Comparison of spontaneous and positive-pressure breathing in supine normal subjects

Peri-operative pulmonary dysfunction and protection

Pulmonary complications are a major cause of peri-operative morbidity and mortality, but have been researched less thoroughly than cardiac complications. It is important to try and predict which patients are at risk of peri-operative pulmonary complications and to intervene to reduce this risk. Anaesthetists are in a unique position to do this during the whole peri-operative period. Pre-operative training, smoking cessation and lung ventilation with tidal volumes of 6-8 ml.kg(-1) and low positive end-expiratory pressure probably reduce postoperative pulmonary complications.

Reversal of Pulmonary Hypertension after Diaphragm Pacing in an Adult Patient with Congenital Central Hypoventilation Syndrome

Introduction

Patients with the congenital central hypoventilation syndrome (CCHS) suffer from life-threatening hypoventilation when asleep, making them dependent on mechanical ventilation (MV) at night or during naps. State-of-art respiratory management consists of intermittent positive-pressure ventilation via a tracheotomy or mask. In some patients hypoventilation is permanent, in which case ventilatory support must be extended to the waking hours. Diaphragm pacing can prove useful in such situations.

Methods and Results

This report describes the case of a 26-year-old woman with CCHS in whom failure to achieve adequate MV led to life-threatening pulmonary hypertension (PH), with a systolic pulmonary artery pressure (PAP) of 80 mmHg and right ventricular hypertrophy, despite optimization of all possible measures and despite extensive therapeutic education efforts. Diaphragm pacing using laparoscopically implanted intradiaphragmatic phrenic nerve stimulation electrodes corrected alveolar hypoventilation and lastingly reversed PH (systolic PAP below 40 mmHg after 2 months, sustained after 2 years). Diaphragm pacing induced shoulder pain, however, involving the chronic use of analgesics. The pacing had to be stopped for tolerance reasons after two years, leading to PH worsening and the need for diurnal MV.

Conclusions

Diaphragm pacing appears likely effective to restore alveolar ventilation and reverse PH in adult CCHS patients.

Rebreathing improves accuracy of ventilatory monitoring

OBJECTIVE

Our objective was to determine if rebreathing would reduce the gradient between arterial and end-tidal CO2 tension during positive-pressure ventilation.

METHODS

DESIGN

Experimental investigation.

SETTING

Anesthesiology laboratory.

SUBJECTS

A total of 10 dogs of either sex.

INTERVENTIONS

Anesthesia (sodium pentobarbital) and muscle relaxation (pancuronium) were induced and animals were tracheally intubated and ventilated with a standard anesthesia ventilator and breathing circuit with CO2 absorber and then with a Mapleson D circuit with a fresh gas flow rate (VF) equal to alveolar ventilation plus the sampling flow rate of two capnometers. Rebreathing was varied by adjusting the respiratory rate (RR) so that minute ventilation (VE) to VF ratio was 1:1, 2:1, 3:1, and 4:1.

RESULTS

CO2 production (ATPD) was determined as the product of expired concentration of CO2 and VE (BTPS). Alveolar ventilation (VA) was calculated by dividing the product of CO2 production and barometric pressure corrected for ambient temperature and water vapor pressure at body temperature by PaCO2. Tidal volume, RR, airway gas temperature, concentration of CO2 in gas at the tracheal tube and inlet/outlet of the mechanical ventilator, body temperature, arterial gas tensions and pH, heart rate, arterial blood pressure, and cardiac output were measured. Minute ventilation, mean arterial blood pressure and end-expiratory CO2 tension (PECO2) (BTPS) were calculated. During positive-pressure ventilation, concentration of inspired CO2 was zero with standard circuitry, and significantly increased with Mapleson D when VE:VF ratio was 1:1 (0.56 +/- 0.19%), 2:1 (1.97 +/- 1.30%), 3:1 (2.56 +/- 1.05%), and 4:1 (3.01 +/- 1.45%) (p < 0.05). PECO2 was 34.8 +/- 3.2 mm Hg during ventilation with the standard circuit, and significantly increased during ventilation with Mapleson D when VE:VF ratio was increased from 1:1 (35.4 +/- 2.5 mm Hg) to 2:1 (40.2 +/- 3.6 mm Hg) and was not further increased at a VE:VF ratio of 3:1 (41.8 +/- 2.7 mm Hg) or 4:1 (41.3 +/- 2.4 mm Hg). The selected fresh gas flow rate was appropriate, because PaCO2 remained unchanged regardless of VE:VF ratio, indicating PaCO2 was dependent on VF, not on VE. The gradient between PaCO2 and PECO2 during ventilation with the standard circuit was 6.6 +/- 3.0 mm Hg; during ventilation with Mapleson D, it decreased significantly when VE:VF ratio was increased from 1:1 (6.5 +/- 3.6 mm Hg) to 2:1 (2.9 +/- 1.5 mm Hg), but was not significantly reduced further at 3:1 (1.7 +/- 1.1 mm Hg) or 4:1 (1.8 +/- 0.5 mm Hg) (p < 0.05).

