Effects of pulmonary gas embolism on circulation and respiration in the dog. IV. Origin of arterial hypoxemia during pulmonary gas embolism

Venous air embolism during surgery, especially cesarean delivery

Venous air embolism (VAE) is the entrapment of air or medical gases into the venous system causing symptoms and signs of pulmonary vessel obstruction. The incidence of VAE during cesarean delivery ranges from 10 to 97% depending on surgical position or diagnostic tools, with a potential for life-threatening events. We reviewed extensive literatures regarding VAE in detail and herein described VAE during surgery including cesarean delivery from background and history to treatment and prevention. It is intended that present work will improve the understanding of VAE during surgery.

Venous air embolism during transurethral resection of the prostate

Venous air embolism during transurethral surgery is a rare event. There have been case reports in the anesthesia and urology literature of fatal air embolism during transurethral prostate resection and transurethral incision of the bladder neck. We present a case of nonfatal venous air embolism during transurethral prostate resection in which incorrect assembly of the bladder irrigation-resectoscope-drain system led to a rapid entrainment of air into the open venous channels of the prostate bed.

Venous air embolism: a review

Venous air embolism (VAE) can be a lethal complication of surgical procedures, during which (1) venous pressure at the site of surgery is subatmospheric or (2) gas is forced under pressure into a body cavity. Though classically associated with neurosurgery, VAE is also a potential complication of laparoscopic, pelvic, and orthopedic procedures. It is, therefore, essential for the practicing anesthesiologist to recognize and treat venous air entrainment. An in-depth review of the pathophysiology, clinical presentation, detection, prevention, and treatment of VAE is presented.

Gas Embolism: Part I. Venous Gas Emboli

Gas emboli syndromes occur in many different settings, and their medical significance ranges from being life-threatening emergencies to being totally innocuous. We discuss venous gas embolization in Part I of this review, and it can result from a variety of traumatic, diagnostic, therapeutic, and surgical interventions. The pathophysiological consequences depend on where the gas bubbles impact and obstruct the circulation—by creating an “air lock” in the right ventricle, by obstruction of pulmonary arterioles, and sometimes with passage into the arterial circulation (so called paradoxical emboli). Various monitoring techniques are available and are known to be useful in high-risk patients. Nevertheless, the diagnosis can be difficult to establish. Myriad and generally nonspecific clinical manifestations may be present; the patient may often exhibit signs and symptoms suggestive of other acute cardiopulmonary or central nervous system events. The classically described “mill-wheel murmur” is actually a rare finding, and it is transient at best. There are no specific diagnostic tests available, and clinicians, must depend on a high level of suspicion in the appropriate settings. Rapid identification of the problem, with prevention of further gas entry into the venous circulation, should be a routine measure. The left lateral decubitus position, administration of 100% oxygen, and hyperbaric oxygenation should all be considered, and they have been shown to be effective treatment modalities.

Detection of venous air embolism by continuous intraarterial oxygen monitoring

In a recent study, we compared a new intraarterial fiberoptic "optode" probe to continuously measure arterial oxygen and carbon dioxide tensions and pH with intermittently drawn blood samples in patients undergoing surgery. In one patient with a diagnosis of Arnold-Chiari type I malformation with outflow obstruction of the fourth ventricle, a major pulmonary air embolism occurred while the patient was undergoing suboccipital craniectomy and cervical laminectomy in the prone position. Three hours after the incision the optode-displayed oxygen tension decreased from a stable value of 225 +/- 8 mm Hg to 63 mm Hg over a 10-minute period. During the same interval, carbon dioxide tension increased and end-tidal carbon dioxide decreased; shortly thereafter, transcutaneous oxygen tension decreased also. Within 20 minutes after the inspired gas mixture was changed to 100% oxygen, the patient's respiratory variables returned to near baseline. No further complications ensued. This is the first time continuously monitored arterial oxygen tension values during a pulmonary embolism have been reported. With further refinement, intraarterial optode probes will add another valuable method of detecting pulmonary air embolism.

