The proper use of mechanical ventilators is related to many problems; however, the primary one is the knowledge of the physician, nurse, inhalation therapist, and other paramedical personnel with a given ventilator. It is important to remember that there are certain inherent problem with all ventilators. These problem must be carefully searched for and avoided

Baker, James P.

VCU Scholars Compass

Proper Use of Mechanical Ventilators* JAMES P. BAKER, M. D. Associate Professor of M edicine, Director, R espiratory Intensive Care Unit , Medical College of Virginia. Richmond , Virginia The widespread use of mechanical ve ntil ators over the past ten to fifteen years has grea tl y com-plicated respiratory ca re. As ven til ators become more sophisticated, the requi re ment for physician knowledge becomes grea ter, since most of thi s equip-ment can cause major harm as well as provide ade-quate ventilation. There are two basic types of mec han ical venti-lators: the pressure cycled and the vol ume cycled . Each has inherent asse ts and problems. The pressure cycled ventilator is generally operated from a com-pressed gas source and delive rs to the patient vary-ing mixtures of oxygen , depending upon the source gas and the pressure-flow relati ons of the machine and the patient's a irways . This is a major problem with most pressure cycled ventil ators. The concen-trations of oxygen are unknown unless they are care-fully measured and controlled. In add itio n, the pres-sure cycled ventil ator cycles off when it arrives at a pressure set by the operator, thus tidal volumes can be greatly varied as lung compliance changes . There-fore , one must measure the oxygen delivery and tidal volume generated by such ven til ators to provide adequate oxygenation and ventilation. The major assets of pressure cycled ventilators are th at they are relatively inexpensive, small in size, and very versatile. The volume ventilators, by contrast, deliver to the patient a controlled tidal volume, independent of the pressure required to generate such a volum e. With constant volume delivery, minor variati ons in lung compliance and airways resistance are less likely to result in major ventil ato ry changes. Most volume ventil ators have a device which directly con-trols oxygen delivery to the pat ient , or a system with • Prese nted at th e 25th Annua l Stoneburn er Lecture Series, February 26 , 1972, at the Medica l Co ll ege of Vir-ginia , Richmond. MCV QUARTERLY 8(2): 123-126, 1972 a nomogram which allows one to arrive at the in-spired oxyge n percentage. However, there are some which req uire that oxygen percentage also be meas-ured as in the pressure cycled ventilators. It can be recognized that the primary factor in the proper use of mechanical ventilators is physi-cian and / or inhalat ion therapist knowledge of a par-t ic ular machine, and their ability to relate the patient to such a machine . In considering what a mechanical ventil ato r should do, the following equation demon-strates that to increase a patient's alveolar oxygen percentage, one can either increase the inspired oxygen percentage, decrease oxygen consumption, or increase alveolar ventilation. F ., o, = Fro, - vo,/V.,. F.,u, = <';O alveolar 0 2. ~, ., , = (Ila inspired Oz. Yo, = O, consumption. V_, = effective alveolar ventilation . Mechanical ventil ators function in several ways to provide these changes. The inspired oxygen concen-tration can be controlled at any level necessary for a given patient. By assuming part of the work of breathing, mechanical ventilators will decrease oxygen consumption, particul arly in those patients who have a remarkable increase in the work of b reat hing. Additionally, alveolar ventilation can be increased or decreased with a mechanical ventilator by adjusting the patient's tidal volume and/ or respiratory rate. Since all of these factors are of direct importance in oxygenation, it is valuable to remember this equation. Also important to keep in mind is that the level of ca rbon dioxide tension is controlled by the level of alveolar ventilation. As alveolar ventil ation is increased, carbon dioxide ten-sion will be decreased and vice versa. (Since we are discussing the proper use of mechanical ventilators, I have chosen to disregard the use of ventilators for intermittent positive pressure breathing therapy.) 