Search PubMed⌕ Search

Biomedical subjects

J B Downs

Publications and source records attributed to J B Downs.

At least 73 records · Page 4Linked to original sources
PubMed1981

Postoperative atelectasis: Intermittent positive pressure breathing, incentive spirometry, and face-mask positive end-expiratory pressure.

Postoperative atelectasis has been treated with inspiratory maneuvers in an attempt to increase functional residual capacity. We compared the effect of intermittent positive pressure breathing (IPPB), incentive spirometry, and 5-cm H2O positive end-expiratory pressure (PEEP) applied with a face mask on the transpulmonary pressure (PL) at the end of expiration of eight patients 24 to 34 hours after aortocoronary bypass graft insertion. Intermittent positive pressure breathing and PEEP increased expiratory PL during therapy. After IPPB, expiratory PL fell below control values and then returned toward, but did not reach, control values After PEEP was discontinued, expiratory PL returned to control values within the next 30 minutes. The results suggest that face-mask PEEP will increase functional residual capacity, that incentive spirometry has little or effect, and that IPPB may decrease lung volume after treatment.

Humans↗
PubMed1981

Ionized calcium concentration and cardiovascular function after cardiopulmonary bypass.

Patients who required cardiopulmonary bypass were studied to determine the postoperative incidence of hypocalcemia and to quantify the effects of intravenous (IV) calcium chloride on ionized calcium (Ca++) concentration in blood and on cardiac function. Patients either received no calcium chloride postoperatively (control), or received it as an intermittent IV bolus (5 mg/kg) or as a constant infusion (0.5 mg/kg/min) whenever Ca++ concentration was less than 1.8 mEq/L. Hemodynamic profiles were determined every 15 minutes during the first two postoperative hours. Regardless of Ca++ concentration and therapy, cardiac indices, stroke indices, and vascular resistances of all patients never differed significantly. No variable changed consistently, other than Ca++ concentration, in those patients receiving calcium chloride. We conclude that postoperative hypocalcemia occurs frequently after cardiopulmonary bypass surgery, but not to the degree that would be expected to cause cardiovascular depression, and is readily corrected with IV calcium chloride. Myocardial depression occurred in all patients, but likely resulted from other causes.

Calcium↗
PubMed1981

Brain tissue pressure measurement during sodium nitroprusside infusion.

The effect of acute 25% decrease in mean arterial pressure (MAP) produced by sodium nitroprusside (SNP) infusion on mean intracerebral and cerebral perfusion pressures was examined in 7 swine with intracranial hypertension. All animals were anesthetized (alpha-chloralose), paralyzed (pancuronium bromide), orotracheally intubated and mechanically ventilated to maintain normocapnia. An epidural balloon was incrementally inflated to produce the desired brain tissue pressure (BTP) which was measured by an intracerebral microtransducer implanted contralateral to the balloon. MAP was measured from the carotid artery; cerebral perfusion pressure (CPP) was calculated as: MAP - BTP. During mildly (14 +/- 1 (SE) mm Hg) or moderately (37 +/- 3 mm Hg) increased BTP, SNP infusion increased BTP (17 +/- 1 and 44 +/- 3 mm Hg, respectively) (p less than 0.05). During severely increased BTP (70 +/- 4 mm Hg), SNP decreased BTP (53 +/- 4 mm Hg, p less than 0.05). Despite this variable BTP, CCP consistently decreased during SNP infusion. BTP and supratentorial subarachnoid cerebrospinal fluid pressure correlated closely; however, the supratentorial subarachnoid pressure consistently underestimated BTP. These findings suggest that estimation of BTP obtained from subarachnoid screw-type devices may not accurately record the pressure within cerebral tissue and the SBNP should not be administered to patients with known or suspected intracranial hypertension unless an intracranial (either BTP or subarachnoid) pressure is being monitored.

Animals↗
PubMed1981

Humidification of rapidly flowing gas.