Hypoxaemia following sustained low-volume venous air embolism in sheep

In six upright (head above thorax) anaesthetised sheep, serial blood gas measurements were made over a 100-minute period during which repeated small-volume air emboli were injected intravenously to lower and maintain the end-tidal CO2 concentration approximately 0.5% below its initial baseline level. With constant volume ventilation and an inspired N2O:O2 ratio of 2:1, the arterial PCO2 progressively increased and the arterial PO2 progressively decreased with significant arterial hypoxaemia ensuing in three out of the six animals. It is suggested that during neurosurgery performed in the sitting position and with an inspired oxygen concentration of 33%, the degree of cardio-respiratory disturbance caused by venous air embolism should be assessed by continuous monitoring not only of end-tidal CO2 concentration but also of arterial oxygen saturation using pulse oximetry.

Observations on the mechanism of hypoxaemia in acute minor pulmonary embolism

An automated computer analysis of ventilation-perfusion lung scans was used to derive graphical data from lung scans of 11 patients with acute minor pulmonary embolism, free of pre-existing cardiorespiratory disease, and with no evidence of intrapulmonary complication or pleural effusion. In each case the analysis showed the presence of areas of lung, remote from those affected by the pulmonary embolism, that had a pathological disturbance of ventilation-perfusion matching with relative overperfusion. Such a disturbance would cause hypoxaemia. When the extent of the mismatching was calculated in terms of relative blood flow and alveolar ventilation it correlated well with the degree of arterial hypoxaemia. It is proposed that in acute minor pulmonary embolism the development of ventilation-perfusion mismatching in areas of lung unaffected by the embolic event may be an important mechanism of hypoxaemia.

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.

Gas exchange abnormalities produced by venous gas emboli

Bubbles of either He, N2 or SF6 were infused intravenously into anesthetized dogs at a rate of 0.2 ml/kg/min. Alterations in pulmonary gas exchange were quantitated by the invert gas elimination method during control, steady state infusion and resolution phases. The hypoxemia produced was predominantly due to regions of low VA/Q rather than pure shunt, and the increase in physiological dead space was due to the addition of high VA/Q regions rather than 'pure' dead space. The VA/Q distribution returned to normal within 30 minutes of stopping the He or N2 bubbles, but remained abnormal for longer periods with SF6 bubbles. The net elimination of insoluble gases (such as He or N2) was only slightly impaired by bubble emboli, provided the cardiac output remained constant. Early pulmonary edema from bubble embolization was documented by increased wet weight/dry weight ratio, but the increased lung water was not apparent on histological examination. This form of pulmonary embolus is unique in that there is a constant fraction of the vasculature blocked although any given region with embolus is undergoing continuous resportion of the bubble. This produced a partial obstruction of the affected gas exchange units which manifests as regions of high VA/Q rather than pure dead space.

Effects of pulmonary gas embolism on circulation and respiration in the dog. V. Effect of changing breathing gas composition on pulmonary gas embolization

It was found in previous investigations that during venous gas infusion at low rates (1--5 ml/min-1) circulatory and respiratory variables reached a constant level after about 10--15 min. The present study demonstrates that this steady state can be disturbed by changing the composition of the breathing gas mixture. Alteration from air to 21% O2 in helium rapidly increased the embolic effects up to a maximum within 1.5--2 min; in the next 5--8 min the circulatory and respiratory variables returned to their previous levels during air breathing. Reverse effects occurred when changing from 21% O2 in helium to air. Similar phenomena were seen after switching from air to pure oxygen and from 21% O2 helium to pure oxygen. However, the extent of the circulatory and respiratory changes differed depending on the composition of the respective alternating breathing gas mixtures and on the initial embolic level as determined by infusion rate and kind of infusion gas. Gas movements between intravascular bubbles and alveolar space might be responsible for these changes.

Effects of pulmonary gas emoblism on circulation and respiration in the dog. VI. Influence of body position on the effects of pulmonary gas embolism

In the present study the influence of body position on the effects of venous gas infusion was examined. The body position of anesthetized dogs ventilated artifically varied between supine, right-side-down and left-side-down position. Without venous gas infusion the change of body position hardly affected pulmonary arterial pressure (Pap) and alveolar CO2 fractional concentration (FACO2). However, during venous gas infusion a change of body position immediately elicited a rapid increase Pap and a decrease in FACO2 completed within a few seconds. Thereafter both variables gradually returned to their initial levels as before the change of body position. The extent of change in Pap and FACO2 due to alteration of body position depended on the initial embolic level (determined by the rate of infusion and the nature of the gas used) and on the change of body position.