Reduction of hypoxic pulmonary vasoconstriction during trichloroethylene anesthesia

Respiratory effects of trichloroethylene

Trichloroethylene (TCE) is a chlorinated solvent that has been used widely around the world in the twentieth century for metal degreasing and dry cleaning. Although TCE displays general toxicity and is classified as a human carcinogen, the association between TCE exposure and respiratory disorders are conflicting. In this review we aimed to systematically evaluate the current evidence for the respiratory effects of TCE exposure and the implications for the practicing clinician. There is limited evidence of an increased risk of lung cancer associated with TCE exposure based on animal and human data. However, the effect of other chlorinated solvents and mixed solvent exposure should be further investigated. Limited data are available to support an association between TCE exposure and respiratory tract disorders such as asthma, chronic bronchitis, or rhinitis. The most consistent data is the association of TCE with autoimmune and vascular diseases such as systemic sclerosis and pulmonary veno-occlusive disease. Although recent data are reassuring regarding the absence of an increased lung cancer risk with TCE exposure, clinicians should be aware of other potential respiratory effects of TCE. In particular, occupational exposure to TCE has been linked to less common conditions such as systemic sclerosis and pulmonary veno-occlusive disease.

Reduced pulmonary vascular reactivity after cold exposure to acute hypoxia: a role of nitric oxide (NO)

Exposure to high altitude causes pulmonary hypertension and alterations in pulmonary vascular reactivity. Among the environmental factors, cold exposure has been suggested to be involved in the development of pulmonary hypertension. However, little information is available about pulmonary vascular reactivity after cold exposure. We examined whether cold exposure can cause changes in pulmonary vascular reactivity to acute hypoxia and the possible participation of endogenous nitric oxide. We measured mean systemic (Psa) and pulmonary artery pressures (Ppa) in conscious rats after 1-week cold exposure (3.5 +/- 1.0 degrees C). Subsequently, we investigated hypoxic pulmonary vasoconstriction (HPV) with and without endogenous NO inhibition using N(G)-nitro-L-arginine methyl ester (3 mg/kg) or 7-nitroindazole (1 mg/kg). Cold exposure for 1 week caused a small but significant increase in Ppa, but not in Psa. Neither Ppa nor Psa showed significant changes after both NO inhibitions in rats exposed to cold. However, cold exposure caused a blunted HPV and an increase in plasma nitrite-nitrate concentration compared with rats kept in a neutral environment (24.0 +/- 1.0 degrees C). In addition, NO inhibition by N(G)-nitro-L-arginine methyl ester partially restored the blunted HPV in rats exposed to cold, but not 7-nitroindazole, a selective inhibitor of neuronal NO synthase. We concluded that cold exposure alters pulmonary vascular reactivity to acute hypoxia, and augmented endothelial NO bioactivity plays a counterregulatory role in response to acute hypoxia during cold exposure in rats.

Comparison of cardiopulmonary response to endogenous nitric oxide inhibition in pigs inhabited at three levels of altitude

Nitric oxide (NO) plays an important role for the pulmonary circulation in normal and chronic hypoxia. We examined effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary and systemic vascular resistance in unanesthetized pigs living at three levels of altitude to evaluate the role of NO in adaptation to a hypoxic environment. Unanesthetized male adult pigs in three areas [Matsumoto, Japan (680 m above sea level, n = 5); Xing, China (2,300 m, n = 5); and Maxin, China (3,750 m, n = 5)] were prepared for vascular monitoring. Pulmonary (P(pa)), and systemic artery pressure (P(sa)) were monitored, and pulmonary artery wedge pressure (P(cwp)) and cardiac output (CO) were measured before and after treatment with a non-selective NOS inhibitor, N(w)-nitro-L-argine (NLA; 20 mg/kg). Pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR) were (P(pa)-P(cwp))/CO and P(sa)/CO, respectively. Related to altitude baseline P(pa) was elevated. After NLA administration, P(pa) and P(sa) increased and CO decreased in all animals, resulting in increases in PVR and SVR. However, there were no significant differences in the increase in PVR and SVR in the three groups of pigs. Thus, endogenous NO production contributes to regulate the basal pulmonary vascular tone, but the development of hypoxic pulmonary hypertension appears to be independent of the NO pathway in adult pigs.

