Resuscitation of dogs from severe acute carbon monoxide poisoning

Pulmonary Phototherapy for Treating Carbon Monoxide Poisoning

RATIONALE

Carbon monoxide (CO) exposure is a leading cause of poison-related mortality. CO binds to Hb, forming carboxyhemoglobin (COHb), and produces tissue damage. Treatment of CO poisoning requires rapid removal of CO and restoration of oxygen delivery. Visible light is known to effectively dissociate CO from Hb, with a single photon dissociating one CO molecule.

OBJECTIVES

To determine whether illumination of the lungs of CO-poisoned mice causes dissociation of COHb from blood transiting the lungs, releasing CO into alveoli and thereby enhancing the rate of CO elimination.

METHODS

We developed a model of CO poisoning in anesthetized and mechanically ventilated mice to assess the effects of direct lung illumination (phototherapy) on the CO elimination rate. Light at wavelengths between 532 and 690 nm was tested. The effect of lung phototherapy administered during CO poisoning was also studied. To avoid a thoracotomy, we assessed the effect of lung phototherapy delivered to murine lungs via an optical fiber placed in the esophagus.

MEASUREMENTS AND MAIN RESULTS

In CO-poisoned mice, phototherapy of exposed lungs at 532, 570, 592, and 628 nm dissociated CO from Hb and doubled the CO elimination rate. Phototherapy administered during severe CO poisoning limited the blood COHb increase and improved the survival rate. Noninvasive transesophageal phototherapy delivered to murine lungs via an optical fiber increased the rate of CO elimination while avoiding a thoracotomy.

CONCLUSIONS

Future development and scaling up of lung phototherapy for patients with CO exposure may provide a significant advance for treating and preventing CO poisoning.

Increased carbon monoxide clearance during exercise in humans

PURPOSE

Hyperventilation increases the clearance of carbon monoxide (CO) from blood; thus, we hypothesized that CO elimination would be enhanced with exercise. Accordingly, this study examined the effect of exercise on the half-life of carboxyhemolobin elimination.

METHODS

Six healthy subjects (three males and three females) with mean ± SD ages of 23 ± 4 yr were exposed to CO sufficient to raise blood carboxyhemolobin concentration to 10-14% on five separate days. The half-life for CO elimination was measured breathing room air at rest and during exercise at three intensities.

RESULTS

Comparisons showed that the half-life decreased with exercise from that during rest in all subjects. The half-life was also measured during 100% oxygen breathing at the lowest exercise intensity of 63 ± 15 W and found to be the least of all measured (23 ± 4 min).

CONCLUSIONS

1) Exercise increased isocapnic ventilation, thereby decreasing the half-life of CO elimination. 2) The half-life of CO elimination represents a hyperbolic function of ventilation [y = y0 + (a / x)], and so increasing ventilation by exercise reaches a point of diminishing returns. 3) Breathing 100% oxygen during mild exercise is as effective in eliminating CO as treatment with hyperbaric oxygen. 4) Moderate exercise under room air conditions is as effective in eliminating CO as breathing oxygen at rest. Thus, the combination of mild exercise, hyperventilation, and normobaric hyperoxia (100% oxygen inhalation) may be considered the "triple therapy" for CO elimination in some patients.

Rapid elimination of CO through the lungs: coming full circle 100 years on

At the start of the 20th century, CO poisoning was treated by administering a combination of CO(2) and O(2) (carbogen) to stimulate ventilation. This treatment was reported to be highly effective, even reversing the deep coma of severe CO poisoning before patients arrived at the hospital. The efficacy of carbogen in treating CO poisoning was initially attributed to the absorption of CO(2); however, it was eventually realized that the increase in pulmonary ventilation was the predominant factor accelerating clearance of CO from the blood. The inhaled CO(2) in the carbogen stimulated ventilation but prevented hypocapnia and the resulting reductions in cerebral blood flow. By then, however, carbogen treatment for CO poisoning had been abandoned in favour of hyperbaric O(2). Now, a half-century later, there is accumulating evidence that hyperbaric O(2) is not efficacious, most probably because of delays in initiating treatment. We now also know that increases in pulmonary ventilation with O(2)-enriched gas can clear CO from the blood as fast, or very nearly as fast, as hyperbaric O(2). Compared with hyperbaric O(2), the technology for accelerating pulmonary clearance of CO with hyperoxic gas is not only portable and inexpensive, but also may be far more effective because treatment can be initiated sooner. In addition, the technology can be distributed more widely, especially in developing countries where the prevalence of CO poisoning is highest. Finally, early pulmonary CO clearance does not delay or preclude any other treatment, including subsequent treatment with hyperbaric O(2).

Reduction of carboxyhaemoglobin levels in the venous blood of cigarette smokers following the administration of carbogen

Cigarette smokers have high carboxyhaemoglobin levels which can promote tumour radioresistance. Inhalation of carbogen gas shortens the half-life of carboxyhaemoglobin, increasing tumour radiosensitivity in animal models. Breathing 2.5% carbogen for 30 min results in a greater reduction in venous blood COHb levels than breathing 5% carbogen for 7 min.

