither appears High-frequency ventilation is a method of ventilation deliver­ to have a clear-cut advantage from a muscle function standpoint ing small tidal volumes, approximating or even less than ana­ (11). Of greatest importance is the recognition of the way in tomical dead space, at rates greater than 60 breaths per minute which equipment and technical factors influence respiratory (16). The advantage of this ventilatory mode is predominately muscle function regardless of mode. related to the low mean airway pressure required to achieve ade­ quate ventilation. Lower airway pressures provided by this tech­ New ventilatory modes nique are thought to reduce the risk of baratrauma and adverse Two newer modes of positive-pressure ventilation have been cardiac effects. The three distinct modes of high-frequency described recently: 1) mandatory minute volume ventilation; ventilation, each having unique physiologic characteristics, and 2) inspiratory pressure support, or pressure support, ven­ include: high-frequency positive-pressure ventilation, high- tilation. Mandatory minute volume ventilation is a mode of frequency jet ventilation, and high-frequency oscillation.

64 Henry Ford Hosp Med J—Vol 35, No 1, 1987 Mechanical Ventilation—Popovich Although high-frequency ventilation offers an exciting and domen. Abnormal chest walls, such as in kyphoscoliosis, may innovative physiologic technique, clinical experience remains require the molding of special domes to provide sealing of the limited, and the clinical indications for the modes of high-fre­ shell-patient interface. quency ventilation remain to be established. To date, high-fre­ New designs in cuirass-like ventilators, such as chest wrap quency ventilation appears most promising in patients with devices, eliminate the constricting plastic chest shell and replace bronchopleural fistula and major air leaks (17). The mode has it with a half-cylinder mesh large enough to cover the thorax and also shown promise in neonatal ventilation for infant respiratory part of the abdomen. The patient is placed in a plastic wrap, distress syndrome and persistent fetal circulation (18). Clinical which can be sealed at the neck, arms, and legs and encom­ use of high-frequency ventilation on patients with respiratory passes the wire cage. Tubing access on the wrap allows attach­ failure should be reserved for controlled, prospective clinical ment of a cycling negative-pressure generator This in effect cre­ trials that allow an assessment of the risk/benefit considerations ates a nonconstricting, easily applied negative-pressure device in these patients and also in selected patients in whom conven­ that can support ventilation in patients without high minute ven­ tional ventilation has failed or is significantly likely to fail. tilation or ventilatory pressure.

Negative-Pressure Ventilation Oscillating beds Oscillating (rocking) beds are devices that angle the patient Negative-pressure ventilators are machines that create nega­ from a semi-upright to recumbent position approximately 12 to tive intrathoracic pressure in cycles to ventilate the patient. The 24 times per minute. The movement from the recumbent to the prototype ofthis method of ventilation is the caisson, or "iron- semi-upright position assists the diaphragm to descend and lung," ventilator, but discussions of negative-pressure ven­ produces negative intrathoracic pressure, thus providing in­ tilators should include all devices that ventilate by producing a spiration. Movement from the semi-upright to the recumbent negative intrathoracic pressure. This includes cuirass, chest position causes abdominal contents to move cephalad, produc­ wrap, and pneumobelt devices, as well as oscillating (rocking) ing a corresponding movement of the diaphragm and thus beds and chairs and diaphragmatic pacemakers. Although expiration. modes of ventilation ordinarily do not refer to negative-pressure ventilators, several methods of this form of ventilation should Although relatively inefficient, oscillating beds may be useful be reviewed as "new" modes or at least as rediscovered "old" for patients with predominately diaphragmatic weakness and modes. chronic respiratory failure, a condition in which the oscillating bed was first used during the poliomyelitis epidemics of the Negative-pressure ventilators have been used successfully to eariy 1950s, Despite creating considerable movement, this provide mechanical ventilation to patients with respiratory mode is generally well tolerated by patients, especially during failure due to chest wall restriction and to neuromuscular disease sleep when the physiologic effects of diaphragmatic weakness (19,20). Theoretically, these devices may rest the diaphragm by are at their greatest. limiting diaphragmatic energy expenditure. Therefore, indi­ viduals with chronic diaphragmatic fatigue, such as certain pa­ tients with respiratory muscle fatigue and chronic obstmctive Diaphragmatic pacemakers pulmonary disease, might be improved by such support (10), Diaphragmatic, or electrophrenic, pacing is a technique de­ Future studies are necessary for evaluation of this concept. veloped by Dr William Glenn at Yale University (22). This tech­ Negative-pressure ventilators are not ideal ventilatory de­ nique is the implantation of electrodes onto the phrenic nerve, vices. They are generally inefficient in producing the desired which are connected to subcutaneously imbedded radio-fre­ minute ventilation, although iron lung ventilation achieves quency receivers. A radio frequency transmitter, which in this reasonable efficiency. They are somewhat burdensome case is the ventilator, gives electrical stimuli via small antennae in application and contribute to a sense of "claustrophobia" placed overlying the receivers on the chest of the individual. and "restriction" according to many patients. Due to the This creates an electrical stimulus to the phrenic nerve, which asynchrony ofthese ventilators with upper airway muscles, eg, subsequently rhythmically contracts the diaphragm and pro­ the genioglossus, which need to contract during inspiration to vides ventilation. The volume of the breath is dependent on the maintain upper airway patency, upper airway obstmction may be magnitude and characteristic of the electrical stimulus and the precipitated in patients with such a predilection (infants or obese response of the muscle. For full ventilatory support, the tech­ patients) (21). nique generally needs to be applied to both phrenic nerves to prevent muscle fiber fatigue and injury. A considerable amount of time and effort is needed to establish adequate ventilation by Cuirass (chest shell) ventilators this technique. The technique has had its most common applica­ The chest shell ventilator is a dome-shaped apparatus which is tion in individuals with respiratory failure due to quadriplegia, fitted over the anterior chest and upper abdomen. Negative-pres­ although it has been applied in individuals with central hypoven­ sure is generated by a pump and transmitted to the chest under tilation (21,22). An added benefit of this mode of negative-pres­ the shell, resulting in negative intrathoracic pressure. Ventila­ sure ventilation is a possible training effect on the diaphragm tion is achieved by controlling the ventilator (pump) rate and the (22). Use ofthis technique is limited to specialized centers with negative pressure developed within the chest shell. The chest personnel of required surgical and medical expertise and experi­ shell must be large enough to fit over the chest and upper ab- ence in this form of ventilation.