CONCLUSIONS

Rebreathing with a Mapleson D circuit and a VF equal to VA permitted normal CO2 elimination. Arterial PCO2 to PECO2 gradient decreased significantly during rebreathing, thus improving the reliability of capnography for estimating arterial PCO2. Consideration should be given to using the Mapleson D as a rebreathing circuit.

Atelectasis and neuromuscular respiratory failure

Atelectasis occurred in 17 of 20 patients treated with assisted ventilation for respiratory failure due to neuromuscular disease. A retrospective review of chest roentgenograms and medical records indicated that atelectasis occurs early in the course of respiratory failure and has a predilection for the lower lobes. Atelectasis was associated with infection and persisted despite therapy, often for more than four weeks. This study demonstrates that atelectasis in this condition still occurs frequently despite modern practices of mechanical ventilatory assistance. Infection, occurring soon after intubation, is identified as the principal cofactor.

Effect of posture on inter-regional distribution of pulmonary ventilation in man

Regional ventilation per unit alveolar volume (V/VA) and regional lung expansion (FRCR/TLCR) were measured in twelve normal male human subjects in seated, supine, lateral decubitus and prone postures using a gamma camera and inhalation of the radioactive gases 81Krm (half-life 13 sec) and 85Krm (half-life 4.4 h). FRCR/TLCR decreased from superior to inferior in all postures except prone where it was uniform; V/VA increased from superior to inferior except in the prone position where it was uniform. In the horizontal axis FRCR/TLCR and V/VA were uniformly distributed except for cranial to caudal gradients (with lower values caudally) in supine and lateral decubitus postures. In the prone posture V/VA tended to be higher in caudal lung zones.

Effect of posture on inter-regional distribution of pulmonary perfusion and VA/Q ratios in man

Using a gamma camera regional pulmonary perfusion per unit alveolar volume (Q/VA) and ventilation-perfusion ratios (VA/Q) were measured in fourteen normal male volunteers in upright, supine, lateral decubitus, prone and prone suspended postures with inhalation and intravenous infusion of radioactive 81Krm (half-life 13 sec) and inhalation of radioactive 85Krm (half-life 4.4 h). In the vertical axis, Q/VA increased from upper to lower lung regions in upright, lateral decubitus, prone and prone suspended postures but was uniform in supine and within the dependent lung in decubitus postures. Horizontally, Q/VA decreased from cranial to caudal in lateral decubitus, prone suspended, and near the diaphragm in supine. Vertically, VA/Q decreased from upper to lower in all postures except in supine and within the dependent decubitus lung where it increased. VA/Q tended to increase from cranial to caudal in horizontal postures. The effects of gravity on Q/VA and VA/Q vertical distributions are modified when FRC is low (supine, lower decubitus lung) because perfusion is more uniformly distributed. Horizontal gradients of Q/VA and VA/Q are more pronounced under conditions of high local lung volume but also occur in postures where FRC is low.

Effect of age on the regional perfusion and washouts of injected xenon in normal men

The aim of this work was to localize the alveoli with low ventilation-perfusion ratio which are responsible for the age related increase of the ideal alveolar-arterial O2 partial pressure difference. For this purpose, we measured: (1) the washouts of perfused 133 Xenon (Xe) in the whole lung and in 6 horizontal slices of the right lung, and (2) the topographical distribution of perfusion (Qi) in 27 healthy, non-smoking seated men, between 18 and 65 years. The distribution of Qi is unaffected by age. The global and regional washouts slowed with age, the trends being the same in the 6 investigated regions. This data was interpreted as indicating that in our subjects the low VA/Q units are not situated predominantly at the base, but are scattered throughout the lung.

The role of intermittent positive pressure breathing in respiratory physiotherapy

This article reviews the value and limitations of I.P.P.B. as opposed to other treatment methods available to the respiratory physiotherapist. In the treatment of patients with airways obstruction I.P.P.B. is presented as having a limited role. Alternative measures such as the use of simple nebulisation, relaxation therapy and manual methods to remove secretions are described as being preferable to I.P.P.B. in most cases. I.P.P.B. has some value in the early post-operative care of some patients undergoing surgery which results in limitation to spontaneous deep breathing postoperatively. However other methods, notably incentive spirometry should also be considered for these patients. In the treatment and prevention of respiratory complications associated with neuromuscular and skeletal disorders I.P.P.B. can play a significant role in counteracting the possible effects of hypoventilation. I.P.P.B. should be considered as one method among many which is available to the physiotherapist to assist in the treatment of respiratory complications associated with medical and surgical conditions.