Lung gas tensions and active regulation of ventilation/perfusion ratios in health and disease

A survey of changing trends in modelling radiation lung injury in mice: bringing out the good, the bad, and the uncertain

Within this millennium there has been resurgence in funding and research dealing with animal models of radiation-induced lung injury to identify and establish predictive biomarkers and effective mitigating agents that are applicable to humans. Most have been performed on mice but there needs to be assurance that the emphasis on such models is not misplaced. We therefore considered it timely to perform a comprehensive appraisal of the literature dealing with radiation lung injury of mice and to critically evaluate the validity and clinical relevance of the research. A total of 357 research papers covering the period of 1970-2015 were extensively reviewed. Whole thorax irradiation (WTI) has become the most common treatment for studying lung injury in mice and distinct trends were seen with regard to the murine strain, radiation dose, intended pathology investigated, length of study, and assays. Recently, the C57BL/6 strain has been increasingly used in the majority of these studies with the notion that they are susceptible to pulmonary fibrosis. Nonetheless, many of these investigations depend on animal survival as the primary end point and neglect the importance of radiation pneumonitis and the anomaly of lethal pleural effusions. A relatively large variation in survival times of C5BL/6 mice is also seen among different institutions pointing to the need for standardization of radiation treatments and environmental conditions. An analysis of mitigating drug treatments is complicated by the fact that the majority of studies are limited to the C57BL/6 strain with a premature termination of the experiments and do not establish whether the treatment actually prevents or simply delays the progression of radiation injury. This survey of the literature has pointed to several improvements that need to be considered in establishing a reliable preclinical murine model of radiation lung injury. The lethality end point should also be used cautiously and with greater emphasis on other assays such as non-invasive lung functional and imaging monitoring in order to quantify specific pulmonary injury that can be better extrapolated to radiation toxicity encountered in our own species.

Endothelin-1 concentrations and optimisation of arterial oxygenation and venous admixture by selective pulmonary artery infusion of prostaglandin E1 during thoracotomy

In order to improve arterial oxygenation and venous admixture during one lung ventilation, the effect of selective infusion of prostaglandin E1 into the pulmonary artery of the ventilated lung was investigated in 12 adult patients undergoing thoracotomy. Patients' bronchi were intubated with a Mallinckrodt bronchial tube and ventilated with 66% oxygen in air. Cardiopulmonary factors such as systemic and pulmonary arterial pressures, pulmonary vascular resistance, blood gas analyses, cardiac output and airway pressure were measured before and during one-lung ventilation; venous admixture (Qs/Qt) was calculated. Serial blood samples were taken from the pulmonary artery of the ventilated lung for endothelin-1 estimation. One lung ventilation reduced the mean PaO2 from 42.1 (2.3) kPa to 11.8 (1.4) kPa (p < 0.001) and increased Qs/Qt from 10.8 (3.2)% to 39.2 (4.7)% (p < 0.001). Pulmonary vascular resistance also increased from 167 (24) dyne.s.cm-5 to 262 (38) dyne.s.cm-5 (p < 0.05) with a corresponding increase of plasma endothelin-1 (p < 0.05). After 30 min of one lung ventilation, PGE1 was infused continuously into the pulmonary artery of the ventilated lung at a rate which increased incrementally from 0.04-0.10 micrograms.kg-1.min-1. PaO2 increased to 20.7 (2.6) kPa (p < 0.01) and Qs/Qt decreased to 30.6 (3.5)% (p < 0.05). During the infusion, pulmonary vascular resistance reduced to 173 (30) dyne.s.cm-5 (p < 0.01). The plasma endothelin-1 concentration reached a plateau between the end of one lung ventilation (before closing the thorax) and 6 h postoperatively. This correlated inversely with the pre-operative forced expiratory volume in one second (r = -0.68, p < 0.005), declining to normal values 72 h after operation. The selective infusion of PGE1 into the pulmonary artery of the ventilated lung corrected pulmonary vasoconstriction and improved both arterial oxygenation and venous admixture during one lung ventilation for thoracotomy.