Modern ventilatory systems that provide intermittent mandatory ventilation (IMV) and continuous positive airway pressure (CPAP) require high rates of flow of dry gases. Because inadequately humidified gas may contribute to respiratory morbidity, the authors conducted a laboratory study of the performance characteristics of four commercially available humidifiers during simulated clinical conditions. Performance was assessed with gases flowing continuously during simulated IMV and during controlled mechanical ventilation (CMV). A separate investigation assessed the thermal stability of and the resistance to flow through each device. It was found that the performances of the devices tested varied widely. With CMV, three of four humidifiers delivered humidified gas at 37 degrees C; however, with the greater total rate of flow of gas during simulated IMV, only one device provided humidified gas at 37 degrees C. Similarly, the thermal stability of the devices varied widely: two humidifiers' temperatures varied little during the study period; the temperatures on the other two devices varied more than 10 degrees C. The effectiveness of humidification and warming varied inversely with the rate of flow. For all devices, resistance to flow of gas was less than than 5 cm H2O/L . sec at rates of flow less than or equal to 60 L/min. The authors conclude that the performance characteristics of commercially available humidifiers vary widely.

Critical Care↗
PubMed1981

Temporal responses of functional residual capacity and oxygen tension to changes in positive end-expiratory pressure.

PEEP is widely accepted as a therapy for some forms of acute respiratory failure (ARF). PEEP increases functional residual capacity (FRC), decreases intrapulmonary shunt fraction, and improves arterial oxygenation. The time required for FRC and arterial oxygen tension (PaO2) to stabilize after an adjustment in the level of PEEP is not clearly established. Therefore, to determine the temporal relationship between PEEP, FRC, and PaO2 after adjusting the level of PEEP, aspiration pneumonitis was produced in swine. The FRC and the PaO2 decreased within seconds after intratracheal instillation of 0.1 N HCl; FRC of all animals was restored to its control value after the application of PEEP, 5 cm H2O, but PaO2 remained low. It was necessary to increase PEEP to 20 cm H2O and FRC to twice the control value to return arterial oxygenation to control levels. After PEEP was applied, an average of 15 sec was required to increase FRC; the less compliant the lung, the more rapid the change. After PEEP was removed, FRC stabilized within an average of 22 sec. When PEEP, 25 cm H2O, was removed, arterial oxygenation decreased suddenly and substantially which suggests that PEEP, especially at higher levels, should not be discontinued, even momentarily, for nonessential maneuvers.

Animals↗
PubMed1981

Inspiration: expiration ratio. Is mean airway pressure the difference?

Mechanical ventilation with different ratios of inspiratory to expiratory times (I:E ratio) and levels of positive end-expiratory pressure (PEEP) were applied to 9 dogs after the aspiration of fresh water and then were compared. The dogs were ventilated with an I:E ratio of 2:1 with 0 PEEP; an I:E ratio of 2:1 with 5 torr PEEP; and an I:E ratio of 1:2 with 5 torr PEEP. Peak airway pressures were regulated to maintain a mean airway pressure of 13 mm Hg. Arterial oxygenation was better with an I:E ratio of 1:2 with PEEP than with an I:E ratio of 2:1 either with or without PEEP (60 +/- 15 torr as compared with 42 +/- 11 torr and 43 +/- 10 torr). Cardiac index was depressed with all ventilatory modes, but oxygen delivery did not significantly differ among all the modes of ventilation. A ratio of prolonged I:E was not seen to benefit blood gas tension.

Airway Resistance↗
PubMed1981

Adult body temperature and heated humidification of anesthetic gases during general anesthesia.

To determine the effects on body temperature of heating and humidifying inspired anesthetic gases to 37 C and 100% relative humidity, 42 men who had major surgical procedures under general anesthesia were studied. Group 1 (control) consisted of 10 patients who inspired gases from a standard semicircle absorber system with no added humidity. The mean nasopharyngeal temperature decreased significantly from 36.2 +/- 0.1 C to 34.9 +/- 0.2 C. Ten other patients, group 2, inspired gases that were heated and humidified. Mean nasopharyngeal temperature was maintained at 36.4 +/- 0.2 C with no significant changes throughout the study. One patient in this group became hypothermic, but only transiently. Ten patients, group 3, were allowed to become hypothermic before gases were heated and humidified. The mean nasopharyngeal temperature increased significantly from 34.7 +/- 0.2 C to 36.0 +/- 0.3 C during 4 hours of heated humidification. For groups 4 and 5, six patients each, gases were heated and humidified on alternate hours. The responses of the two groups demonstrated a causal relationship between the heating and humidifying of inspired gases and an increased mean nasopharyngeal temperature. It is concluded that heating and humidifying gases to 37 C and 100% relative humidity effectively maintains normothermia and rewarms hypothermic adults during general anesthesia.

