Roger S. Wilson

Hemodynamic responses to mechanical ventilation with PEEP: the effect of hypervolemia.

The hemodynamic effects of prolonged mechanical ventilation with positive end-expiratory pressure (PEEP), with and without blood volume augmentation, were studied in 18 beagles anesthetized with halothane (0.7 per cent end-tidal). Addition of 12 cm H2O PEEP during mechanical ventilation in normavolemic dogs was associated with reductions of transmural cardiac filling pressures, cardiac index and stroke index to 50 per cent of control values. Circulatory adaptation did not occur. Filling pressures and flow remained unchanged during the ensuing 8 hours when PEEP was maintained. They returned to control levels when PEEP was discontinued, except for the transmural right ventricular end-diastolic pressure, which remained elevated above control levels. Systemic vascular resistance was unchanged, but pulmonary vascular resistance doubled upon addition of PEEP. Following autologous whole blood transfusion (25 ml/kg) during mechanical ventilation with PEEP, cardiac index returned to, and remained at, control levels. After PEEP was discontinued, cardiac index increased acutely and remained elevated for the remainder of the study period (as long as 7 hours). Comparable transfusion during mechanical ventilation without PEEP elevated cardiac index only transiently. Right atrial, pulmonary capillary wedge, and right and left ventricular end-diastolic pressures showed marked increases relative to atmospheric with PEEP and after transfusion. Calculated transmural pressures demonstrated clear reductions with application of PEEP, followed by increases to control levels with transfusion and further increases to above control when PEEP was discontinued. Study of ventricular function curves revealed that changes in filling pressures and not to changes in ventricular contractility. Transmural pulmonary arterial diastolic pressure rose throughout the 12 hours of study, despite return of pulmonary vascular resistance to control level with removal of PEEP. Thus, acute decreases in cardiac filling pressure, cardiac index, and stroke index persist consequent to application of PEEP, and circulatory adaptation does not occur. The apparent hemodynamic deterioration may be reversed by blood volume augmentation, but when PEEP is discontinued, hypervolemia with consequent increases in filling pressures and a move along a ventricular function curve will occur. Changes in cardiac index will depend upon the overall state of right and left ventricular contractility.

Transperitoneal versus retroperitoneal approach for aortic reconstruction: A randomized prospective study*

A prospective, randomized study was conducted to compare the retroperitoneal versus transperitoneal approach for elective aortic reconstruction. One hundred thirteen patients (transperitoneal = 59, retroperitoneal = 54) were randomized between March 1987 and October 1988. In addition, to assess the changing course of patients undergoing aortic reconstruction similar data were gathered retrospectively on a group of 56 patients undergoing aortic reconstruction by the same surgeons performed via a transperitoneal approach in 1984 to 1985. Randomized patients were identical in age, male to female ratio, smoking history, incidence and severity of cardiopulmonary disease, indication for operation, and use of epidural anesthetics. Details of operation including operative and aortic cross-clamp times, crystalloid and transfusion requirements, degree of hypothermia on arrival at the intensive care unit, and perioperative fluid and blood requirements did not differ significantly for patients undergoing transperitoneal versus retroperitoneal reconstruction. Respiratory morbidity, as assessed by percent of patients requiring postoperative ventilation, deterioration in pulmonary function tests, and the incidence of respiratory complications, was identical in randomized patients. Other aspects of postoperative recovery including recovery of gastrointestinal function, the requirement for narcotics, metabolic parameters of operative stress, the incidence of major and minor complications, and the duration of hospital stay were similar for randomized patients undergoing transperitoneal and retroperitoneal reconstruction. When compared to retrospectively reviewed patients having aortic reconstruction, randomized patients undergoing transperitoneal and retroperitoneal operations had highly significant (p less than 0.001) reductions in postoperative ventilation, transfusion requirements, and length of hospital stay. Such trends were all independent of transperitoneal versus retroperitoneal approach.(ABSTRACT TRUNCATED AT 250 WORDS)

