Supplemental oxygen impairs detection of hypoventilation by pulse oximetry

STUDY OBJECTIVE

This two-part study was designed to determine the effect of supplemental oxygen on the detection of hypoventilation, evidenced by a decline in oxygen saturation (Spo(2)) with pulse oximetry.

DESIGN

Phase 1 was a prospective, patient-controlled, clinical trial. Phase 2 was a prospective, randomized, clinical trial.

SETTING

Phase 1 took place in the operating room. Phase 2 took place in the postanesthesia care unit (PACU).

PATIENTS

In phase 1, 45 patients underwent abdominal, gynecologic, urologic, and lower-extremity vascular operations. In phase 2, 288 patients were recovering from anesthesia.

INTERVENTIONS

In phase 1, modeling of deliberate hypoventilation entailed decreasing by 50% the minute ventilation of patients receiving general anesthesia. Patients breathing a fraction of inspired oxygen (Fio(2)) of 0.21 (n = 25) underwent hypoventilation for up to 5 min. Patients with an Fio(2) of 0.25 (n = 10) or 0.30 (n = 10) underwent hypoventilation for 10 min. In phase 2, spontaneously breathing patients were randomized to breathe room air (n = 155) or to receive supplemental oxygen (n = 133) on arrival in the PACU.

MEASUREMENTS AND RESULTS

In phase 1, end-tidal carbon dioxide and Spo(2) were measured during deliberate hypoventilation. A decrease in Spo(2) occurred only in patients who breathed room air. No decline occurred in patients with Fio(2) levels of 0.25 and 0.30. In phase 2, Spo(2) was recorded every min for up to 40 min in the PACU. Arterial desaturation (Spo(2) < 90%) was fourfold higher in patients who breathed room air than in patients who breathed supplemental oxygen (9.0% vs 2.3%, p = 0.02).

CONCLUSION

Hypoventilation can be detected reliably by pulse oximetry only when patients breathe room air. In patients with spontaneous ventilation, supplemental oxygen often masked the ability to detect abnormalities in respiratory function in the PACU. Without the need for capnography and arterial blood gas analysis, pulse oximetry is a useful tool to assess ventilatory abnormalities, but only in the absence of supplemental inspired oxygen.

Prevention of postoperative pulmonary complications with CPAP, incentive spirometry, and conservative therapy

Continuous positive airway pressure (CPAP) administered at intervals with a mask and incentive spirometry (IS) were compared with a regimen of coughing and deep breathing (CDB) to determine which promoted the most rapid recovery of pulmonary function after upper abdominal operations in 65 adults. Postoperatively, FRC of patients in all groups was similar relative to preoperative values. However, mean FRC of patients who received CPAP increased more rapidly than did mean FRC of those receiving CDB when compared to the values obtained following operation (p less than 0.05). Incentive spirometry did not increase FRC to a greater extent than did CDB. Roentgenographic evidence of atelectasis 72 hours postoperatively was observed in 23 percent of CPAP patients (five of 22) and 42 percent and 41 percent of patients who received CDB (eight of 19) and IS (nine of 22). Two patients (3 percent) developed pneumonia. The low incidence of pneumonia regardless of the type of therapy may be attributable to vigorous, vigilant respiratory care in a population at high risk for developing pneumonia. Frequency and supervision of respiratory therapy may be more important than the type of therapy delivered after upper abdominal operations. Mask CPAP offers advantages because it requires no effort from the patient, and therapy is not painful.

High level positive end expiratory pressure (PEEP) in acute respiratory insufficiency

Twenty-eight patients developed severe, progressive acute respiratory insufficiency despite aggressive application of conventional respiratory therapy. Application of increased PEEP (18 torr or greater) resulted in a significant decrease in QA/QT. Selection of the optimal levle of PEEP for each patient required serial determinations of QA/QT and measurement of cardiovascular response. The overall survival rate was 61 percent. Acute respiratory insufficiency was a proximate cause of death in only one patient. Four of the patients (14 percent) developed a pneumothorax following institution of high PEEP therapy. Cardiac output was not affected adversely at any level of PEEP up to 32 torr (44 cm H2O). We conclude that high levels of PEEP can be therapeutic for patients with refractory respiratory failure when combined with intermittent mandatory ventilation and careful cardiovascular monitoring. As with any therapy, the optimum dose should be tailored to each patient according to his needs and response.