Influence of hypoxia and hypercapnia on the kinetics and hypokaliaemic effect of salbutamol in the rabbit

1. The aims were to document the influence of moderate hypoxia or hypercapnia on salbutamol kinetics and its hypokaliaemic effect, following its administration through the intravenous (60 micrograms/kg), intratracheal (60 micrograms/kg), and oral (2400 micrograms/kg) routes (n = 5). In control animals, PaO2 was around 85 mmHg and PaCO2 20 mmHg; in hypoxic animals PaO2 was around 40 mmHg and in the hypercapnic rabbit PaCO2 was 50 mmHg. 2. Following the intravenous administration of salbutamol, the apparent volume of distribution increased two-fold (p < 0.05) in animals with hypoxia and hypercapnia. Consequently, its half life was enhanced (p < 0.05). Given via the trachea, salbutamol bioavailability was decreased by hypoxia. 3. When salbutamol was given orally, hypoxia or hypercapnia increased the area under salbutamol plasma concentration as a function of time (p < 0.05). 4. In control animals, the salbutamol hypokaliaemic effect was greater when administered orally than through the other routes. Compared with control animals, the experimental conditions reduced the hypokaliaemic effect of salbutamol only when given orally. 5. It is concluded that salbutamol kinetics and dynamics can be altered by hypoxia and hypercapnia.

About mechanisms of prostaglandin E1 induced deterioration of pulmonary gas exchange in COPD patients

Prostaglandin E1 (PGE1) has been reported to attenuate COPD-related pulmonary hypertension and to slightly lower the arterial oxygen tension (PaO2). In order to infer the involved mechanisms, the effects of intravenous infusion of PGE1 on pulmonary haemodynamics, diffusing lung capacity for CO (DLCO), membrane diffusing capacity (Dm), pulmonary capillary blood volume (Vc), physiological shunt (Qps/Qt), arterial blood gases and other lung functional indices were evaluated in 20 COPD patients with pulmonary hypertension and, excluding right catheterization, in 14 control subjects. The examines were studied at baseline and during infusion of 20-30 ng kg-1 min PGE1 or placebo. In control subjects PGE1 only caused systemic arterial pressure decrease (-17.8%). In COPD patients, as expected, PGE1 increased cardiac index (16.2%), but decreased systemic arterial pressure (-21.2%), pulmonary arterial pressure (-27.9%), pulmonary vascular resistance (-45.4%) and PaO2 (-10.4%), worsening their hypoxaemia. However, the effect of PGE1 on DLCO was an increase (11.9%), due to an increase in Vc (15.2%) and less markedly in Dm (4.9%). Physiological and anatomical shunts were increased with PGE1 (20.2% and 14.8%) and the overall ventilation/perfusion ratio decreased from 0.89 to 0.79. Decrements in PaO2 correlated with increments in Qps/Qt (r = 0.86). In conclusion, in COPD patients studied, PGE1 increased DLCO, which compensated for the deleterious effect of increased cardiac output on alveolar-capillary gas equilibration. Therefore, worsening of hypoxaemia during PGE1 infusion was related with increased right-to-left shunt and deterioration of ventilation-perfusion relationship.

Elevated pulmonary artery systolic storage volume associated with improved ventilation-to-perfusion ratios in acute respiratory failure

The possibility that an elevated pulmonary artery systolic storage volume (PASSV) correlates with improved overall ratios of ventilation-to-perfusion and hence benefits gas exchange in acute respiratory failure was examined. We examined this by assessing the correlation between PASSV and both the physiologic dead space to tidal volume ratio (VD/VT) and intrapulmonary shunt fraction (Qsp/Qt). The VD/VT and Qsp/Qt were used as an index of distribution of ventilation-to-perfusion as well as efficiency of pulmonary gas exchange. Twenty-eight patients suffering from acute respiratory failure were included. All required mechanical ventilation. PASSV was calculated from the pulmonary artery (PA) compliance and mean PA systolic distending pressure. Pulmonary arteriolar pressures were computed by Fourier analysis. PA compliance was derived from the PA time constant and the PA resistance. Storage volume fraction of stroke volume index (PASSV/SVI) was used to compare individual variations. There were inverse linear relationships between PASSV/SVI and VD/VT (r = 0.693, p less than 0.0001), and between PASSV/SVI and Qsp/Qt (r = 0.427, p = 0.012). Also, a direct correlation was found between VD/VT and PA time constant (r = 0.503, p = 0.002). The patients were divided into two groups based on PASSV/SVI to evaluate the effect of other hemodynamic data on PASSV. Comparison of the two groups revealed that VD/VT and Qsp/Qt were lower (p less than 0.0001, and p = 0.018, respectively), PA time constant was higher (p less than 0.001), and right ventricular stroke-work index was higher (p = 0.005) in the group with a high PASSV/SVI. There were no differences in other hemodynamic data between the two groups. These data suggest that an elevated PASSV may indeed benefit gas exchange in acute respiratory failure.