Pulmonary barotrauma during mechanical ventilation

In the treatment of acute respiratory failure, pulmonary barotrauma (subcutaneous emphysema, pneumothorax, and pneumomediastinum) developed in ten patients (10%) receiving IPPV without PEEP and in seven patients (11%) receiving IPPV with PEEP. Pre-existing chronic obstructive pulmonary disease seemed to predispose to the development of pulmonary barotrauma. Necropsy findings revealed the presence of pneumonitis and pulmonary edema in the majority of patients, in addition to pulmonary emphysema in some. Available information fails to demonstrate a correlation between the incidence of pulmonary barotrauma and the magnitude of air-way pressure required for adequate mechanical ventilation. The addition of PEEP to IPPV during therapy for acute respiratory failure does not in-crease the incidence of pulmonary barotrauma.

Effect of mechanical ventilation with end-inspiratory pause on blood-gas exchange.

The effects of end-inspiratory pause (EIP) on gas exchange were measured in 10 adult patients with acute respiratory insufficiency while maintained on mechanical ventilation. Four inspiratory patterns were studied with a constant tidal volume (10 to 15 ml/kg body weight), respiratory rate (9 to 12 breaths/ min), FIO2 (0.5) and end-expiratory pressure. Inspiratory flow rate (&OV0312;insp) and EIP time were varied to produce a control pattern (&OV0312;insp = 60 L/min, EIP = 0), 2 EIP patterns of 0.6 and 1.2 seconds with a similar &OV0312;insp and a “slow” flow pattern (&OV0312;insp = 30 L/min) without EIP. The control pattern was applied before and after each study period.Arterial oxygenation was unchanged with both EIP and “slow” flow patterns when compared to control. Dead-space ventilation (VD/VT) and Paco2 were significantly decreased (p<0.01) as EIP was increased from 0 to 1.2 seconds, but remained unchanged with slow inspiratory flow. Thus, EIP improved the efficiency of ventilation with no apparent improvement in oxygenation in patients with acute respiratory insufficiency.

The Oxygen Cost of Breathing Following Anesthesia and Cardiac Surgery

The amount of oxygen consumed by respiratory muscles during spontaneous respiration was measured in ten patients immediately following mitral valve replacement. The cost of breathing was calculated as the difference (Δ02) between mean &OV0312;02‘s measured during spontaneous and mechanical ventilation. In every patient, mean &OV0312;02 was greater during spontaneous than during mechanical ventilation; 150.5 = 8.1 ml/m2/min (mean = SE) vs. 121.1 = 6.0 ml/m2/min. The mean Δ&OV0312;02, or oxygen cost, was 29.7 = 6.6 ml/m2/min (range 3.8–64.6 ml/m2/min), which represents 18.6 per cent of &OV0312;02 utilized by respiratory muscles, as opposed to the accepted norm of 1–3 per cent. This cost was predictable from preoperative vital capacity; the lower the vital capacity (expressed as per cent predicted vital capacity), the greater the oxygen cost of breathing. The results confirm one beneficial aspect of mechanical ventilation during the immediate postoperative period, in climinaling the oxygen cost of breathing and reducing total &OV0312;02.

Bronchial injury during lung isolation in a pediatric patient.

I ndications for single-lung ventilation in the pediatric patient include lung isolation and enhanced surgical exposure. The choice of technique is limited because double-lumen endobroncbial tubes, standard in adults, are useful only for larger children. Alternatives for infants and young children include selective endobronchial intubation and insertion of a bronchial blocker. There are disadvantages to each approach. We describe a case in which the use of a blocker was associated with bronchial rupture.

Comparison of thoracic and lumbar epidural infusions of bupivacaine and fentanyl for post-thoracotomy analgesia.