Contribution of nitric oxide to adaptation of tibetan sheep to high altitude

We examined the effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary hemodynamics in awake sheep living at low and high altitudes to evaluate the role of NO in adaptation to an hypoxic environment. Unanaesthetized male sheep in three places--Matsumoto, Japan (680 m above sea level), Xing, China (2300 m) and Maxin, China (3750 m)--were prepared for measurements of pulmonary artery (Ppa) and pulmonary vascular resistance (PVR) before and after the NOS inhibition. The non-selective NOS inhibitor, Nw-nitro-l-argine (NLA, 20 mg/kg) was used. Baseline Ppa became elevated with an increase in altitude. After NLA administration, PVR significantly increased in animals of all groups. However, the increase in PVR after NLA in tibetan sheep at 3750 m was significantly higher than those in other groups. We conclude that augmented endogenous NO production may contribute to regulating the pulmonary vascular tone in tibetan sheep (3750 m) adapted to high altitude.

Alarming hypoxemia during one-lung ventilation in a patient with respiratory bronchiolitis-associated interstitial lung disease

PURPOSE

To report a patient with respiratory bronchiolitis-associated interstitial lung disease (RB-ILD) who developed severe hypoxemia during one-lung ventilation (OLV).

CLINICAL FEATURES

A 27-yr-old female, ex-smoker presented with productive cough and dyspnea of 18-month duration. The chest x-ray revealed diffuse abnormalities involving both lungs consisting of interstitial emphysema with irregular shadowing. Preoperative PaO(2) was 88 mmHg and pulmonary function tests showed moderate obstructive disease. The patient underwent right open lung biopsy. After induction of anesthesia, a left double lumen tube was inserted and its position verified with auscultation and fibreoptic bronchoscopy. Upon initiation of OLV, the patient developed severe hypoxemia and the PaO(2) dropped from 500 mmHg during two-lung ventilation (TLV) to 50 mmHg. Hypoxemia was readily corrected by resuming TLV.

CONCLUSION

The severe hypoxemia during OLV in this patient with RB-ILD may be attributed to impaired hypoxic pulmonary vasoconstriction. Other causes were not excluded. Caution is warranted when initiating OLV in these patients.

Pulmonary scintigraphy in canine lobar and sublobar airway obstruction

The intrapulmonary distribution of the inhalation-to-perfusion ratio (I/P) was studied after placement of an obstruction in a sublobar bronchus (SLO) or in a lobar bronchus (LO) in 17 anesthetized prone (sternal recumbent) dogs. Placement of an SLO or LO did not induce any significant changes in the standard ventilatory and hemodynamic parameters measured. With the use of 99mTc aerosol inhalation combined with 99mTc perfusion lung scintigraphy, parameters quantifying the inhalation (I) to perfusion (P) mismatching at regional as well as at intraregional levels were calculated. The SLO increased the relative ventilation to the lung containing the obstruction, induced a shift of blood from the obstructed segment to the rest of the same lung, and increased the ratio between the mean I and the mean P (regional mismatching factor) in both the obstructed segment and the lung containing the obstruction. The LO diverted air and blood away from the obstructed lobe to the contralateral lung, but blood to a lesser extent. The LO decreased the regional mismatching factor in the obstructed lobe but also in the lung containing the obstruction. It also increased significantly the intraregional I to P mismatching in the obstructed lobe as well as in the rest of the same lung. After withdrawal of LO and reinsufflation of the collapsed lobe, blood continued to leave this lobe, while the withdrawal of SLO allowed recovery of the initial perfusion. This suggests that both hypoxic vasoconstriction and mechanical factors such as vascular distortion are involved in the blood shifts observed, and require some delay to be relieved. It is concluded that in dogs compensating mechanisms such as collateral ventilation are very effective. In case of SLO, I to the lung involved is increased. In case of LO, vasoconstriction in response to alveolar hypoxia could not fully compensate for the decreased I to the obstructed lobe.

