Peak velocity and flow quantification validation for sensitivity-encoded phase-contrast MR imaging

RATIONALE AND OBJECTIVES

Phase-contrast (PC) magnetic resonance imaging (MRI) technique has important clinical applications in blood flow quantification and pressure gradient estimation by velocity measurement. Parallel imaging using sensitivity encoding (SENSE) may substantially reduce scan time. We demonstrate the utility of PC-MRI measurements accelerated by SENSE under clinical conditions.

MATERIALS AND METHODS

Accuracy and repeatability of a SENSE-PC implementation was evaluated by comparison with a commercial PC sequence with five normal volunteers. Twenty-six patients were then scanned with SENSE-PC at reduction factors (R = 1, 2, and 3). Blood flow and peak velocity were measured in the aorta and pulmonary trunk in 16 patients and peak velocity was measured at the coarctation of 10 patients. Quantitative flow, shunt ratio, and peak velocity measurements obtained with different reduction factors were compared using correlation, linear regression, and Bland-Altman statistics. All studies were approved by an Institutional Review Board, and informed consent was acquired from all subjects.

RESULTS

The correlation coefficients for all comparisons were >0.962 and with high statistical significance (P < .01). Linear regression slopes ranged between 0.96 and 1.11 for flow and 0.88 to 1.05 for peak velocity. For flow, the Bland-Altman statistics yielded a total mean difference ranging from -0.02 to 0.05) L/minute with 2 standard of deviation limits ranging from -0.52 to 0.75 L/minute. For peak velocity, the total mean difference ranged from -0.10 to -0.004) milliseconds with 2-SD limits ranging from -0.062 to 0.46 milliseconds. R = 3 to R = 1 comparisons had greater 2-SD limits than R = 2 to R = 1 comparisons.

CONCLUSION

SENSE PC-MRI measurements for flow and pressure gradient estimation were comparable to conventional PC-MRI.

Reduction of respiratory system resistance of rabbits with surfactant deficiency using a novel ultra thin walled endotracheal tube

The ultra thin walled, two-stage endotracheal tube (UTW-TS-ETT) is very flexible, nonkinking, and has a widened extralaryngeal portion. The UTW-TS-ETT has a greater ID/OD (internal diameter/outer diameter) ratio than a comparable standard endotracheal tube (ST-ETT) because of its thinner wall: 0.2-0.25 mm in UTW-TS-ETT, compared to 0.55-0.8 mm in ST-ETT. The authors hypothesized that in an animal model of lung disease, significant reductions in respiratory system resistance (Rrs) of 30-40% would be achieved using the UTW-TS-ETT, compared to Rrs achieved with the comparable ST-ETT. This study compared the pulmonary mechanics of rabbits (N = 17, body weight 3.4-4.7 kg) before and after induction of surfactant deficiency, using either ST-ETT (OD 4.9 mm, ID 3.5 mm) or UTW-TS-ETT (OD 5.0 mm, ID 4.6 mm). Animals were sedated, paralyzed, and ventilated by an ETT placed through a tracheotomy incision. Surfactant deficiency was induced by lavaging the lungs with normal saline (10 ml/kg). Pulmonary mechanics were measured on identical ventilator settings for each ETT used at baseline and at 45 min after lavage. Compared to ST-ETT, UTW-TS-ETT reduced Rrs by 50.6 +/- 8.7% in normal lungs (significantly more than 40%; p < 0.01), and by 41.47 +/- 16.2% in surfactant deficient lungs (significantly more than 30%; p < 0.05). Tidal volume increased with UTW-TS-ETT in all animals but did not achieve statistical significance. The UTW-TS-ETT did not induce significant changes in respiratory system compliance, PaO2, PaCO2, or pH. It is concluded that UTW-TS-ETT significantly reduces Rrs in rabbits with either normal lungs or with surfactant deficient lungs. This novel ETT may be beneficial for ventilated patients with increased Rrs, by effecting a decrease in Rrs and thus reducing the work of breathing and improving ventilation efficiency.

