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Editorial Indian Pediatrics 1998; 35:595-600 |
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Neonatal Ventilation: Present and Future Directions |
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Continuous Positive Airway Pressure (CPAP) The use of CP AP has become an important adjunct in neonatal respiratory management and is increasingly being used either prior to intubation or immediately after extubation. CP AP improves oxygenation by an increase in the functional residual capacity (FRC), and in some respects, functions similar to surfactant(2). Thus, physiologically, efficient application of CP AP makes sense, but recent randomized trials assessing the role of nasal and nasopharyngeal CP AP in neonates, both early CPAP(3) and after extubation(4), have shown conflicting results. Because of this, there is no justification to recommend routine post-extubation CP AP. Instead, it seems reasonable to extubate (particularly small babies) to supplemental oxygen first, and consider CP AP as a first line of treatment if there is inadequate alveolar ventilation, particularly if this is attributable to post-extubation atelectasis. Although there are different methods of providing CP AP, the best route appears to be through nasal prongs because of ease of application and physiological advantages (less resistance). Nasopharyngeal or endotracheal (ET) CP AP may be counterproductive, especially in small babies, and should not be used. Over-reliance on CP AP may also be dangerous and it should only be used if infants show adequate spontaneous respiratory effort, appear to be tolerating CP AP well, and maintain adequate arterial blood gases (PC02 of < 50 torr, and pH > 7.25). Techniques of Assisted Ventilation Because of newer designs and concepts, it is useful to understand some specific nomenclature currently used. Positive- pressure ventilation in newborns is accomplished through either conventional or high frequency ventilators (capable of cycling at rates more than 150 breaths per minute). Conventional ventilators are either 'pres- sure' or 'volume' types. They can be further classified on the basis of the cycling mechanism, i.e., the way in which the inspiratory cycle is terminated. Thus, in pressure- limited, time-cycled ventilation, a peak inspiratory pressure is set and during inspiration, gas is delivered to achieve that target pressure. After the target is reached, the remainder of the gas volume is released into the atmosphere. As a result, the tidal volume delivery with each breath is variable despite the recorded peak pressure being constant. Inspiration also ends after a preset (limited) time period. In contrast, with volume limited modes, a preset volume is delivered with each breath regardless of the pressure that is needed (unless arbitrarily limited by the clinician). Some ventilators also use airway flow as the basis of cycling, in which inspiration ends when flow has reached a critical low or preset level (flow-cycled ventilation). Ventilators are now in use which provide the capability of using either volume or pressure-limited, or flow-cycled ventilation, depending on the operator's preference. In a recently published randomized controlled trial, it was shown that volume-controlled ventilation was both safe and effective when used in a group of preterm infants with RDS who weighed more than 1200 g. Compared with infants who received time-cycled, pressure-limited ventilation, infants treated with volume controlled ventilation weaned faster and were extubated significantly sooner and had fewer complications(5). High-frequency ventilation is most frequently provided by oscillatory ventilators. (HFOV) which are essentially airway vibrators (piston pump or vibrating diaphragm) that operate at frequencies ranging from 240 to 24,000 breaths per minute. During HFOV, inspiration and expiration are both active processes. A continuous flow of fresh gas (bias gas flow) rushes past the source that generates the oscillation, and a controlled leak or low-pass filter allows gas to exit the system. Pressure oscillations within the airway produce tiny tidal volume fluctuations around a constant distending pressure. This maintains lung volume in a fashion similar to CPAP. The tidal volume is determined by the amplitude of the airway pressure oscillation (ΔP) which, in turn, is dependent on the stroke length of the device that produces the oscillation. Patient- Triggered Ventilation (PTV) refers to a form of mechanical ventilation in which the machine delivered breath is initiated in response to a signal derived from the patient's own inspiratory effort, thus synchronising the onset of both spontaneous and mechanical breaths. Th signal 'event' needs to be an accurate measure of the infant's respiratory drive but should minimize artefact resulting from some other source. Three signals have been utilized to provide PTV to the newborn; impedance, pressure, and flow. Each has inherent advantages and disadvantages(6). Patient-triggered ventilation is available in both pressure-limited and volume-con- trolled modes, and can be accomplished in a variety of ways. Fundamental to all of these methods is a switch in the control of ventilation from clinician to patient. Despite earlier concerns about the ability of smaller babies to trigger ventilation(7), the sophistication of newer generation ventilators allow its application to even the smallest of babies(8). However, despite improvements in the software, some of the ventilators seem to perform less well in one mode than another(9). Types of PTV Assist/Control Ventilation (A/C) misnomer- ed as PTV, is a combination mode in which the ventilator delivers a positive pressure breath. in response to the patient's inspiratory effort (assist) provided it exceeds a preset threshold criteria (pressure, flow or impedance). The back-up rate (control) ensures a minimum mandatory minute ventilation rate in case the patient stops making inspiratory effort. It is probably the best mode to use in the premature infant in the acute phase of illness because it requires the least amount of patient effort, and produces improved oxygenation at the same or lower mean airway pressure than other conventional modes(10). In addition, inspiratory flow is proportional to patient effort. Synchronised Intermittent Mandatory Ventilation (SIMV) refers to a ventilatory mode where the mechanically delivered breaths are cycled at a rate set by the clinician but are synchronised to the onset of the patient's own breath. In between mechanical breaths, the patient breathes freely from the bias flow in the circuit. The flexibility of SIMV in providing a range of