Hypoxic pulmonary vasoconstriction in man: effects of hyperventilation

The pulmonary vasoconstriction response to hypoxia was studied in eight anaesthetized supine subjects. One lung was made hypoxic while the other was ventilated with 100% oxygen. This was achieved by separating the tidal gas-distribution to the lungs by means of a double-lumen tracheal catheter. The hypoxic pulmonary vasoconstriction (HPV) response was estimated from the blood flow diversion away from the hypoxic lung. Blood flow distribution between the lungs was calculated from the regional expired carbon dioxide production, assuming regional carbon dioxide production to be proportional to blood flow. The subjects were studied during six different conditions. Firstly, when ventilated with 100% oxygen to both lungs at a PaCO2 of about 6 kPa. Secondly, with 100% oxygen to the left lung and 5% oxygen in nitrogen to the right (test) lung. The ratio between carbon dioxide output from right and left lung was calculated. These measurements were repeated during two states of hyperventilation (PaCO2 of about 4.5 kPa and 3.5 kPa, respectively) with and without hypoxia (conditions 3-6). During normoventilation, blood flow distribution between the lungs was equal. During hypoxia, blood flow distribution to the hypoxic lung decreased by 35% of the pre-hypoxic value. Furthermore, a decrease in arterial oxygen tension from 51.5 +/- 4.5 to 11.5 +/- 2.1 kPa was observed. During excessive hyperventilation (PaCO2 3.2 +/- 0.2 kPa), blood flow distribution to the hypoxic right lung decreased by only 10% of its pre-hypoxic value. A further decrease in arterial oxygen tension to 8.5 +/- 1.8 kPa was observed. This decrease in PaO2 was possibly due to an increased venous admixture caused by an abolished HPV response. It is concluded that hyperventilation counteracts hypoxic pulmonary vasoconstriction in man.

Effects of dopamine and dobutamine on the distribution of pulmonary blood flow during lobar ventilation hypoxia and lobar collapse in dogs

Hypoxic pulmonary vasoconstriction was induced in the left lower lobe of fifteen dogs by ventilating the lobe with 7% O2 or by absorption collapse, and the distribution of flow between the lobe and the remainder of the lung was measured with electromagnetic flow probes. The lobar to total blood flow ratio was reduced by lobar ventilation hypoxia and decreased further during lobar collapse. In seven dogs, an infusion of 20 micrograms kg-1 min-1 of dopamine produced an increase in total blood flow, an increase in pulmonary artery pressure (P less than 0.01), and an increase in lobar to total flow ratio (P less than 0.05) during both hypoxic states. There was a significant fall in arterial PO2 (P less than 0.01) during ventilation hypoxia. Similar changes in total and lobar to total flow ratio (P less than 0.01) were observed in eight dogs given 20 micrograms kg-1 min-1 of dobutamine, but there were no changes in pulmonary artery pressure. The greater increase in total flow (+ 111%) resulted in a marked increase in mixed venous PO2 and no significant changes in arterial PO2 in this group of dogs. It is concluded that both drugs produce an increase in lobar to total blood flow ratio and shunt fraction, but that the mechanisms causing the redistribution of flow may differ.

Hypoxic pulmonary vasoconstriction in the human lung: the effect of prolonged unilateral hypoxic challenge during anaesthesia

The influence of time on the pulmonary vasoconstrictor response to hypoxia was studied in six subjects during general anaesthesia and artificial ventilation prior to elective surgery. The lungs were intubated separately with a double-lumen bronchial catheter. After preoxygenation of both lungs for 30 min, the test lung was rendered hypoxic for 60 min by ventilation with 5% O2 in N2, with the control lung still being ventilated with 100% O2. Cardiac output was determined by thermodilution, and the distribution of blood flow between the lungs was assessed from the excretion of a continuously infused poorly soluble gas (SF6). The fractional perfusion of the test lung decreased from 53% to 25% of cardiac output within the first 15 min of unilateral hypoxia. The pulmonary artery mean pressure increased by 14% and the pulmonary vascular resistance (PVR) of the test lung increased by 54%. Venous admixture increased from 21% to 39% of cardiac output, while the "true" shunt was maintained at about 15%. Arterial oxygen tension (Pao2) fell from 45 kPa to 12 kPa. Prolonging the unilateral hypoxic challenge caused no further change in the redistribution of the pulmonary blood flow, but cardiac output and pulmonary artery mean pressure continued to increase to 40%-50% above control values after 1 h of hypoxia. The PVR of the test lung remained unchanged. The findings suggest that there is an immediate vasoconstrictor response to hypoxia in the human lung and that there is no further potentiation or diminution, of the response during a 60-min period of hypoxia.