Intratracheal pulmonary ventilation versus conventional mechanical ventilation in a rabbit model of surfactant deficiency

Intratracheal pulmonary ventilation (ITPV) enhances the clearance of CO2 from dead space and lungs by a bias flow of gas administered in the distal trachea. ITPV flow is continuously administered through a separate catheter placed within an endotracheal tube (ETT). After exiting from catheter's tip in the distal trachea, the flow of gas is redirected outward away from the lungs. We hypothesized that, compared with conventional mechanical ventilation (CMV), ITPV may increase minute CO2 clearance (VCO2), reduce the partial pressure of CO2 dioxide in arterial gas (PaCO2), and reduce distal tracheal peak inspiratory pressure (dPIP). We induced surfactant deficiency in 15 adult rabbits by lung lavage with 10 mL/kg normal saline. Animals were ventilated through a double-lumen 4.0 ETT, inserted through a tracheotomy incision. dPIP, distal positive end expiratory pressure, and distal mean airway pressure were monitored, and the mean exhaled CO2 concentration was measured. For ventilator rates (respiratory rate) of 30, 45, and 70 breaths/min, the study included two phases: phase I compared CO2 clearance and PaCO2 between ITPV and CMV using similar ventilatory pressures; phase II evaluated the effectiveness of ITPV in reducing dPIP and tidal volume (Vt), compared with CMV, while maintaining eucapnea. When comparing ITPV and CMV, the following results (mean +/- SD) were achieved at respiratory rate of 30, 45, and 70 breaths/min, respectively. Phase I ITPV resulted in mean percent reduction of PaCO2 by 31.4 +/- 10%, 37.1 +/- 9.7% and 38.3 +/- 9%; mean percent increase in VCO2 by 61.3 +/- 29%, 56 +/- 23, and 98 +/- 40%, compared with CMV. Phase II ITPV resulted in mean percent reduction of dPIP by 35.5 +/- 14%, 38 +/- 10.8%, and 37.2 +/- 13.7%, and mean percent reduction in Vt by 34.7 +/- 12.9%, 36.4 +/- 15%, and 52.7 +/- 10.7%, compared with CMV. The changes in PaCO2, VCO2 (phase I), and dPIP and Vt (phase II) were all significantly more than 25% (p < 0.05). Oxygenation and pH were not significantly different between ITPV and CMV. We conclude that, in a surfactant deficiency rabbit model, ITPV is an efficient mode of assisted ventilation that increases CO2 clearance and reduces ventilator pressures required for adequate ventilation. We speculate that ITPV can minimize lung barotrauma associated with mechanical ventilation.

Predictors of Neonatal Mortality in 1,500–1,999 g Premature Infants with Respiratory Failure

Despite the introduction of new ventilation techniques and surfactant therapy, some premature infants still experience severe respiratory failure and either die or survive with severe bronchopulmonary dysplasia. Extracorporeal membrane oxygenation is currently not offered for preterm infants with a birth weight less than 2,000 g, mainly because of the potential high risk for intracranial hemorrhage. The aim of this study was to determine risk predictors for mortality alone and for mortality or major lung morbidity in 1,500-1,999 g premature infants with respiratory failure. We reviewed the medical records of all preterm infants (n = 459) with respiratory failure and a birth weight of 1,500-1,999 g treated at five medical centers from 1989 to 1991. Of those infants, 23 (5%) had severe respiratory failure, defined as a requirement for ventilatory support with the fraction of inspired oxygen > or = 0.8 or peak inspiratory pressure > or = 30 cmH2O for > or = 3 hr in the 1st week of life. A mortality of > or = 75% was associated with a single arterial/alveolar oxygen ratio < or = 0.04; pulmonary air leak alone or pulmonary air leak with a mean airway pressure > or = 12 cmH2O; and arterial oxygen tension < or = 50 mmHg. These risk predictors may provide a basis for the selection of patients for future clinical trials of extracorporeal membrane oxygenation in this high-risk group of 1,500-1,999 g premature infants with severe respiratory failure.