Comparative effects of dopamine and dobutamine on the pulmonary haemodynamic response to hypoxia in dogs ventilated with a hypoxic gas mixture

1. The effects of dopamine and dobutamine on the pulmonary haemodynamic response to hypoxia were studied in nine anaesthetized dogs under hypoxaemia induced by ventilation with hypoxic gas mixture. 2. From the present study, only dopamine infusion reduced arterial oxygenation during hypoxic hypoxaemia, whereas catecholamine maintained it at the same level during normoxaemia. 3. A redistribution of perfusion to the pulmonary vasculature may be related to a reduction of arterial oxygenation with dopamine infusion during hypoxic hypoxaemia. 4. It is presumed that an increase in pulmonary blood flow would emphasize an inhibition of hypoxic pressor response in the pulmonary vasculature, which may not be a direct effect of dopamine.

Partial reversal of hypoxic pulmonary hypertension by heparin

Chronic hypoxia produces pulmonary hypertension and an increase in medial thickness of pulmonary arteries that reach maximal values after 10 days of hypoxia. We previously showed that heparin given during the first 10 days of hypoxia reduced the development of both pulmonary hypertension and vascular remodeling in the guinea pig. To determine if heparin could reverse established hypoxic pulmonary hypertension and vascular remodeling, we administered heparin by continuous subcutaneous infusion (20 U/kg/h) for the last 7 days of a 21-day exposure to hypoxia (10% O2, balance N2) and compared these animals with normal saline-infused hypoxic control and room air-exposed animals. Hypoxia increased pulmonary artery pressure from 11 +/- 1 mm Hg (mean +/- SEM) in room air animals to 20 +/- 2 mm Hg (p less than 0.05) in saline-treated hypoxic control animals. Heparin reduced pulmonary artery pressure to 16 +/- 1 mm Hg (p less than 0.05 versus hypoxic control and room air control animals). Total pulmonary resistance (TPR) increased with hypoxia from 0.043 +/- 0.003 mm Hg x min x kg-1 x ml-1 in room air to 0.090 +/- 0.004 in hypoxia (p less than 0.05), and in the rise in TPR was also partially reversed by heparin to 0.068 +/- 0.0003 (p less than 0.05). The percentage of medial thickness of alveolar duct arteries increased from 5.8 +/- 0.6% in room air to 9.5 +/- 0.1% (p less than 0.05) after 3 wk of hypoxia, and heparin therapy partially reversed the increase in medial thickness to 7.2 +/- 0.7% (p less than 0.05 versus both hypoxia control and room air).(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in regional ventilation and perfusion of the lung after endoscopic laser treatment

To determine whether endoscopic laser treatment improves both ventilation and perfusion in patients with advanced lung cancer, krypton-81m ventilation and technetium-99m labelled macro-aggregate perfusion scanning was performed immediately before and two or four days after treatment in a consecutive series of 28 patients. Twelve patients had not received any other treatment before laser therapy and 16 had undergone previous treatments that included radiotherapy. Ventilation and perfusion were quantified by expressing the number of counts in the affected lung as a percentage of the total counts. Ventilation and perfusion improved after laser treatment in 23 patients (82%). The mean ventilation score in the affected lung rose by 50% (p less than 0.001) and the mean perfusion score rose by 24% (p less than 0.001). Incremental changes in ventilation and perfusion scores were positively correlated (r = 0.80). Mean spirometric values, six minute walking distance, the Karnofsky performance index, and breathlessness and wellbeing scores also improved significantly. Patients with main bronchial obstruction who had had no radiotherapy showed the most striking improvements. It is concluded that the removal of intraluminal tumour from the bronchial tree leads to matched improvements in ventilation and perfusion in most patients and that this is associated with valuable improvement in symptoms.