Protamine induced arterial hypoxaemia: the relationship to hypoxic pulmonary vasoconstriction

Protamine administration may induce arterial hypoxaemia in dogs and humans. However, the responsible mechanism has not been established. Protamine, as it is a pulmonary vasoactive substance, may interfere with normal hypoxic pulmonary vasoconstriction (HPV) and cause arterial hypoxaemia. This possibility was tested in dogs utilizing a one lung hypoxic model. One lung hypoxic ventilation decreased pulmonary blood flow in the hypoxic lung from 1022 +/- 96 ml X min-1 (mean +/- SEM) to 846 +/- 39 ml X min-1 (p less than 0.05) while increasing blood flow from 833 +/- 85 ml X min-1 to 1109 +/- 101 ml X min-1 (p less than 0.05) in the normoxic lung, resulting in 24 per cent effective diversion of blood flow. Protamine infusion, after heparinization, markedly elevated pulmonary vascular resistance in both lungs but preferentially in the normoxic lung (102 +/- 27 per cent increase in normoxic lung, 60 +/- 6.4 per cent increase in hypoxic lung) and significantly reversed the pulmonary blood flow shift induced by one lung hypoxic ventilation (effective diversion of blood flow was reduced to four per cent). Concurrently, arterial PO2 further decreased. Our results demonstrate that protamine interferes with effectiveness of pre-existing HPV and suggest that this mechanism, at least in part, may be responsible for arterial hypoxaemia observed after protamine infusion. The marked generalized pulmonary vasoconstriction with protamine appears to be the direct force that interferes with pre-existing auto-regulatory HPV. In addition to the well known haemodynamic effects of protamine, protamine infusion may also cause arterial hypoxaemia in those patients in whom HPV plays a significant role in maintaining arterial oxygenation.

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.

Effect of halothane on the pulmonary vascular response to hypoxia in dogs

The pulmonary vascular response to alveolar hypoxia with and without halothane exposure was measured in dogs. Hypoxia increased pulmonary artery pressure (Ppa) and pulmonary vascular resistance (PVR) so that in each case inverse linear relationships were found with arterial oxygen saturation. These responses were highly significant and reproduceable but varied greatly between individuals. Halothane administration resulted in an increased slope of PVR against oxygen saturation due to the fact that reduction in cardiac output exceeded the reduction in Ppa. An analysis of variance showed that it was possible to predict accurately the hypoxic PVR and Ppa responses under halothane anaesthesia from the control hypoxic responses. Animals with low PVR responses showed the greatest arterial oxygen desaturation with hypoxia, whereas high responders showed least oxygen desaturation. Thus it was possible to predict those individuals at risk from severe arterial oxygen desaturation under halothane anaesthesia.

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.

Reduction of hypoxic pulmonary vasoconstriction by diethyl ether in the isolated perfused cat lung: the effect of acidosis and alkalosis

Hypoxic pulmonary vasoconstriction is a protective mechanism diverting pulmonary blood flow away from hypoxic areas toward more optimally oxygenated lung units. Venous admixture is reduced and arterial oxygenation improved. Hypoxic pulmonary vasoconstriction was demonstrated during acidosis, alkalosis and normal pH in the isolated perfused cat lung under conditions of constant flow and constant left atrial and airway pressures. Two per cent diethyl ether markedly reduced hypoxic vasoconstriction under all acid-base conditions, the hypoxic pressor response returning after wash-out of diethyl ether. Modification of hypoxic pulmonary vasoconstriction during acid-base disturbances and possible implications of concurrent anaesthetic administration are discussed.