123 124 Jn considering the mechanical venti lators, one must think of the indication for their use: I. To substitute for the normal work of breath-ing. 2. To assume a n increased work of breathing with increased airway or ti ssue resistance or increased dead space ventilation . 3. To change to a more effecti ve pattern of ventil ation . 4 . T o stabilize the chest wall aft er chest trauma. 5. To manage pulmo na ry congestion or edema. 6. T o improve oxygenation when severe hypox-emia cannot be corrected by other means. Although some of these indications might be better sati sfi ed with a particular type of respirator, with proper knowledge and experience a pressure or volume cycled ventilator might reasonably well pro-vide for any of these indic ations. This is a very im-portant consideration as people become more and more dependent upo n the more expensive volume cycled venti lators. It may be easier to ma nage a patient with multipl e complications wi th these types of ve ntil ators, however, it is in many circumstances unnecessary. Therefore, it is appropriate to contrast the complications which occur with these two types of ventilators. Complications of Mechanical Ventilation I . Problem s of tubes or trac heostomies 2 . Reduction of venous return 3. Atelectas is 4 . Oxygen tox icity 5. Infection 6. Acid-base abnormalities 7. Gastrointestinal co mplications 8. T echnical a nd mechanical co mplications 9 . Emotional problems I 0. Miscellaneous Intubation of a major bronchus leading to ate-lectasis of the other lung, tube dislodgment, laryn-geal damage or edema, or post-ex tubation laryngeal problems are well known to anesthesiologists. The tracheosto my avoids these problems which occur above the tracheostomy site; however, it co ntributes the additio nal problems of an open wound and a rigid tube through the neck, which might lead to bleeding, tracheostomy-s ite infec tion , o r necrosis with erosion of vessels . Tracheal stenosis may foll ow either tracheostomy or indotracheal tube intubatio n. There BAKER: PROPER USE OF MECHANICAL VENTILATORS is little difference in the frequency of these complica-tions with a pressure or volume cycled venti lator. Reduction in venous return resulting in de-creased cardiac output, in our experience, has been much more commonly a problem in patients who are hypo vo lemic. Auto nomic dysfunction and pro-longed hypoxemia also a re factors with this com-plication. To avoid this, one must monitor the blood pressure and urine output , and reduce mean inspira-tory pressure, in addit io n to providing adequate fluid volume. This is probably the major indication for the use of a volume ventilator to deliver the tidal volume rapidly, pa rticularly in patients with high compliance losses or high a irway resista nce. In these insta nces, a pressure cycled ventilator may require too lo ng a time to provide an adequate inspiration ma inta ining a positive intrathorac ic pressure for ex-cessive pe riods. M ost ventilators are fitted with a negat ive pressure device which operates du ring the end of ex ha lation to enh ance venous return. This in our hands has been of little o r no value and , at least in patients with chron ic obstructive pulmonary disease, is rel atively contraindicated since it produces closure of the poorly supported a irways, leading to miliary atelectasis. Atelectasis occurs in patients on prolonged me-chanical ventil ation , particularly in those with a high-inspired oxygen percentage. It may be mani-fested in numerous ways; however, typically it is indic ated by an increase in oxygen requirement and decreas ing compliance . Compliance loss is indicated by increasing inspiratory pressures to provide the same tidal volume or a decrease in tidal volume with a fixed inspiratory pressure . To avoid this problem, a proper respiratory pattern which provides for over-all venti lation of the lungs must be established. Con-tinuous pos itive pressure ventil ation , infl ation hold , and intermittent sighing a re all approaches to this problem . Most volume ventilators have the abil ity to perform one or mo re of these functions . However, they can also be readily handled with a pressure cycled ve ntil ator with proper inhalation therapy or nursing ca re to provide for intermittent deep breath-ing of patients and adequate a irway toilet . A very impo rtant factor in prevention of atelectasis is ade-quate humidification of inspired gases. Chest physio-the ra py, suctio n, a nd bronchoscopy a re also helpful in the avoidance a nd treatment of this problem. I would like to mention that