Effects of exercise on gas exchange in patients recovering from acute severe asthma

The effect of exercise on gas exchange was studied in 17 patients (seven male and 10 female) recovering in hospital from acute severe asthma. On admission the patients (mean age 26 (SD 8) years) had a mean peak expiratory flow (PEF) of 26.8% (5.7%) predicted, a mean arterial oxygen tension (PaO2) of 8.63 (1.26) kPa, and a mean alveolar-arterial oxygen difference (A-aDO2) of 5.98 (1.34) kPa. Once resting symptoms had resolved (after 3.7 (1.6) days) patients performed a constant load exercise test (100 watts for men, 75 watts for women) on a bicycle ergometer for five minutes. PEF was measured before exercise and at five minute intervals for 30 minutes after exercise. Blood gas tensions were measured on capillary blood before, during, and 10 minutes after exercise. Treatment was not interrupted for this study. Mean PEF at rest was 87.6% (21.1%) predicted and showed no significant change after exercise. Mean PaO2 was 13.13 (1.37) kPa before exercise; it showed no significant change during exercise (13.93 (1.34) kPa) or 10 minutes after exercise (13.50 (2.15) kPa). Mean A-aDO2 also showed no change, being 1.82 (1.31) kPa before exercise, 1.79 (1.27) kPa during exercise, and 2.53 (0.93) kPa after exercise. It is concluded that moderate exercise carried out shortly after treatment for acute severe asthma is unlikely to result in worsening gas exchange during or after exercise if resting PEF, PaO2 and A-aDO2 have attained normal or near normal values.

Enhancement of hypoxic pulmonary vasoconstriction by low dose almitrine bismesylate in normal humans

The effect of almitrine bismesylate on hypoxic pulmonary vasoconstriction (HPV) remains controversial. We therefore investigated in a double-blind, placebo-controlled, randomized design the effects of low dose of almitrine bismesylate (4 micrograms/kg/min given intravenously) on blood gases, pulmonary hemodynamics, and ventilation-perfusion (VA/Q) distributions in normal subjects breathing a hypoxic mixture (FIO2, 0.125), room air (FIO2, 0.21), and oxygen (FIO2, 1.0) in a random sequence. In the placebo group (7 subjects), no change was recorded. In the almitrine group (10 subjects), arterial PO2 improved during hypoxia (from 42 +/- 2 to 47 +/- 1 mm Hg, p less than 0.05, mean +/- SEM) and normoxia (from 99 +/- 3 to 104 +/- 2, p less than 0.05). Pulmonary arterial mean pressure and pulmonary vascular resistance index increased with almitrine during hypoxia, respectively, from 20 +/- 1 to 23 +/- 1 mm Hg (p less than 0.01) and from 207 +/- 22 to 283 +/- 35 dyne.s.cm-5.m2 (p less than 0.01), and during normoxia, respectively, from 12 +/- 1 to 14 +/- 1 mm Hg (p less than 0.05) and from 90 +/- 11 to 137 +/- 13 dyne.s.cm-5.m2 (p less than 0.05). The VA/Q distribution improved during hypoxia, with a shift of the blood flow distribution to better oxygenated lung units with higher VA/Q ratios. We conclude that in normal humans low dose of almitrine improves gas exchange by an enhancement of HPV.

Platypnea induced by worsening of VA/Q inhomogeneity in the sitting position in chronic obstructive lung disease

We report a patient with COPD who experienced severe dyspnea with a decreased PaO2 in the sitting position, largely improved in the ventral prone position (platypnea). By using an isotopic method, we showed that this phenomenon is the result of a deleterious effect of the sitting position on regional ventilation/perfusion matching. We speculated that hypoxic vascular pulmonary constriction was deficient in our patient. This hypothesis is supported by a dramatically better matching of the VA/Q ratio after treatment with almitrine bismesilate, a drug that could potentiate hypoxic pulmonary vasoconstrictive reflexes.