Predictors of neonatal mortality in 1,500-1,999 g premature infants with respiratory failure. Basis for ECMO Therapeutic Trial

Despite the introduction of new ventilation techniques and surfactant therapy, some premature infants still experience severe respiratory failure and either die or survive with severe bronchopulmonary dysplasia. Extracorporeal membrane oxygenation is currently not offered for preterm infants with a birth weight less than 2,000 g, mainly because of the potential high risk for intracranial hemorrhage. The aim of this study was to determine risk predictors for mortality alone and for mortality or major lung morbidity in 1,500-1,999 g premature infants with respiratory failure. We reviewed the medical records of all preterm infants (n = 459) with respiratory failure and a birth weight of 1,500-1,999 g treated at five medical centers from 1989 to 1991. Of those infants, 23 (5%) had severe respiratory failure, defined as a requirement for ventilatory support with the fraction of inspired oxygen > or = 0.8 or peak inspiratory pressure > or = 30 cmH2O for > or = 3 hr in the 1st week of life. A mortality of > or = 75% was associated with a single arterial/alveolar oxygen ratio < or = 0.04; pulmonary air leak alone or pulmonary air leak with a mean airway pressure > or = 12 cmH2O; and arterial oxygen tension < or = 50 mmHg. These risk predictors may provide a basis for the selection of patients for future clinical trials of extracorporeal membrane oxygenation in this high-risk group of 1,500-1,999 g premature infants with severe respiratory failure.

Pulmonary sequelae at six months following extracorporeal membrane oxygenation

Mechanical assisted ventilation for neonatal respiratory failure is associated with residual lung disease. Because ECMO rests the lungs, it has been suggested that ECMO will prevent chronic lung disease in survivors. To determine whether or not ECMO survivors have evidence of pulmonary sequelae, we studied 19 infants who were treated with ECMO for neonatal respiratory failure. Ten infants still required supplemental oxygen or pulmonary medications or both to treat clinical lung disease during the first six months of life. Thoracic gas volume was normal. Pulmonary mechanics in ECMO survivors were compared with those of 13 preterm infants with BPD at similar age. We conclude that a significant proportion of ECMO survivors have residual abnormalities in pulmonary mechanics at 6 months of age. We speculate that neonatal lung injury due to meconium aspiration and other causes is a more important determinant of abnormal pulmonary sequelae than the method of treatment.

Serial measurement of pulmonary mechanics assists in weaning from extracorporeal membrane oxygenation in neonates with respiratory failure

Extracorporeal membrane oxygenation (ECMO) is a highly invasive therapy for intractable neonatal respiratory failure, and serious complications may occur with increasing duration of bypass. Weaning from bypass is empirical at present. Thus, there is a need to accurately predict when infants can be successfully decannulated. We hypothesized that pulmonary mechanics would reflect lung recovery and, therefore, predict successful weaning from ECMO. We measured pulmonary mechanics daily in 22 neonates, at gestational age of 37.8 +/- 0.6 weeks (SE) requiring ECMO for severe respiratory failure (oxygen index 66 +/- 6). Pulmonary resistance (Rpul), dynamic compliance (Cdyn), and tidal volume (VT) were measured. Rpul did not predict lung recovery. Cdyn within 24 hours of starting ECMO was 0.3 +/- 0.04 ml/cm H2O. Cdyn within 24 hours of weaning from ECMO was 1.2 +/- 0.09 ml/cm H2O (p less than 0.001). All 22 infants had Cdyn greater than 0.6 ml/cm H2O at the time of decannulation, but four infants (20 percent) with Cdyn less than 0.6 ml/cm H2O could not be weaned from ECMO within 20 hours (p less than 0.01). Thus, a minimum Cdyn of 0.6 ml/cm H2O is associated with successful weaning from ECMO. Cdyn of 0.8 ml/cm H2O provided better overall discrimination between those who could be successfully weaned from ECMO. We conclude that serial measurement of dynamic pulmonary compliance predicts successful weaning from ECMO.