Henry Ford Hosp Med J—Vol 35, No 1, 1987 Mechanical Ventilation—Popovich 65 Sunimary 10. Grassino A, Macklem PT. Respiratory muscle fatigue and ventilatory failure. Am Rev Med 1984:35:625-47. Although new modes of mechanical ventilation are appear­ 11. Weisman IM, Rinaldo JE, Rogers RM, Sanders MH. Intermittent man­ ing, the newer approaches often do not offer clearly significant datory ventilation. Am Rev Respir Dis 1983;127:641-7. advantages over older modes of mechanical ventilation. The 12. Hewlett AM, Piatt AS, Terry VG. Mandatory minute volume: A new con­ roles of such new modes of ventilation await clinical studies cept in weaning from mechanical ventilation. Anesthesia 1977:32:163-9. showing the benefits and riskso f such techniques. New applica­ 13. Laaban JP, Lemaire F, Baron JF, et al. Influence of caloric intake on the respiratory mode during mandatory minute volume ventilation. Chest 1985; tions of old modes of ventilation and refinements in cunent ap­ 87:67-72. paratus frequently offer more immediate promise to the clinical 14. Maclntyre NR. Respiratory function during pressure support ventilation. sphere of practice. Chest 1986:89:677-83. 15. Prakash O. Meij S. Cardiopulmonary response to inspiratory pressure References support during spontaneous ventilation vs conventional ventilation. Chest 1985:88:403-8. 1. Cane RD, Shapiro BA. Mechanical ventilatory support. JAMA 16. Popovich J Jr High-frequency ventilation: Current status. Henry Ford 1985:254:87-92. Hosp MedJ 1986;34:51-5. 2. Popovich J Jr The physiology of mechanical ventilation and the mechan­ 17. Carlon GC, Ray C Jr, Klain M. et al. High frequency positive-pressure ical zoo: IPPB. PEER CPAR Med Chn N Am 1983:67:621-31. ventilation in the management of a patient with bronchopleural fistula. Anesthe­ 3. Pontoppidan H. Gef finB , Lowenstein E. Acute respiratory failure in the siology 1980;52:160-2. adult, N Engl J Med 1972;287(pt 2):743-52. 18. Bland RD. Kim MH, Light MJ, Woodson JL. High frequency mechanical 4. Marini JJ, Capps JS, Culver BH. The inspiratory work of breathing during ventilation in severe hyaline membrane disease: An altemative treatment? Crit assisted mechanical ventilation. Chest 1985;87:612-8. Care Med 1980;8:275-80. 5. Marini JJ, Rodriguez M, Lamb V. The inspiratory workload of patient- 19. Garay SM, Turino GM, Goldring RM. Sustained reversal of chronic initiated mechanical ventilation. Am Rev Respir Dis 1986;134:902-9. hypocapnia in patients with alveolar hypoventilation syndromes: Long term 6. Christopher KL, Neff TA, Bowman JL. Eberie DJ, Irvin CG, Good JT Jr maintenance with noninvasive nocturnal mechanical ventilation. Am J Med Demand and continuous flow intermittent mandatory ventilation systems. Chest 1981;70:269-74. 1985:87:625-30. 20. O'Leary J, King R, LeBlanc M, Moss R. Liebhaber M, Lewiston N. 7. Gibney RTN, Wilson RS, Pontoppidan H. Comparison of work of Cuirass ventilation in childhood neuromuscular disease. J Pediatr 1979: breathing on high gas flow and demand valve continuous positive airway pres­ 94:419-21. sure systems. Chest 1982;82;692-5. 21. Glenn WWL, Gee JBL, Cole DR, Farmer WC, Shaw RK, Beckman CB. 8. Heenan TJ, Downs JB, Douglas ME, Ruiz BC, Jumper L. Intermittent Combined central alveolar hypoventilation and upper airway obstruction: Treat­ mandatory ventilation: Is synchronization important? Chest 1980:77:598-602. ment by tracheostomy and diaphragm pacing. Am J Med 1978:64:50-60. 9. Rochester DR, Braun NMT, Laine S. Diaphragmatic energy expenditure 22. Glenn WWL, Hogan JF, Loke JSO, Ciesielski TE, Phelps ML, Rowedder in chronic respiratory failure: The effect of assisted ventilation with body respi­ R. Ventilatory support by pacing of the conditioned diaphragm in quadriplegia. rators. Am J Med 1977:63:223-32. N Engl JMed 1984;310:1150-5.

66 Henry Ford Hosp Med J—Vol 35. No 1. 1987 Mechanical Ventilation—Popovich