Hypoxic pulmonary vasoconstriction and pulmonary gas exchange in normal man

Blood gases, hemodynamics and ventilation were measured in 7 healthy volunteers at baseline while breathing room air (FIO2 0.21), during hypoxia (FIO2 0.125, 15 min) and after nifedipine 20 mg sublingually at FIO2 0.21 (45 min) and at FIO2 0.125 (15 min). Distributions of ventilation-perfusion ratios (VA/Q) were determined, using the multiple inert gas elimination technique, at baseline, during hypoxia, and again during hypoxia after nifedipine intake. Hypoxia was associated with an average increase in pulmonary vascular resistances by 104%, which was partially inhibited by nifedipine. The inert gas data showed a mild deterioration in the distribution of VA/Q ratios during hypoxia. However, when blood flow and ventilation were constrained to the baseline normoxic values in the distributions recovered during hypoxia ('normalization procedure') a slight improvement in VA/Q matching could be evidenced, which was blunted during hypoxia after nifedipine. This was interpreted as the functional effect of hypoxic pulmonary vasoconstriction (HPV). Using the 'normalized' distributions, we computed the relationship between the decrease in compartmental blood flow that occurred during hypoxia and the corresponding alveolar PO2, and calculated the gain due to HPV feedback using equations of the control theory. The contribution of HPV to the stability of compartmental VA/Q was greatest for alveolar PO2 values around 60 mm Hg, but at best the feedback had only a moderate efficiency.

Right ventricular stroke-work. An index of distribution of pulmonary perfusion in acute respiratory failure

To evaluate the possibility that an elevated right ventricular stroke-work index observed in acute respiratory failure correlates with less dead space ventilation, 20 patients requiring mechanical ventilation were studied. Ratio of physiologic dead space to tidal volume was used as an index of distribution of pulmonary perfusion. An inverse linear relationship was found between right ventricular stroke-work index and dead space ratio (r = -0.74, p less than 0.001). No correlation was present between cardiac index and dead space ratio. To evaluate the effects of other hemodynamic data on the distribution of pulmonary perfusion, the patients were divided into two groups based on a dead space ratio. Comparison of the two groups revealed right ventricular stroke-work index was significantly higher in the low dead space ratio group (p less than 0.001). There were no differences in cardiac index, pulmonary-artery pressure, or right atrial pressures. These data suggest that elevated right ventricular stroke-work index in acute respiratory failure is associated with more even distribution of pulmonary perfusion, and that patients with a high dead space ratio were characterized by right ventricular dysfunction when compared to patients with a low dead space ratio.

Improvement in ventilation-perfusion matching by almitrine in COPD

Almitrine, a peripheral chemoreceptor stimulating drug, was given 100 mg orally to six patients with advanced chronic obstructive pulmonary disease (COPD), and its effects on hemodynamics, blood gases, lung mechanics, and the distribution of ventilation/perfusion ratios (VA/Q), determined by the inert gas elimination technique, were investigated. Arterial Po2 increased from 52 +/- 4 to 59 +/- 3 mm Hg, mean +/- SEM, p less than 0.01, arterial Pco2 decreased from 46 +/- 3 to 43 +/- 3 mm Hg, p less than 0.05, and venous admixture from 30 +/- 6 to 19 +/- 3 percent, p less than 0.02. No change occurred in ventilation, variables of lung mechanics, systemic and pulmonary hemodynamics, except an increase in pulmonary vascular resistance (from 364 +/- 103 to 438 +/- 99 dyne.s.cm-5, p less than 0.05). A reduction in VA/Q inequality could be demonstrated with a redistribution of blood flow into the lungs by a diversion of 15 percent of total blood flow from units with low VA/Q (between 0.08 and 0.4) to units with normal VA/Q (between 0.5 and 1.8). These changes might be explained by an enhancement of hypoxic pulmonary vasoconstriction. Pharmacologic peripheral chemoreceptor stimulation, at an infra-ventilatory analeptic dosage, might be of therapeutic interest to patients with respiratory insufficiency due to VA/Q inequality.