Airway pressure release ventilation

Airway pressure release ventilation (APRV) delivers continuous positive airway pressure (CPAP) and may support ventilation simultaneously. This investigation tested whether, after acute lung injury (ALI), APRV promotes alveolar ventilation and arterial oxygenation without increasing airway pressure (Paw) above the CPAP level and without depressing cardiac function. Ten anesthetized dogs randomly received either intermittent positive-pressure ventilation (IPPV) or APRV. APRV was delivered with a continuous-flow CPAP system. Expiration occurred when a switch in the expiratory limb opened and Paw decreased to near-ambient, which decreased lung volume. After baseline data collection, ALI was induced by infusing oleic acid iv. Two hours later, IPPV and APRV were administered randomly, and data were collected. With normal lungs, APRV and IPPV achieved similar gas exchange and hemodynamic function. During ALI, arterial oxygenation was improved, and peak Paw which did not exceed the CPAP level, was lower during APRV. Similar minute ventilations were delivered by both modes but resulted in lower PaCO2 with APRV. Thus, APRV decreased physiologic deadspace ventilation. Hemodynamic status was similar during both modes. Therefore, APRV is an improved method of oxygenation and ventilatory support for patients with ALI that will allow unrestricted spontaneous ventilation and may decrease the incidence of barotrauma.

Airway pressure release ventilation during acute lung injury: a prospective multicenter trial

OBJECTIVE

To evaluate the feasibility of airway pressure release ventilation (APRV) in providing ventilatory support to patients with acute lung injury of diverse etiology and mild-to-moderate severity.

DESIGN

Prospective, multicenter, nonrandomized crossover trial.

SETTING

ICUs in six major referral hospitals.

PATIENTS

Fifty adult patients with respiratory failure requiring mechanical ventilation and positive end-expiratory airway pressure.

INTERVENTIONS

After optimization of continuous positive airway pressure (CPAP), conventional ventilation and APRV were administered sequentially for 30 mins. During APRV, the CPAP level and airway pressure release level were adjusted to prevent hypoxemia, while the degree of ventilatory support was adjusted by altering the frequency of pressure release.

MEASUREMENTS AND MAIN RESULTS

Circulatory and ventilatory pressures, arterial blood gases and pH, heart rate, and respiratory rate were measured. Alveolar ventilation was augmented adequately in 47 of 50 patients by APRV. Adjustment of APRV required an increase in mean CPAP from 13 +/- 3 (SD) to 21 +/- 9 cm H2O and a release pressure of 6 +/- 5 cm H2O. This airway pressure pattern produced a mean airway pressure comparable to that pressure achieved during conventional ventilation. Failure of APRV in three patients could be attributed to an inadequate level of CPAP or an inadequate APRV rate. While maintaining oxygenation of arterial blood and circulatory function, APRV allowed a substantial (55 +/- 17%; p less than .0001) reduction in peak airway pressure compared with conventional positive pressure ventilation adjusted to deliver a comparable or lower level of ventilatory support.

CONCLUSIONS

APRV is a feasible alternative to conventional mechanical ventilation for augmentation of alveolar ventilation in patients with acute lung injury of mild-to-moderate severity.

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.

Airway pressure release ventilation (APRV). A human trial

After operative coronary revascularization, 14 consenting adults received conventional positive pressure ventilation (PPV). When they were hemodynamically stable, data were collected during PPV and then during airway pressure release ventilation (APRV). During APRV, airway pressure (Paw) was reduced periodically at the lowest frequency which produced normal PaCO2. As anesthesia resolved, the rate of APRV breaths was decreased until patients breathed only with CPAP. During PPV and APRV, pHa, PaO2/FIO2, and hemodynamic variables were similar. All patients were weaned from APRV without complication. Optimal ventilator design for patients with acute lung injury would provide CPAP as a primary intervention and secondarily would augment alveolar ventilation. The APRV supported oxygenation and ventilation in patients with mild acute lung injury, yet with much lower peak airway pressure than produced by PPV.