Anesthesia, Inhalation↗
PubMed1980

Assessment of cardiac filling pressure during continuous positive pressure ventilation.

Before and after 10 dogs were near-drowned with fresh water, cardiac filling pressures were measured during spontaneous respiration, controlled mechanical ventilation with ambient expiratory airway pressure, continuous positive-pressure ventilation (CPPV) with 20 ml H2O PEEP, and CPPV alone. Pulmonary arterial occlusion and left ventricular end diastolic pressures were measured and compared. Intrapleural pressure was subtracted from values for each of these pressures to calculate respective transmural filling pressures. Mechanical ventilation and CPPV each decreased thoracic venous return, but only CPPV increased pulmonary arteriolar resistance. The increase of both airway pressure and pulmonary arteriolar resistance, in turn, increased both right atrial and pulmonary arterial occlusion pressures, but decreased left ventricular filling. Thus, measurement of pulmonary arterial occlusion pressure alone did not allow accurate assessment of cardiac filling pressure. The authors found that measurement of intrapleural pressure was necessary to obtain an accurate reflection of left ventricular filling pressure during CPPV. Momentary interruption of CPPV to measure any pressure was of no value in assessing vascular filling and caused pulmonary edema in several animals. Therefore, the authors recommend that vascular pressures be measured and evaluated without interruption of positive airway pressure.

Animals↗
PubMed1980

Intermittent mandatory ventilation and weaning.

Oxygen, PEEP, and mechanical ventilatory therapy should be administered to patients in varying amounts and should be removed gradually and independently. The method of determining optimal PEEP, oxygen, and ventilation is not unlike that recommended for many other therapies. Nine years of prospective evaluation have demonstrated the numerous clinical advantages of this technique, and relatively few complications have been associated with it. Reduced FIO2 may promote resistance to atelectasis and allow rapid discontinuation of mechanical ventilation and PEEP. Similarly, optimal levels of PEEP may improve matching of ventilation and perfusion and assist lung mechanics so that FIO2 and mechanical ventilation may be reduced. Minimal mechanical ventilatory support eliminates iatrogenic respiratory alkalosis, and weaning from ventilatory support may be initiated early. This, in turn, minimizes the detrimental effects of mechanical ventilation on acid-base balance and cardiovascular function, as well as lessening barotrauma. We think that this approach has simplified the clinical management of patients with compromised repiratory function and decreased their mortality.

Humans↗
PubMed1980

Cardiopulmonary effects of intermittent mandatory ventilation.

IMV is a combination of spontaneous and mechanical ventilation. For numerous reasons, IMV is potentially more advantageous than conventional techniques. By maintaining spontaneous breathing, mechanical augmentation can be titrated to adjust alveolar minute ventilation levels to normal, thereby decreasing the incidence of respiratory alkalemia. There are major differences between the cardiopulmonary effects of IMV and conventional mechanical ventilation. Spontaneous inspiration decreases Ppl and results in better distribution of inspired gas, a better V/Q, and less physiological dead space. In addition, transmural filling pressures, venous return, and cardiac output are more normal than during conventional mechanical ventilation. Maintenance of spontaneous ventilation lowers mean Paw and pulmonary vascular resistance. If venous admixture occurs, it can be minimized by titrating PEEP. Thus, more effective therapy for hypoxemia is possible. If spontaneous breathing is to persist and be efective, work-of-breathing must be minimized. This can be accomplished best when a continuous flow of gas provides optimal CPAP to maintain FRC and to minimize the effects of decreased compliance without depressing cardiac function.

Cardiac Output↗
PubMed1980

Intermittent mandatory ventilation; is synchronization important?

Intermittent mandatory ventilation (IMV) allows patients to breathe spontaneously between mechanically supported breaths. Recently, several manufacturers have implied that nonsynchronous application of the mechanical breath may depress cardiovascular function and increase pulmonary barotrauma. Because it is technically difficult and expensive to synchronize mechanical ventilation to spontaneous breathing, we sought to determine whether there is any significant difference in cardiopulmonary function during synchronous and nonsynchronous mandatory ventilation. Our investigation failed to support the hypothesis that synchronization of spontaneous and mechanically mediated breathing is physiologically beneficial.

