Ventilator Modes: Old and New

Mechanical ventilatory support

This chapter reviews the most updated knowledge regarding mechanical ventilation, its' indications and its' features both diagnostic and therapeutic. Further, the various modes of mechanical ventilation are described. The reader will also gain insight into the pathophysiology of various disease processes and the mode of ventilation that may be the most helpful in their treatment. Weaning is also discussed as well as a relatively new type of ventilation, non-invasive.

Continuous dose furosemide as a therapeutic approach to acute respiratory distress syndrome (ARDS)

BACKGROUND

Acute respiratory distress syndrome (ARDS) is seen in a variety of clinical settings in critically ill patients. ARDS has been defined as a clinical syndrome characterized by progressive hypoxemia, tachypnea, and generalized patchy bilateral pulmonary infiltrates in the absence of cardiac failure. Furosemide has been shown to improve pulmonary gas exchange and intrapulmonary shunt in animal models of ARDS by preferential perfusion of nonedematous lung units. We hypothesized that continuous dose furosemide would improve lung injury during resuscitation from oleic acid-induced lung injury in canines.

METHODS

Ten mongrel dogs were anesthetized and given intravenous oleic acid (0.1 mg/kg) to induce lung injury. Once lung injury was established (2 h) the control animals (n = 6) were continued on standard supportive therapy, and the study animals (n = 4) were started on continuous dose furosemide at 0.2 mg/kg/h. Cardiac filling pressures were maintained in all animals by infusion of isotonic saline solution. Data collected included lung injury score (LIS), cardiac index (CI), stroke volume index (SVI), pulmonary capillary wedge pressure (PCWP), urine output (UO), volume of resuscitation (VR), and pulmonary shunt fraction (Qs/Qt). Data were collected at baseline, established lung injury (2 h), and end of protocol (6 h). Data were compared between groups at various stages of the model using one-way analysis of variance with repeated measures.

RESULTS

All 10 animals survived the protocol. There was no difference between the experimental and control groups at baseline or established lung injury (2 h) for CI, SVI, PCWP, or VR. There was a significant improvement in PO2/FIO2 and reduction of PEEP values in the furosemide group. There was also a statistically significant difference between experimental and control groups in LIS, Qs/Qt, and urine volumes.

CONCLUSIONS

Continuous dose furosemide therapy improves LIS, PO2/FIO2, and Qs/Qt and decreases PEEP requirements in this oleic acid model of ARDS.

The acute respiratory distress syndrome

ARDS is a complex response of the lung to direct (inhalational) and indirect (hematogenous) insults. It is easy to be pessimistic about the benefit of the approaches we have described, but there is evidence that overall survival has improved in recent years. To maintain this progress, new therapies for ARDS must be rigorously evaluated, and their routine use should be recommended only after careful scrutiny of the evidence. Such a course will eliminate the unnecessary risks and costs often associated with unproved therapies.

The use of neuromuscular blocking drugs in the intensive care unit: a US perspective

Surprisingly little is known about the use of neuromuscular blockers (NMBs) in intensive care units (ICUs) in the USA. Recently, Klessig et al. surveyed anesthesiologists/intensivists in the USA and found that the 55% who responded used NMBs in the ICU in an average of 10 patients per ICU per month. Anxiolytics and analgesics were administered concomitantly with NMBs, but a majority of respondents did not use electrophysiologic measures of the degree of blockade. Another survey of predominantly medical ICUs also demonstrated widespread use of NMBs, but internists did not use sedation/analgesia as frequently as anesthesiologists for patients receiving NMBs, and infrequently monitored the degree of neuromuscular blockade. Because these were retrospective surveys, we decided to monitor prospectively the use of NMBs in our ICUs. The use of NMBs was ascertained by daily review of pharmacy records and, when use was documented, the patients' hospital records were reviewed. Where information was missing or not found, attending physicians were interviewed. On average, one patient per month per ICU received NMBs. Approximately 5% of neonatal and pediatric, and 1% of adult, ICU patients received NMBs. Eighty-three percent of patients received NMBs to facilitate mechanical ventilation, and mortality was high (51%) in those critically ill patients. More than half the patients were treated for < or = 24 h, the remainder for 2 days to > 3 weeks. Twitch monitors were used for monitoring the degree of neuromuscular blockade in adult patients, and all patients received sedatives/analgesics. We estimated that the risk of clinically significant, prolonged neuromuscular blockade following the discontinuation of NMBs was 5% per year.(ABSTRACT TRUNCATED AT 250 WORDS)

Volume-assured pressure support ventilation (VAPSV). A new approach for reducing muscle workload during acute respiratory failure

This study reports the preliminary clinical evaluation of a new mode of ventilation--volume-assured pressure support ventilation (VAPSV)--which incorporates inspiratory pressure support (PSV) with conventional volume-assisted cycles (VAV). This combination optimizes the inspiratory flow during assisted/controlled cycles, reducing the patient's respiratory burden commonly observed during VAV. Different from conventional PSV, VAPSV assures precise control of tidal volume (VT) in unstable patients. Eight patients with acute respiratory failure (ARF) were submitted to assisted ventilation under VAV and VAPSV. Patient's ventilatory workload (evaluated through the pressure-time product, mechanical work per liter of ventilation, and work per minute) and patient's ventilatory drive (occlusion pressure--P0.1) were significantly reduced during VAPSV. This "relief" was more evident among the most distressed patients (p < 0.001), allowing a reduction of more than 60 percent in muscle load, without the need of increasing peak tracheal pressure. Mean inspiratory flow (VT/TI), VT, and effective dynamic compliance were significantly increased during VAPSV, whereas the effective inspiratory impedance decreased. These mechanical advantages of VAPSV allowed a reduction of intrinsic PEEP, whenever it was present. Blood gas values were similar in both periods. We concluded that VAPSV is a promising form of ventilatory support. At the same time that it was able to safely assure a minimum preset VT, VAPSV reduced patient workload and improved synchrony between the patient and the ventilator during ARF.