Chapter 109

The past three decades have seen immense advances in the technologies used to replace the functions of failing organs or systems. During this time, mechanical ventilation has evolved from being used only as a last rite, when there was little hope of survival, to being a widely used and versatile technique that allows thousands of patients to recover from respiratory failure every year. Extracorporeal membrane oxygenation (ECMO), reintroduced into neonatal and pediatric intensive care medicine in the 1980s, is rapidly reaching similar status, as its indications and, perhaps more importantly, its limitations are better defined. More recently, external and implantable ventricular assist devices have undergone sufficient development to become viable alternatives for the continued support of patients with severe circulatory dysfunction, often as a bridge toward cardiac transplantation. While these technologies offer many possibilities, intensive care specialists must use them wisely by carefully selecting the patients who can benefit, by providing patients and families with realistic assessments of their potential, and by carefully evaluating the results to define better indications and to improve efficacy.

Today, any modern intensive care unit has at its disposal a wide array of ventilator models, many of them capable of effectively ventilating the lungs of adults as well as those of premature infants. Although these devices certainly make the task easier, their application to the treatment of infants and children requires more than a passing knowledge of respiratory physiology—it requires careful attention to monitoring the patient’s responses.

All commonly used methods of mechanical ventilation are based on the same principle: A mechanical device forces a gas mixture into the lungs during inspiration and then allows the passive recoil of lungs and chest wall to force gas out of the lungs during expiration. In most cases, the device uses a bellows or other type of pneumatic mechanism to inject a gas mixture into the airway lumen, either through a cannula inserted in the trachea or through a mask applied around the mouth and nose (positive-pressure ventilation). More rarely, the device lowers pressure around the patient’s chest and abdomen (negative-pressure ventilation), decreasing pleural and alveolar pressures relative to atmospheric pressure and drawing gas into the alveoli. Usually, a cannula must be placed in the trachea to prevent subatmospheric airway pressures from collapsing the pharynx and larynx during inspiration. Contrary to common misconception, positive- and negative-pressure ventilation apply similar strains to the lung tissue for a given level of lung inflation.3 Thus, it seems there would be no difference in the degree of lung injury caused by both methods, provided that tidal volume and functional residual capacity are kept equal. This consideration is, of course, limited by the practicality of its assumptions. Without entering a several-decade-long controversy on which method of ventilation is better,4 it seems clear that, by reducing rather than increasing pleural pressure relative to the atmosphere, negative-pressure ventilation may have a more beneficial effect on venous return to the heart than positive-pressure ventilation.5...

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