Ventilatory pattern, intrapleural pressure, and cardiac output

Continuous positive-pressure ventilation may decrease cardiac output. However, a few reports have separated the effects of positive and end-expiratory pressure (PEEP) from those of mechanical ventilation. Ten surgical patients requiring mechanical ventilatory support had catheters inserted for measurement of right atrial pressure (RAP), pulmonary artery occlusion pressure (PAOP), intrapleural, radial artery, airway, and atrial filling pressures, and cardiac output. All patients breathed spontaneously between mechanical breaths delivered every 30 seconds by intermittent mandatory ventilation (IMV). Measurements were made with 0, 5, and 10 cm H2O PEEP, and during intermittent positive-pressure ventilation (IPPV) with 12 breaths/min without PEEP. Airway pressure (Paw), intrapleural pressure, RAP, and PAOP were increased by PEEP and IPPV. Intrapleural pressure increased most during IPPV (p less than 0.001). Atrial filling pressures and cardiac output were unaffected by PEEP but decreased during IPPV (p less than 0.001). Patients receiving IMV maintained negative intrapleural pressure, atrial filling pressure, cardiac output and, therefore, O2 delivery, regardless of PEEP level. The authors conclude that patients requiring mechanical respiratory support, with or without PEEP, may maintain better cardiopulmonary function when allowed some spontaneous ventilatory activity.

Cardiovascular effects of conventional positive pressure ventilation and airway pressure release ventilation

The hemodynamic sequelae of conventional positive pressure ventilation (CPPV), airway pressure release ventilation (APRV), and spontaneous breathing were compared with continuous positive airway pressure (CPAP) in ten anesthetized dogs who had ventilatory failure with and without parenchymal lung injury. The APRV corrected respiratory acidosis without significantly effecting arterial blood oxygenation, venous admixture, cardiovascular function, or tissue oxygen utilization. Application of CPPV precipitated marked depressions in blood pressure, stroke volume, and cardiac output. A concomitant decrease in venous admixture did not compensate for these adverse cardiovascular effects. Deterioration of tissue oxygen delivery resulted in oxygen supply-demand imbalance during CPPV. The results of this experimental study indicate that if ventilatory augmentation of subjects who require CPAP is desired, APRV will enhance alveolar ventilation without compromising circulatory function and tissue oxygen balance, whereas CPPV will impair cardiovascular function significantly.

Effect of interfacing between spontaneous breathing and mechanical cycles on the ventilation-perfusion distribution in canine lung injury

BACKGROUND

Improved matching between ventilation and perfusion (VA/Q) has been proposed to be a major advantage of partial ventilatory support compared with controlled mechanical ventilation. This study was designed to determine whether a difference in gas exchange exists between partial ventilatory support techniques that allow unsupported spontaneous breathing to occur during any phase of the mechanical ventilatory cycle and those that provide mechanical support for each spontaneous inspiratory effort.

METHODS

Ten anesthetized dogs with oleic acid-induced lung injury received, in random order, pressure-support ventilation (PSV) and airway pressure-release ventilation (APRV) with and without spontaneous breathing using equivalent airway pressure limits. Gas exchange was assessed by conventional blood gas analysis and by estimating the VA/Q distributions using the multiple inert-gas elimination technique.

RESULTS

During APRV, spontaneous breathing accounted for 10 +/- 1% of the total expiratory minute ventilation. Breath-to-breath ventilatory support with PSV resulted in the highest total expiratory minute ventilation (P < 0.05). During spontaneous breathing with APRV, cardiac output increased from 3.9 +/- 0.3 to 4.6 +/- 0.41.min-1 (P < 0.05), arterial oxygen tension from 75 +/- 3 to 107 +/- 8 mmHg (P < 0.05), and oxygen delivery from 567 +/- 47 to 719 +/- 73 ml.kg.min-1 (P < 0.05). PSV did not increase cardiac output, arterial oxygen tension, and oxygen delivery. Spontaneous breathing did not increase oxygen consumption. During APRV spontaneous breathing accounted for a 13 +/- 2% decrease (P < 0.05) in blood flow to shunt units (VA/Q < 0.005) and a 14 +/- 2% increase (P < 0.05) in the perfusion of normal VA/Q units (0.1 < VA/Q < 10). Pulmonary blood flow distribution to shunt and normal VA/Q units was similar during PSV and APRV without spontaneous breathing. Dead space (VA/Q > 100) ventilation decreased by 6% during APRV with spontaneous breathing compared with PSV (P < 0.05).

CONCLUSIONS

Spontaneous breathing superimposed on mechanical ventilation contributes to improved VA/Q matching and increased systemic blood flow. Apparently, the spontaneous contribution to a mechanically assisted breath during PSV is not sufficient to counteract the VA/Q maldistribution of positive pressure lung insufflation during acute lung injury.