oxygen toxicity usually occurs only with high-inspired oxygen per-centages a nd it is greatly enhanced by prolonged BAKER: PROPER USE OF MECHANICAL VENTILATORS 125 exposure. There are obviously multiple factors in-volved. It is of interest in our experience that if it does occur, it appears largely reversible if the patient survives. Particularly important to mention is that most all pressure cycled ventilators deliver higher oxygen concentrations than one would suspect. A common misconception is that on the air dilute setting these ventilators deliver 40 % oxygen. Therefore, one must set up this type of respirator by measuring oxygen delivery to avoid oxygen toxicity. This is done either by operating the pressure cycled ventila-tor with compressed air and adding oxygen, or by adding compressed air to an oxygen-driven machine while monitoring inspired oxygen percentage. One must be aware that as the pattern of the patient's breathing changes, there will be varying percentages of oxygen delivered by these types of ventilators. Infection is a major problem with mechanical ventilators since the warm, humid inspired gases, tub-ing, nebulizers, and humidifiers provide good culture media for many gram negative organisms. Me-chanical ventilators have been found to be a source of gram negative infection in many hospitals. In our experience in culturing respirators, we have found that no respirator had organisms cultured from it prior to the organism being grown from the patient's sputum. Thus if respirator tubing is changed at least daily and sterilized properly prior to each use, the probability that these machines will result in spread of infection decreases. The practice of using a ma-chine on more than one patient must be condemned unless everything from the respirator head out to the patient is changed between patients. Infection is managed with proper identification of the organism and specific therapy. Inadequate humidification of inspired gases is one of the major problems in the use of mechanical ventilators. At room temperature (70 °F), a gas saturated with water vapor contains approximately 20 mg of water per liter of gas. At body temperature, 44 mg of water are required to saturate a liter of gas. Since normal tracheal air is 100% humidified at body temperature, it is practically impossible to provide adequate quantities of water to equal the normal condition without passing the inspired gas mixture through a heated humidifier or nebulizer. This fact makes the use of heated humidifiers or nebulizers absolutely necessary with ventilators to avoid drying and damaging airway mucosa and ob-struction of airways with inspissated secretions. Most patients who have chronic respiratory failure wi ll have a compensated respiratory acidosis. As carbon dioxide is removed with an efficient me-chanical venti lator, severe post-hypercapneic meta-bolic alkalosis may be produced. The ability of di ssolved carbon dioxide to traverse the blood brain barrier with great ease and the resistance of bicar-bonate to cross th e same barrier may result in cen-tral nervous system alkalosis if PaC02 is too rapidly reduced. This can lead to convulsions, coma, and death, and must be considered in all patients who are being placed on mechanical venti lators. For this reason, the arterial pH rather than the PaC02 should be the controlled variable. There is evidence to in-dicate that patients on mechanical ventilators will retain fluid , therefore , patients must be weighed daily and electrolytes must be measured frequently. A significant number of gastrointestinal compli-cations are seen in patients severely ill with respira-tory failure. The most life threatening is gastrointes-tinal bleeding. This, while a significant problem, has not been too common in our experience . A more common problem is gastric distension or paralytic ileus in patients with severe lung disease. This may result in limitation of movement of the diaphram and a decrease in venti lation as well as basilar lung atelectasis. Technical and mechanical problems must al-ways be considered. The ventilator which becomes disconnected from a patient or malfunctions is of no value to the patient. Thus there must be a positive connection between the patient and the respirator for the machine to function properly. We have adopted the use of the Morch tracheostomy tube and have had little difficulty with disconnection following this change. Since such problems may be life threat-ening