ventilatory support makes it useful, both as a primary means or as a method for weaning(11). However, reducing the SIMV to a very slow rate « 20 per minute) may be unwise particularly when discontinuation of mechanical ventilation is imminent, as this may impose significant work of breathing for an intubated baby, thus contributing to weaning failure. This disadvantage of slow rate SIMV can be compensated by some further means of breath support such as pressure support ventilation. Pressure Support Ventilation (PSV) is designed to assist the patient's spontaneous breathing with an inspiratory pressure 'boost'. In this respect PSV closely resembles assist/ control ventilation. How- ever, being flow-cycled, PSV is better customised to support and synchronise with patient effort because the patient has full control of both how much to breathe (inspiratory flow rate) and for how long (inspiratory time). In this sense, it is more physiological and mimics the patient's own spontaneous breaths. Although PSV can be used for full ventilation (PSV max), in practice it is generally used as a weaning mode, mostly in conjunction with volume-cycled SIMV(12). Proportional Assist Ventilation (P A V) and Mandatory Minute Ventilation (MMV) Two other promising synchronised ventilatory strategies are proportional assist ventilation, in which the ventilator generates pressure proportional to patient's effort, and mandatory minute ventilation, whereby the clinician programs a target minute ventilation and the ventilator provides supplemental breaths if the patient fails to achieve the target value. With P A V, the more the patient 'pulls', the more pressure the machine generates. Thus, the responsibility for determining the level and pattern of breathing is shifted entirely from the clinician to the patient(13). Several P A V delivery systems are currently under development for clinical use, but so far no data is available relating to its use in the neonatal population. With MMV, the patient can do all, some, or none of the work of breathing while adequate alveolar ventilation is assured. Rescue Strategies for Management of Ventilatory Failure Despite improvements in ventilatory techniques, conventional ventilation will still fail in certain specific situations where alternative treatment such as ECMO(14), high-frequency ventilation (HFV)(15), or inhalational nitric oxide (INO)(16) administration may have potential benefit. Hence, each Unit practising neonatal ventilation should agree on a criterion for judging treat- merit failure. A commonly used parameter which serves as a good clinical indicator for such purposes is: PAW x FiO2 x 100 Oxygenation Index (OI) = ------------------------------- PaO2 (PAW = Mean Airway Pressure) A 01 of > 40 indicates severe respiratory compromise, whereas an 01 between 25 and 40 suggests failure to respond -to the existing mode of ventilatory support and the need for rescue therapy(17). Although, each of the modalities (HFV, ECMO, INO and lately liquid ventilation) have been in use for some time, and individual centers have shown enthusiasm for their application, there is still no irrefutable evidence to advocate their universal use in the management of neonatal respiratory failure. Moreover, coincident with improvements in conventional ventilation, the utilization of these methods is already declining in many Units (personal communication). In our experience, each of these rescue modalities seems more effective than others in specific clinical situations, and perhaps choosing a disease-appropriate strategy rather than the type of ventilation (or treatment), may be the more important determinant of clinical success. Weaning from Mechanical Ventilation (Art or Science?) Weaning babies from mechanical ventilation is one of the commonest decision making process in neonatal medicine, but there is no consensus as to what is the most appropriate time to discontinue mechanical ventilation. Moreover, with the different styles of ventilation in use, the individual methodology for weaning may differ among. clinicians or institutions according to experience and practise. Physiologic essentials for. successful extubation include reliable respiratory drive, neuromuscular competence, and reduced respiratory system load. With the availability of pulmonary mechanics testing at the bedside, investigators have used a number of physiological parameters such as tidal volume, minute ventilation (ticial volume x rate), and ratio of respiratory rate to tidal volume (rapid shallow breathing index) as a part of 'protocol' based weaning compared to those based on clinical perception. Recently, we assessed the feasibility of using the ratio of spontaneously generated to mechanically generated' minute ventilation to predict successful extubation. In this study, if the spontaneous minute ventilation exceeded 50% of the mechanical minute ventilation, babies were extubated. Using this method, 49 of 57 preterm infants were successfully extubated, giving. a: positive predicted value of 86%(18). A randomized controlled trial is underway to determine the best application of this tool. Pulmonary Mechanics Testing With the increasing complexity of neonatal respiratory care, pulmonary mechanics testing is emerging as a valuable tool to aid clinical decision making in the management of ventilated infants(19). This is based on the rationale that early identification of pulmonary problems, and institution of appropriate therapeutic or ventilatory adjustments will improve the dysfunction and/ or reduce chronic lung injuries. Besides assessment of acute respiratory distress and evaluation of mechanical ventilation, potential benefits of real-time pulmonary graphics include assessment of suitability for weaning, monitoring of complex treatments such as ECMO or INO, and follow-up of chronic lung disease. The two forms of respiratory graphics most widely utilized in clinical practice are scalar waveforms and loops, both showing simultaneous relations between pressure, volume, and air flow(20). Although there are as yet no published clinical trials to suggest that pulmonary mechanics testing reduces mortality or morbidity, it has in conjunction with clinical, radiological and blood gas monitoring - changed neonatal ventilation for 'good judgement' to 'informed judgement'. It is not surprising that pulmonary mechanics testing is increasingly becoming an essential element in the assessment of patient status, therapeutic evaluation, and the 'fine tuning' of ventilator settings to customise management according to the problems and responses of individual patient.
1.
Sinha SK, Donn SM. Advances in neonatal Conventional Ventilation. Arch Dis Child 1996; 75: F135-F140. |