Comparison of continuous positive airway pressure, incentive spirometry, and conservative therapy after cardiac operations

Of 38 patients undergoing median sternotomy for cardiac operations all developed profound restrictive defects in pulmonary function during the first 72 h after tracheal extubation. Although decreased lung volumes were refractory to correction by vigorous, aggressive pulmonary therapy during this period, frequent and supervised treatment may prevent further deterioration in pulmonary function. The overall incidence of pneumonia was only 3% (1/38). Continuous positive airway pressure delivered by mask proved to be a nearly effortless form of postoperative respiratory therapy that was less painful than incentive spirometry or coughing and deep breathing, and therefore may be preferable.

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.

Continuous positive airway pressure modulates effect of inhaled nitric oxide on the ventilation-perfusion distributions in canine lung injury

OBJECTIVES

The present study was designed to evaluate if continuous positive airway pressure (CPAP) augments the effect of nitric oxide (NO) inhalation on matching between ventilation and perfusion (VA/Q) during acute lung injury.

DESIGN

Prospective, randomized study.

SETTING

A research laboratory at a university medical center.

SUBJECTS

Ten anesthetized mongrel dogs with oleic acid-induced lung injury.

INTERVENTIONS

Zero or 40 parts per million of NO in the inspiratory gas, with and without 10 cm H2O CPAP in random order.

MEASUREMENTS AND MAIN RESULTS

Gas exchange was assessed by estimating the VA/Q distributions using the multiple inert gas elimination technique. Application of CPAP decreased blood flow to shunt units by 26 +/- 2 percent (mean +/- SD) and increased the fraction of cardiac output to normal VA/Q units (VA/Q ratio of 0.1 to 10) by 26 +/- 2 percent (p < 0.05). Inhalation of NO during CPAP accounted for a further 10 +/- 2 percent decrease in the blood flow to shunt units and an 8 +/- 2 percent increase in the fraction of the cardiac output to normal VA/Q units (p < 0.05). Inhalation of NO alone had no significant effect on the VA/Q distributions. Inhalation of NO decreased mean transmural pulmonary artery pressure (Ppatm) both without (Ppatm from 30 +/- 2 to 23 +/- 2 mm Hg; PVR from 323 +/- 44 to 228 +/- 43 dynes.s .cm-5; p < 0.05) and with CPAP (Ppatm from 25 +/- 2 to 20 +/- 2 mm Hg; PVR from 255 +/- 30 to 173 +/- 31 dynes.s.cm-5; p < 0.05).

CONCLUSIONS

Although pulmonary vascular resistance can be lowered with NO inhalation alone, recruitment of gas exchange units with CPAP is necessary to produce a beneficial effect of NO inhalation on VA/Q matching and oxygenation. When recruitment of gas exchange units with CPAP brings gaseous NO in contact with enough pulmonary blood vessels, NO-induced vasodilation will augment VA/Q matching by a steal mechanism.

Change in pulmonary venous admixture with varying inspired oxygen

Pulmonary venous admixture (Qsp/Qt) was analyzed as a function of fractional concentration of inspired O2 (FIO2) in 30 patients who required postoperative mechanical ventilation. Pulmonary and radial artery blood-gas tensions and pH were measured and Qsp/Qt was calculated with FIO2 ranging from 0.21 to 1. In all patients, Qsp/Qt decreased when FIO2 was increased from 0.21 to 0.4 and then stabilized to an FIO2 of approximately 0.6. As the FIO2 was increased to 1, Qsp/Qt increased. Since the inhalation of gas mixture with FIO2 greater than or equal to 0.6 increased right-to-left intrapulmonary shunting of blood, we recommend respiratory function be evaluated during inhalation of a clinically useful concentration of O2 rather than at an FIO2 of 1.

Effect of endogenous and inhaled nitric oxide on the ventilation-perfusion relationships in oleic-acid lung injury

Previous investigations have shown that the ventilation-perfusion (VA/Q) mismatch caused by acute lung injury can be alleviated either by inducing vasodilation in ventilated lung units with inhaled nitric oxide (NO) or by inhibiting the synthesis of endogenous NO, which opposes hypoxic pulmonary vasoconstriction. To determine the effects of a combination of these interventions, we evaluated cardiopulmonary function and VA/Q distributions in 10 dogs with oleic acid-induced lung injury. Each animal received, in random order, zero or 40 ppm of NO in inspiratory gas, with and without intravenous infusion of NG-monomethyl-L-arginine (L-NMMA) (5 mg/kg/h). The multiple inert-gas elimination technique was used to estimate VA/Q distributions. Systemic L-NMMA administration alone did not affect VA/Q inequality and gas exchange, but increased pulmonary and systemic vascular resistance. Inhaled NO improved gas exchange by redistributing blood flow from shunt units to lung units with a nearly ideal VA/Q ratio, without affecting pulmonary or systemic vascular resistance. Improved VA/Q matching and gas exchange was most pronounced when NO was inhaled in the presence of systemic L-NMMA. Inhalation of NO reversed the pulmonary but not the systemic vasoconstriction caused by L-NMMA. These results suggest that endogenous NO release is not limited to hypoxic lung regions in animals with oleic acid-induced lung injury. Inhaled NO reversed L-NMMA-induced pulmonary vasoconstriction and improved VA/Q matching by selectively dilating the pulmonary vasculature in ventilated lung units.