The effect of PCO2 on hypoxic pulmonary vasoconstriction

Lung areas with a low V/Q ratio cause hypoxaemia. The low alveolar oxygen concentration may cause local hypoxic pulmonary vasoconstriction (HPV) which reduces perfusion, raises the V/Q ratio, and hence reduces the tendency to a low PaO2. By changing PCO2, the HPV response can be altered. We examined this relationship in anaesthetized dogs by using a tracheal divider to separate hypoxic (nitrogen ventilated) from oxygenated (100 per cent oxygen ventilated) lung. Relative perfusion was assessed from total 133Xe exhaled from each lung area after intravenous infusions. When PaCO2 was changed by changing ventilation, we found that an increasing PaCO2 increased HPV and also PaO2. At a PaCO2 of 3.3 kPa, HPV was abolished and PaO2 fell. We also changed PaCO2 by altering PICO2 to one or both lung areas while ventilation remained constant throughout the experiment. Again as PaCO2 increased, HPV and PaCO2 increased. When PaCO2 fell and end tidal carbon dioxide in the hypoxic lung (PETCO2) remained elevated by maintaining PICO2 in the hypoxic lung and removing CO2 from the oxygenated lung) HPV was maintained. Thus it is the alveolar concentration of CO2 in the hypoxic lung which is important in modifying HPV. We conclude that in this model a low PETCO2 (3.3 kPa) in hypoxic lung will reduce HPV, and will result in more severe hypoxaemia. This may have relevance in both anaesthetized and intensive care unit patients when a higher PaO2 may be obtained by increasing hypoxic lung PETCO2. The effect of PETCO2 on PaO2 will be influenced by other variables, but when hypoventilated or hypoxic exist, increasing PETCO2 may reinforce hypoxic pulmonary vasoconstriction and thus may increase PaO2.

Continuous ketamine infusion for one-lung anaesthesia

The mechanism which normally affects distribution of blood flow through unventilated areas of the lung is hypoxic pulmonary vasoconstriction; this acts to divert the blood to well ventilated alveoli, resulting in a better ratio of ventilation to perfusion. Several reports have focused attention on the reduction or abolition of this reflex in the unventilated lung by most of the volatile anaesthetic agents used in clinical practice. This response was not abolished by the intravenous anaesthetic agents. One hundred and ten patients undergoing elective pulmonary resection were studied to evaluate the effect of a continuous infusion of ketamine during one-lung anaesthesia, by observing the changes in PaO2 as a reflection of shunt. Ketamine was chosen as the intravenous agent for its positive inotropic and chronotropic action. Additionally, by providing both analgesia and hypnosis, we were able to administer inspired oxygen concentrations of 50-100 per cent without concern that the patient might have recall for events during operation. We have demonstrated that in all cases a PaO2 in excess of 9.31 kPa (70 torr) was achieved with ketamine and FIO2 1.0 as well as an increase in shunt fraction from 25.9 per cent (FIO2 0.5) to 36.0 per cent (FIO2 1.0). We feel that ketamine provides a satisfactory alternative to the volatile agents for one-lung anaesthesia in patients where relative hypoxaemia might be unacceptable during operation.

Effects of intrapulmonary CO2 and airway pressure on phrenic activity and pulmonary stretch receptor discharge in dogs

Intrapulmonary CO2 and stretch sensitivity were studied in anesthetized, thoracotomized dogs in which both lungs were independently ventilated. The left pulmonary artery was occluded so that changes in left (Test) lung CO2 did not alter systemic arterial PCO2. Adequate gas exchange was maintained in the right lung, which was vagally denervated. In one series of experiments, neural activity in the C5 root of the phrenic nerve was integrated to assess ventilatory drive at various levels of Test lung CO2 and airway pressure both during cyclic ventilation and static lung inflation. In a second series, single unit activity in vagal afferents from pulmonary stretch receptors (PSR) was recorded. Both phrenic activity and PSR discharge were strongly affected by changes in airway pressure but not by changes in Test lung CO2 between 2 and 7%. However, when test lung CO2 was decreased below 2%, phrenic activity decreased and PSR activity increased. These changes were invariably accompanied by an increase in peak airway pressure during cyclic ventilation suggesting that lung mechanics had been altered. The results indicate that, in this preparation, the effects of lung stretch on ventilatory drive spans a wide range whereas intrapulmonary CO2 exerts an effect only at very low levels. It appears that the reflex effects of both stimuli can be accounted by an effect on PSR activity.