Childhood sequelae of infant lung disease: exercise and pulmonary function abnormalities after bronchopulmonary dysplasia

To determine the long-term pulmonary sequelae and effect on exercise tolerance of bronchopulmonary dysplasia (BPD), we studied 10 children at a mean age of 10.4 years, who had been born prematurely, survived respiratory distress syndrome, and subsequently developed BPD, and compared them with eight age-matched normal children born at term. Pulmonary function tests and graded exercise stress tests were performed. Residual volume, the ratio between residual volume and total lung capacity, vital capacity, forced expiratory volume in 1 second, forced expiratory flow between 25% and 75% of vital capacity, and maximal expiratory flows at 80%, 70%, and 60% of total lung capacity were all abnormal (P less than 0.02) in the children with BPD, compared with control values. Pre-exercise transcutaneous CO2 tension was higher (P less than 0.05) in the BPD group than in the control group. At maximal workload, tcPCO2 remained high in patients with BPD compared with control values (P less than 0.05). Arterial oxygen saturation at maximal workload fell below pre-exercise levels in the BPD group (P less than 0.05) but not in control children. There were no differences in maximal oxygen consumption between the BPD group and control children. Exercise-induced bronchospasm occurred in 50% of the BPD group, but not in the control group. We conclude that long-term survivors of BPD have evidence of airway obstruction, hyperinflation, and airway hyperreactivity, compared with a control group. Aerobic fitness was not significantly different in the BPD and control groups, but was achieved in the BPD group at the expense of a fall in SaO2 and a rise in tcPCO2.

Primary hyperinsulinemia reduces surface active material flux in tracheal fluid of fetal lambs

We sought to test the hypothesis that hyperinsulinemia per se alters the flux of surface active material (SAM) into tracheal fluid by continuously infusing insulin (0.24 +/- 0.04 units/kg/hr, mean +/- S.E.) from 112 through 135 days gestation into five chronically catheterised fetal lambs, from which tracheal fluid could be collected. Serum insulin levels in these fetuses (95 +/- 10 microunits/ml) were greater than in five chronically catheterised control fetuses of the same gestational age (10 +/- 1 microunits/ml, P less than 0.001) and in the mothers (38 +/- 6 microunits/ml, P less than 0.001). Serum glucose levels in the insulin-treated fetuses (10 +/- 1 mg/dl) were lower than in the control fetuses (19 +/- 1 mg/dl, P less than 0.001) and in the mothers (60 +/- 3 mg/dl, P less than 0.001). Arterial blood gases (pH 7.37 +/- 0.01, PO2 23.3 +/- 0.05 mm Hg, PCO2 41.5 +/- 0.9 mm Hg) and hematocrit (33 + 1% at 127 days gestation and 31 +/- 1% at 135 days gestation) in the insulin treated fetuses were not different from the controls. SAM flux into the tracheal fluid of the insulin-treated fetuses was 1 microgram/kg/hr, coefficient of variation 373%. This was lower than SAM flux in the control fetuses (26 micrograms/kg/hr, coefficient of variation 28%, P less than 0.01). Moreover, among the control fetuses, SAM began to appear in tracheal fluid at 119 days gestation and was present in all five fetuses by 125 days gestation, whereas SAM did not begin to appear in the insulin-treated fetuses until 127 days gestation and did not appear at all in three of them.

Drug 'administration' via breast milk

In the current popularity of breast-feeding, certain pitfalls for the infant tend to be overlooked. Physicians should be particularly aware with respect to which drugs and toxins can be transmitted by the lactating mother, and which cannot. In aid of such awareness, the authors tabulate the excretion characteristics and compatibility with breast-feeding of more than 200 common drugs and substances.