Isocapnic hyperventilation increases carbon monoxide elimination and oxygen delivery

Hyperventilation with mixtures of O2 and CO2 has long been known to enhance carbon monoxide (CO) elimination at low HbCO levels in animals and humans. The effect of this therapy on oxygen delivery (DO2) has not been studied. Isocapnic hyperventilation utilizing mechanical ventilation may decrease cardiac output and therefore decrease DO2 while increasing CO elimination. We studied the effects of isocapnic hyperventilation on five adult mechanically ventilated sheep exposed to multiple episodes of severe CO poisoning. Five ventilatory patterns were studied: baseline minute ventilation (RR. VT), twice (2. RR) and four times (4. RR) baseline respiratory rate, and twice (2. VT) and four times (4. VT) baseline tidal volume. The mean carboxyhemoglobin (HbCO) washout half-time (t1/2) was 14.3 +/- 1.6 min for RR. VT, decreasing to 9.5 +/- 0.9 min for 2. RR, 8.0 +/- 0.5 min for 2. VT, 6.2 +/- 0.5 min for 4. RR, and 5.2 +/- 0.5 min for 4. VT. DO2 was increased during hyperventilation compared with baseline ventilation for 2. VT, 4. RR, and 4. VT ventilatory patterns. Isocapnic hyperventilation, in our animal model, did not alter arterial or pulmonary blood pressures, arterial pH, or cardiac output. Isocapnic hyperventilation is a promising therapy for CO poisoning.

A simple "new" method to accelerate clearance of carbon monoxide

The currently recommended prehospital treatment for carbon monoxide (CO) poisoning is administration of 100% O(2). We have shown in dogs that normocapnic hyperpnea with O(2) further accelerates CO elimination. The purpose of this study was to examine the relation between minute ventilation (V E) and the rate of elimination of CO in humans. Seven healthy male volunteers were exposed to CO (400 to 1,000 ppm) in air until their carboxyhemoglobin (COHb) levels reached 10 to 12%. They then breathed either 100% O(2) at resting V E (4.3 to 9.0 L min) for 60 min or O(2) containing 4.5 to 4.8% CO(2) (to maintain normocapnia) at two to six times resting V E for 90 min. The half-time of the decrease in COHb fell from 78 +/- 24 min (mean +/- SD) during resting V E with 100% O(2) to 31 +/- 6 min (p < 0. 001) during normocapnic hyperpnea with O(2). The relation between V E and the half-time of COHb reduction approximated a rectangular hyperbola. Because both the method and circuit are simple, this approach may enhance the first-aid treatment of CO poisoning.

Isocapnic hyperpnea accelerates carbon monoxide elimination

A major impediment to the use of hyperpnea in the treatment of CO poisoning is the development of hypocapnia or discomfort of CO2 inhalation. We examined the effect of nonrebreathing isocapnic hyperpnea on the rate of decrease of carboxyhemoglobin levels (COHb) in five pentobarbital-anesthetized ventilated dogs first exposed to CO and then ventilated with room air at normocapnia (control). They were then ventilated with 100% O2 at control ventilation, and at six times control ventilation without hypocapnia ("isocapnic hyperpnea") for at least 42 min at each ventilator setting. We measured blood gases and COHb. At control ventilation, the half-time for elimination of COHb (t1/2) was 212 +/- 17 min (mean +/- SD) on room air and 42 +/- 3 min on 100% O2. The t1/2 decreased to 18 +/- 2 min (p < 0.0005) during isocapnic hyperpnea. In two similarly prepared dogs treated with hyperbaric O2, the t1/2 were 20 and 28 min. We conclude that isocapnic hyperpnea more than doubles the rate of COHb elimination induced by normal ventilation with 100% O2. Isocapnic hyperpnea could improve the efficacy of the standard treatment of CO poisoning, 100% O2 at atmospheric or increased pressures.

The effect of exposure duration on the blood level of glucose, pyruvate and lactate in acute carbon monoxide intoxication in man

Blood glucose, pyruvate and lactate were examined during hospitalization of 13 cases of acute carbon monoxide poisoning developing from short (less than or equal to 1.5 h) or long (10-14 h) exposure. CO intoxication resulted in increased blood levels of all carbohydrate metabolites studied. Increased levels of pyruvate and lactate were much more pronounced and lasted for a longer time following long, compared with short, CO exposure despite similar blood COHb level (about 40% at the beginning of hospitalization). The results showed difference in biochemical effect of short and long single exposure to CO that could not be detected by the measurement of COHb.

Lack of the correlation between biochemical effects on rats and blood carboxyhemoglobin concentrations in various conditions of single acute exposure to carbon monoxide

The relationship between conditions of exposure to carbon monoxide (CO) and biochemical effects was investigated in experiments on rats. The magnitude and the time of biochemical disturbances in the tissues resulting from two different exposures consisting of 1 Vol.-% CO for 4 min and 0.4 Vol.-% CO for 40 min respectively were compared. In both cases, at the end of exposure the same level of blood carboxyhemoglobin (COHb) (about 50%) was reached. The biochemical determinations in the blood (pH, glucose, lactate, pyruvate) and brain tissue (lactate, pyruvate) were carried out immediately after termination of the exposure and after the time periods of restitution. CO exposure resulted in a decreased blood pH, increased level of blood glucose, as well as that of lactate and pyruvate both in blood and brain tissue. These changes were much more pronounced following the "longer-lesser" exposure than after the "shorter-intense" one, although blood concentrations of COHb was the same. The observed phenomenon puts some light on the frequently encountered lack of the correlation between COHb level in blood and severity of CO intoxication in clinical practice.

Use of a one-man, mobile pressure chamber in the treatment of carbon monoxide poisoning

For the past five years a mobile pressure chamber has been used to treat patients suffering from severe carbon monoxide poisoning with oxygen at 2 atmospheres pressure. Of the 25 patients treated, 20 recovered completely and only three died. This apparatus could also be used for hyperbaric oxygen therapy in other conditions for which it is indicated.