The pulmonary vasoconstrictor response to hypoxia during enflurane anesthesia

Previous investigations have shown that diethyl ether, halothane and methoxyflurane inhibit the vasoconstrictor response to hypoxia in isolated rat lungs. A damping effect of diethyl ether and halothane has been observed in the human lung as well. In the intact dog lung, however, other workers have recently denied any effect of halothane and enflurane on this vasoconstrictor mechanism. These findings challenged us to investigate the effect of enflurane on the basoconstrictor response to hypoxia in isolated rat lungs. We found that enflurane administered via the airways reduced the response in proportion to the end-tidal concentration of enflurane, as determined by mass spectrometry.

Pulmonary embolism distribution to ventilated and unventilated lungs in the dog: a cause of hypoxaemia

To examine a possible mechanism which could cause arterial hypoxaemia following pulmonary embolism, we collapsed and did not ventilate one lung in each of eleven dogs, to produce hypoxic pulmonary vasoconstriction. In five dogs (Starch Group), PaO2 fell from 10 to 7.7 kPa (76.6 to 58.4 torr) as shunt fraction (Qs/Qt) rose from 19 to 31 per cent. Mean pulmonary artery pressure (ppa), paCO2 and VD/VT remained constant. Starch emboli (63--74 micron) were taken injected. PPA increased significantly and PaO2 dropped further to 5 kPa (37.8 torr) as Qs/Qt rose to 57 per cent. VD/VT increased and PaCO2 remained constant. Microscopic examination of the lungs showed that three times as many emboli went to the ventilated lung compared to the unventilated lung. Six dogs (Blood Clot Group) received 51Cr labelled autologous blood clot. Changes after emboli in PPA, PaO2, Qs/QT, PaCO2 and VD/VT were similar to the results in the Starch group. 125I serum albumin was then injected and the dogs were sacrificed. The lungs were monogenized separately and the 51Cr and 125I counted. The 51Cr counts indicated 66 per cent of the blood clot emboli went to the ventilated lung. Following embolization, the 125I counts suggested a shift in perfusion to the unventilated lung. We conclude from these results that emboli are preferentially distributed to ventilated lung. After embolization PPA increases. At least in this pulmonary embolism model the increased PPA may overcome hypoxic pulmonary vasoconstriction, redistribute blood to non-ventilated lung and create arterial hypoxaemia.

Changes in blood volume and extravascular water content in isolated perfused rat lungs during ventilation hypoxia

Isolated rat lungs were perfused with homologous blood at constant volume inflow. The effect of ventilation hypoxia on pulmonary vascular resistance, preparation weight and reservoir volume (vascular capacitance) were studied. In some experiments also wet/dry weight ratio of the preparation was estimated (extravascular water content). There was no difference in this last parameter between hypoxic and normoxic lungs, thus alveolar hypoxia had no effect on tissue water content as previously described in intact rats. With forward perfusion small and transient changes in either direction were seen in preparation weight and reservoir volume, even though inflow pressure exceeded 5 kPa during alveolar hypoxia. With backward perfusion marked weight increases were seen, and if inflow pressure in this situation was above 3.5 kPa, the weight change was irreversible, thus indicating outward fluid filtration. It is concluded that the vessels responding to alveolar hypoxia are located on the arterial side of the pulmonary vasculature.

Reduction of hypoxic pulmonary vasoconstriction by nitrous oxide administration in the isolated perfused cat lung

Local pulmonary vasconstriction in response to alveolar hypoxia is a protective mechanism reducing blood flow to poorly oxygenated areas of lung. Pulmonary blood flow is thereby directed to better oxygenated lung units and venous admixture and the resulting arterial hypoxaemia is reduced. The effect of nitrous oxide on the pulmonary pressor response to alveolar hypoxia was assessed in the isolated perfused cat lung preparation under conditions of constant flow and constant left atrial and airway pressures. Nitrous oxide, in concentrations of 50 per cent and 75 per cent, was found to produce a reversible depression of the hypoxic pulmonary pressor response. The importance of hypoxia pulmonary vasconstriction and the possible implications of its reduction by anaesthetic agents are discussed.