Increased artificial deadspace ventilation is a safe and reliable method for deliberate hypercapnia

Sleeping position and responses to a carbon dioxide challenge in convalescent prematurely born infants studied post-term

OBJECTIVES

To test the hypothesis that the ventilatory response to a carbon dioxide (CO2) challenge would be lower in the prone compared to the supine position in prematurely born infants studied post-term. To determine whether there were postural-related differences in respiratory drive, respiratory muscle strength, thoracoabdominal synchrony and/or lung volume.

DESIGN

Prospective cohort study.

SETTING

Tertiary neonatal unit.

PATIENTS

Eighteen infants (median gestational age 31 (range 22-32) weeks) were studied at a median of 5 (range 2-11) weeks post-term.

INTERVENTIONS

The ventilatory responses to three added carbon dioxide (CO2) levels (0% baseline, 2% and 4%) were assessed in the prone and supine positions.

MAIN OUTCOME MEASURES

The airway pressure change after the first 100 ms of an occluded inspiration (P0.1) (respiratory drive) and the maximum inspiratory pressure during crying with an occluded airway (Pimax) (respiratory muscle strength) were measured. The P0.1/Pimax ratio at each CO2 level and slope of the P0.1/Pimax response were calculated.

RESULTS

The mean P0.1 (p<0.05) and P0.1/Pimax (p<0.05) were higher and the functional residual capacity (p=0.031) lower in the supine compared to the prone position. The mean P0.1 and P0.1/Pimax increased independently of position as the percentage CO2 increased (p<0.001). There was no tendency for the differences in P0.1 and P0.1/Pimax between the prone and supine position to vary by CO2 level.

CONCLUSIONS

Convalescent, prematurely born infants studied post-term have a reduced respiratory drive, but not a lower ventilatory response to a CO2 challenge, in the prone compared to the supine position.

Quantitative evaluation of oxygenation in venous vessels using T2-Relaxation-Under-Spin-Tagging MRI

Noninvasive measurement of cerebral venous oxygenation can serve as a tool for better understanding fMRI signals and for clinical evaluation of brain oxygen homeostasis. In this study a novel technique, T2-Relaxation-Under-Spin-Tagging (TRUST) MRI, is developed to estimate oxygenation in venous vessels. This method uses the spin labeling principle to automatically isolate pure blood signals from which T2 relaxation times are determined using flow-insensitive T2-preparation pulses. The blood T2 is then converted to blood oxygenation using a calibration plot. In vivo experiments gave a baseline venous oxygenation of 64.8 +/- 6.3% in sagittal sinus in healthy volunteers (n = 24). Reproducibility studies demonstrated that the standard deviation across trials was 2.0 +/- 1.1%. The effects of repetition time and inversion time selections were investigated. The TRUST technique was further tested using various physiologic challenges. Hypercapnia induced an increase in venous oxygenation by 13.8 +/- 1.1%. On the other hand, caffeine ingestion resulted in a decrease in oxygenation by 7.0 +/- 1.8%. Contrast agent infusion (Gd-DTPA, 0.1 mmol/kg) reduced venous blood T2 by 11.2 ms. The results of this study show that TRUST MRI is a useful technique for quantitative assessment of blood oxygenation in the brain.

Cardiorespiratory effects of added dead space in patients with heart failure and central sleep apnea

BACKGROUND

Inhaled CO(2) has been shown to stabilize the breathing pattern of patients with central sleep apnea (CSA) with and without congestive heart failure (CHF). Added dead space (DS) as a form of supplemental CO(2) was effective in eliminating idiopathic CSA. The efficacy and safety of DS has not yet been evaluated in patients with CHF and CSA.

METHODS

We examined the respiratory and cardiovascular effects of added DS in eight patients with CHF and CSA. The DS consisted of a facemask attached to a cylinder of adjustable volume. During wakefulness, the cardiorespiratory response to 200 to 600 mL of DS was tested. Cardiac output and stroke volume were measured using echocardiography with and without DS. During the nocturnal study, patients slept with and without DS, alternating at approximately 1-h intervals.

RESULTS

Values are expressed as the mean +/- SE. The wakefulness study revealed a plateau in the partial pressure of end-tidal CO(2) (PETCO(2)) and the partial pressure of end-tidal O(2) between DS amounts of 400 and 600 mL. The mean stroke volume index (33 +/- 7 vs 34 +/- 7 mL/m(2), respectively) and the mean cardiac index (1.9 +/- 0.3 vs 1.9 +/- 0.4 L/min/m(2), respectively) were not affected by DS. Neither heart rate nor BP showed a significant change in response to DS of < or = 600 mL. During sleep, DS increased the PETCO(2) (40.7 +/- 2.7 vs 38.9 +/- 2.6 mm Hg, respectively; p < 0.05), reduced apnea (1 +/- 1 vs 29 +/- 7 episodes per hour, respectively; p < 0.01) and arousal (21 +/- 8 vs 30 +/- 8 arousals per hour, respectively; p < 0.05), increased the mean arterial oxygen saturation (SaO(2)) [94.4 +/- 1.0% vs 93.5 +/- 1.1%, respectively; p < 0.01), and reduced SaO(2) oscillations (DeltaSaO(2) from maximum to minimum, 1.8 +/- 0.4% vs 5.5 +/- 0.9%, respectively; p < 0.01).

CONCLUSION

DS stabilized CSA and improved sleep quality in patients with CHF without significant acute adverse effects on the cardiovascular function.