Postoperative hypoxemia: common, undetected, and unsuspected after bariatric surgery

BACKGROUND

Patients undergoing gastric bypass are at greater than ordinary risk for postoperative respiratory insufficiency, presumably related to obstructive sleep apnea (OSA) and patient-controlled analgesia (PCA). This study was proposed to quantify the magnitude of the problem.

METHODS

Fifteen patients undergoing gastric bypass had oxygen saturation (SpO(2)) recorded continuously, but not displayed, for 24h postoperatively; eight also had arterial blood analysis every 4h. All received narcotic PCA. SpO(2)<90% lasting more than 10 s was reviewed. Results are mean+/-SEM.

RESULTS

Mean age was 44+/-4 y, and mean BMI was 48+/-2kg/m(2); 77% had OSA. Every patient had more than one episode with SpO(2)<90% for longer than 30s undetected by routine monitoring; most had multiple episodes. Nadir SpO(2) averaged 75% +/- 8%. Mean longest duration of desaturation below 90% averaged 21+/-15min. Mean PaCO(2) was 37+/-3mm Hg; maximum PaCO(2) was 47mm Hg.

CONCLUSIONS

Severe and prolonged episodes of hypoxemia were a consistent finding, despite aggressive preoperative diagnosis and treatment of OSA, including use of CPAP postoperatively. Although some postoperative hypoventilation was expected, the degree and frequency of desaturation were surprising. No patient exhibited arterial PaCO(2) evidence of hypoventilation. No patient experienced cardiopulmonary arrest/instability, in spite of severe, repeated episodes of hypoxemia. In no instance was a significant hypoxemic episode suspected or detected. Continuous pulse oximetry monitoring, with an audible alarm set for a saturation less than 90% for 10 s, would have alerted providers to 100% of significant hypoxemic episodes. Our recommendation is routinely monitoring (with alarm capability enabled) every bariatric surgical patient, to prevent such occurrence.

Treatment of bronchopleural fistula during continuous positive pressure ventilation

Pneumothorax as a complication of continuous positive pressure ventilation may result in the formation of bronchopleural fistula. If positive end-expiratory pressure cannot be sustained, the functional residual capacity and the ratio of alveolar ventilation to perfusion may decrease, and pulmonary gas exchange may be severely impaired. Recommended therapy includes removal of positive airway pressure and institution of negative intrapleural pressure via thoracostomy tube, but this may cause severe hypoxemia in patients with acute respiratory failure in spite of potentially toxic inspired oxygen concentrations. We applied positive intrapleural pressure equal to the end-expiratory airway pressure of three patients who developed bronchopleural fistula during therapy for acute respiratory failure. Positive intrapleural pressure facilitated resolution of the bronchopleural fistula in each case.

Pulmonary function following severe acute respiratory failure and high levels of positive end-expiratory pressure

In an 18-month period, we treated 561 patients with mechanical ventilation. Fifty-four (10 percent) of these patients had acute respiratory failure, requiring treatment with positive end-expiratory pressure (PEEP) in excess of 20 mm Hg (range, 20 to 40 mm Hg). All patients were allowed to breathe spontaneously between volume-limited mechanical breaths delivered at a rate sufficient to maintain an arterial pH greater than or equal to 7.35. PEEP was applied until calculated pulmonary venous admixture was minimized. Forty-three (80 percent) of these 54 patients were alive and asymptomatic three months after dischage from the hospital, and tests of pulmonary function were performed on ten patients within one year after hospitalization. Abnormalities in pulmonary function appeared to be reversible, and pulmonary function gradually approached normal within one year. It appears that neither acute respiratory failure nor exposure to high airway pressures caused significant permanent pulmonary damage in the ten patients studied.