and each type of respirator has a particular set of associated problems, the therapist or physi-cian must be familiar with the problems which are possible with a particular type of ventilator. Changes in ventilatory or oxygen requirements must always be considered in managing patients on ventilators. Therefore one must measure these modalities fre-quently. Obviously, there are human errors in man-agement and the scope of these is beyond this dis-cussion. The miscellaneous complications which occur are multiple. Major cardiovascular problems, such as arrhythmias, digitalis toxicity, and pulmonary em-boli are not necessarily related to ventilators; how-ever, they must be recognized in patients on ventila-tors. Pneumothorax and mediastinal emphysema 126 must be antic ipated, si nce they may cause marked difficulty and frequently occur in ventilator patients. The other major problem is weaning a patient from a ven til ator. There are several factors to utili ze in deciding when to begin weaning a patient: I. Oxygen requirement: alveolar-arteria l oxy-gen gradient should be less than 300 mm Hg breathing 100% 0 2 2. Tidal volume : grea ter than 10 cc per kg body weight off respirator 3. Vital capacity: greater than twice the tidal volume 4. R atio dead space to tidal vo lume (Yd per Vt): less than 0.5 5. Tnspiratory negative pressure : more than 20 cm H 20 negative pressure These a re obv iously not the only factors in weaning. The clinical judgment of the nurses and inhalati on therapists is of major importance and is relied on heavi ly in our respi ratory care unit. There are sev-eral problems which we routinely see in patients who are difficult to wean from respirators. The pa-tient must be ready emotionally as well as physio-logically to be removed from the respirato r. He should have a rter ial blood gas values controlled near to those that he can support on his own without ass isted ventil ation. The pat ient should be cycling the ventilato r himself; although this is unnecessary , it is helpful. Adequ ate humidification must be pro-vided at all times when he is off the ventilator or he will have further problems with secretions and airway obstruction. Hi s secretions shou ld be under optimal control. One shou ld begin with brief periods off the venti lator, that is, fi ve to ten minutes per hour. Time off the ventil ator is prolonged as the patient's tolerance increases. Even while the pat ient is off the ventil ato r, his lungs should be hyper-expanded freq uently. Arte rial blood gases, vital signs, BAKER: PROPER USE OF MECHANICAL VENTILATORS and pati ent condition must be observed repeatedly during this period. We have always allowed the pa ti ent ini tia ll y to sleep on the ventilator and be weaned only during th e daytime; then he finally is all owed to sleep without ventil atory assistance. An-other ve ry important aspect o f weaning the patient is emotional support. A patient who can communi-cate vocally has a much b~tter opportunity for a reasonable emotional state. Our policy is to put in a ligh tweight pl as tic tracheostomy tube which can be plugged to a llow the patient to ta lk while he is being weaned. This procedure in many circumstances has been of great value. The proper use of mechanical ventilators is related to many problems; however, the primary one is the knowledge of the phys ician, nurse, inhalation therapi st, and other paramedical personnel with a given ventilator. It is important to remember that there are certa in inherent problems with all ventila-tors. These probl ems must be carefully searched for and avo ided. REFERENCES BENDIXEN. H . H .. <'I al. Respira1orv Care. Saint Louis: The C. V . Mosby Company. 1965. COMROE, J. H .. FORSTER, R. E .. D UBOIS. A. B .. BRISCOE, W . A .. ,,No CARLSEN, E .. JR. The Lung. Chicago: Yearbook M edica l Pu blica tions. 1962 . KILB URN. K. H . Shock. seizures. and coma with alk alosis du r ing mechanica l ventilati on. Ann. / 111. Med. 65:977-984. 1966. REINANZ. J . A .. e l 11/. The potential ro le o f inhalation th era py equipment in nosocomial pulmonary infection. ! . C/in. l nve.1·1. 44:83 1- 839. 1965. SLADEN. A .. <' I a l. Pulmonary complications and water re-tention in prolonged mechanica l ventil ation. Ne w Eng. ! . M ed. 279 :448- 453 . 1968. 

Proper Use of Mechanical Ventilators

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Proper Use of Mechanical Ventilators

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