Airway Resistance↗
PubMed1980

Response of pulmonary venous admixture. A means of comparing therapies?

Patients who require cardiopulmonary bypass have a reproducible increase in interstitial pulmonary water and pulmonary venous admixture, a decrease in functional residual capacity, and a mild arterial hypoxemia. Therefore, such patients are good subjects for evaluating therapy with positive end-expiratory pressure and steroids, both of which are controversial. We sought to determine if the response of the pulmonary venous admixture to varying concentrations of inspired oxygen could affect the apparent efficacy of these types of therapy. Results of our study are presented.

Carbon Dioxide↗
PubMed1979

Alteration of oxygen tension and oxyhemoglobin saturation. A hazard of sodium bicarbonate administration.

The administration of sodium bicarbonate solution, which has been advocated for the treatment of metabolic acidosis, may have detrimental side effects. We evaluated oxyhemoglobin saturation and oxygen tensions in eight anesthetized swine before and after freshwater near-drowning and after a rapid intravenous infusion of 7.5% sodium bicarbonate solution (8 mEq/kg). After freshwater aspiration, arterial and venous oxygen tensions and oxyhemoglobin saturation decreased. Administration of sodium bicarbonate resulted in decreased venous and increased arterial, oxygen tensions. Arterial, but not venous, oxyhemoglobin saturation increased. These findings suggest that sodium bicarbonate caused a distinct leftward shift in the oxyhemoglobin dissociation curve, which could impair tissue oxygenation. Therefore, to avoid detrimental effects, sodium bicarbonate should be administered slowly and in a dose sufficient just to correct metabolic acidosis.

Acidosis↗
PubMed1979

Lung expansion, airway pressure transmission, and positive end-expiratory pressure.

Transmission of airway pressure to the intrapleural space and change in functional residual capacity by positive end-expiratory pressure (PEEP) were measured in ten anesthetized swine. Measurements and calculations were performed with varying lung and chest wall compliances. When both compliances were normal, approximately half of the applied airway pressure was transmitted. Aspiration of hydrochloric acid reduced lung compliance approximately fourfold and decreased airway pressure transmission. Increased thoracic compliance also reduced airway pressure transmission. When acid aspiration reduced lung compliance and sternotomy simultaneously increased thoracic compliance, pressure transmission was maximally reduced. Decreases in either thoracic or lung compliance reduced the volume-expanding effects of PEEP. Positive end-expiratory pressure was least effective when thoracic and lung compliances were reduced simultaneously. Careful assessment of both lung and thoracic compliances may be helpful in treating patients requiring elevated airway pressure.

Animals↗
PubMed1979

Renal function and cardiovascular responses during positive airway pressure.

The authors determined cardiovascular, renal, and hormonal responses to increased airway pressure during continuous positive-pressure ventilation (CPPV) and continuous positive airway pressure (CPAP). Nine healthy, hydrated laboratory swine had appropriate catheters placed to allow for measurement of intrapleural, aortic, inferior vena caval, and left ventricular end-diastolic pressures; cardiac output; and urinary flow. Samples of arterial blood were analyzed for oxygen and carbon dioxide tensions, pH, plasma vasopressin, osmolality, and creatinine and sodium concentrations. Urine was analyzed for osmolality and creatinine and sodium concentrations, and volume was recorded. Intrapleural pressure was subtracted from left ventricular end-diastolic pressure to calculate transmural pressure, a reflection of left ventricular filling pressure. Glomerular filtration rate and urinary free-water and osmolal clearances were also calculated. Expiratory left ventricular filling pressure was decreased equally by CPAP and CPPV. However, inspiratory left ventricular filling pressure and cardiac output were decreased by CPPV only. Urinary flow and glomerular filtration rate were decreased equally by CPAP and CPPV. Sodium excretion was decreased and plasma vasopressin increased by CPPV, but not by CPAP. Urinary free water and osmolal clearances were not changed by either ventilatory pattern. Although many of the renal-function variables were affected similarly by CPPV and CPAP, these alterations were not influenced solely by cardiac output or vasopressin, because only CPPV depressed cardiac output and increased vasopressin levels.

Animals↗