Epidural analgesia, via either a thoracic or lumbar route, is commonly used to provide postoperative analgesia following thoracotomy for pulmonary resection, but little data indicate which location is better in terms of postoperative analgesia, side effects, or associated complications. In this study, 45 patients, who were scheduled to have epidural analgesia and undergo a lateral thoracotomy, were randomized to receive either a thoracic or a lumbar catheter. Pain assessments and routine clinical data were recorded to determine if either thoracic or lumbar epidural catheters provided superior analgesia, fewer side effects, or fewer complications. This study found no statistical difference in pain relief or side effects between lumbar and thoracic epidural analgesia for post-thoracotomy pain. An increased infusion rate (6.4 +/- 1.9 v 5.1 +/- 1.4 mL/h, P = 0.02) was required in the lumbar group to achieve equivalent analgesic levels.

Morphine Therapy Controls Autonomic Hyperactivity in Tetanus

Excerpt Cardiovascular dysfunction is a well-recognized complication of generalized tetanus in man (1, 2). Both alpha and beta adrenergic blocking agents have been advocated to control arrhythmias ...

Extrapulmonary factors influencing the ratio of arterial oxygen tension to inspired oxygen concentration in burn patients.

The ratio of arterial oxygen tension to inspired oxygen concentration (PaO2/FIO2) as an index of respiratory function was evaluated in 22 patients with body surface area burns of 15–80%. These results indicate that this ratio is limited in its applicability because extrapulmonary factors, such as cardiac output, oxygen consumption, and arterial oxygen content, can affect this index by alterations in the amount of venous desaturation. Useful estimates of intrapulmonary right to left shunt (Qs/Qt) from PaO2/FIO2 were obtained only when arteriovenous oxygen content differences (aQDO2) were between 3–5 ml/dl. There were aVDO2 values above and below 3–5 ml/dl in at least 35% of the observations. Under these circumstances, PaO2/FIO2 did not correctly reflect changes in Qs/Qt. Blood gases from central venous catheters did not mirror changes in true mixed venous blood and, thus, can lead to erroneous estimations of Qs/Qt. Rational therapy of reduced arterial oxygen saturation requires measurement of both extra- and intrapulmonary factors contributing to arterial desaturation. Measurement of PaO2/FIO2 alone will not estimate these factors.

Ondansetron does not affect alfentanil-induced ventilatory depression or sedation.

Ondansetron is a selective 5-hydroxytryptamine type 3 receptor antagonist effective as an antiemetic in patients experiencing post-operative or cancer chemotherapy-induced nausea and vomiting. Currently, no information is available regarding the interaction of ondansetron with opioids, although a serotonin antagonist might be expected to modify some opioid actions. This study was designed to measure the effects of ondansetron on alfentanil-induced ventilatory depression and sedation in healthy male volunteers. Ventilatory drive (measured as the end-tidal CO2 necessary to produce a minute ventilation of 15 l/min) was determined in 29 subjects using a modification of the Read rebreathing technique. Sedation was measured by asking the subjects to complete visual analog scales. Alfentanil was administered as a bolus (5 micrograms/kg) followed by a continuous infusion (0.25-0.75 micrograms.kg-1.min-1) for at least 90 min. Study medication (ondansetron 8 or 16 mg or vehicle placebo) was then administered in a randomized, double-blind manner, and the alfentanil was infused for an additional 15 min. Measurements of ventilatory drive and sedation were made at baseline, during alfentanil infusion, after study medication, and at 30-min intervals after alfentanil was discontinued. Alfentanil produced significant ventilatory depression (P less than 0.001) and sedation (P less than 0.001) in all three groups. Neither placebo nor ondansetron produced further change in the intensity of either alfentanil effect. After discontinuation of the opioid, both ventilatory depression and sedation decreased, and the rate of recovery was not significantly different between groups. The data indicate that alfentanil-induced sedation and ventilatory depression are not significantly affected by the subsequent administration of ondansetron.