Cardiorespiratory effects of large pneumothorax in conscious and anesthetized dogs

Heart-lung interactions during mechanical ventilation: the basics

The hemodynamic effects of mechanical ventilation can be grouped into three clinically relevant concepts. First, since spontaneous ventilation is exercise. In patients increased work of breathing, initiation of mechanical ventilatory support may improve O₂ delivery because the work of breathing is reduced. Second, changes in lung volume alter autonomic tone, pulmonary vascular resistance, and at high lung volumes compress the heart in the cardiac fossa similarly to cardiac tamponade. As lung volume increases so does the pressure difference between airway and pleural pressure. When this pressure difference exceeds pulmonary artery pressure, pulmonary vessels collapse as they pass form the pulmonary arteries into the alveolar space increasing pulmonary vascular resistance. Hyperinflation increases pulmonary vascular resistance impeding right ventricular ejection. Anything that over distends lung units will increase their vascular resistance, and if occurring globally throughout the lung, increase pulmonary vascular resistance. Decreases in end-expiratory lung volume cause alveolar collapse increases pulmonary vasomotor tone by the process of hypoxic pulmonary vasoconstriction. Recruitment maneuvers that restore alveolar oxygenation without over distention will reduce pulmonary artery pressure. Third, positive-pressure ventilation increases intrathoracic pressure. Since diaphragmatic descent increases intra-abdominal pressure, the decrease in the pressure gradient for venous return is less than would otherwise occur if the only change were an increase in right atrial pressure. However, in hypovolemic states, it can induce profound decreases in venous return. Increases in intrathoracic pressure decreases left ventricular afterload and will augment left ventricular ejection. In patients with hypervolemic heart failure, this afterload reducing effect can result in improved left ventricular ejection, increased cardiac output and reduced myocardial O₂ demand. This brief review will focus primarily on mechanical ventilation and intrathoracic pressure as they affect right and left ventricular function and cardiac output.

Congenital Lobar Emphysema and Tension Pneumothorax in a Dog

Congenital lobar emphysema (CLE) and tension pneumothorax (TPT) are rarely reported in dogs. A case of CLE of the right middle lung lobe predisposing to air trapping, alveolar hyperinflation, and pleural rupture resulting in fatal spontaneous TPT in a 6-month-old mixed breed dog is described. The unique alteration of “bloat line” was observed in this case in addition to compressive atelectasis of all other lung lobes and lack of negative pressure within the thoracic cavity, signifying markedly elevated intrathoracic pressure. Bronchial cartilage hypoplasia and bronchiectasis were confirmed microscopically, which likely led to abnormal dynamic collapse of bronchi during expiration, consequentially leading to increased intrapulmonary pressure, bullous emphysema, and pleural rupture resulting in TPT. TPT consequent to CLE may therefore be considered one of the potential causes of sudden death in young dogs without overt clinical illness.

Clinical manifestations of tension pneumothorax: protocol for a systematic review and meta-analysis

BACKGROUND

Although health care providers utilize classically described signs and symptoms to diagnose tension pneumothorax, available literature sources differ in their descriptions of its clinical manifestations. Moreover, while the clinical manifestations of tension pneumothorax have been suggested to differ among subjects of varying respiratory status, it remains unknown if these differences are supported by clinical evidence. Thus, the primary objective of this study is to systematically describe and contrast the clinical manifestations of tension pneumothorax among patients receiving positive pressure ventilation versus those who are breathing unassisted.

METHODS/DESIGN

We will search electronic bibliographic databases (MEDLINE, PubMed, EMBASE, and the Cochrane Database of Systematic Reviews) and clinical trial registries from their first available date as well as personal files, identified review articles, and included article bibliographies. Two investigators will independently screen identified article titles and abstracts and select observational (cohort, case-control, and cross-sectional) studies and case reports and series that report original data on clinical manifestations of tension pneumothorax. These investigators will also independently assess risk of bias and extract data. Identified data on the clinical manifestations of tension pneumothorax will be stratified according to whether adult or pediatric study patients were receiving positive pressure ventilation or were breathing unassisted, as well as whether the two investigators independently agreed that the clinical condition of the study patient(s) aligned with a previously published tension pneumothorax working definition. These data will then be summarized using a formal narrative synthesis alongside a meta-analysis of observational studies and then case reports and series where possible. Pooled or combined estimates of the occurrence rate of clinical manifestations will be calculated using random effects models (for observational studies) and generalized estimating equations adjusted for reported potential confounding factors (for case reports and series).

DISCUSSION

This study will compile the world literature on tension pneumothorax and provide the first systematic description of the clinical manifestations of the disorder according to presenting patient respiratory status. It will also demonstrate a series of methods that may be used to address difficulties likely to be encountered during the conduct of a meta-analysis of data contained in published case reports and series. PROSPERO registration number: CRD42013005826.

Vented versus unvented chest seals for treatment of pneumothorax and prevention of tension pneumothorax in a swine model

BACKGROUND

Unvented chest seals (CSs) are currently recommended for the management of penetrating thoracic injuries in the battlefield. Since no supporting data exist, we compared the efficacy of a preferred unvented with that of a vented CS in a novel swine model of pneumothorax (PTx).

METHODS

An open chest wound was created in the left thorax of spontaneously air-breathing anesthetized pigs (n = 8). A CS was applied over the injury, then tension PTx was induced by incremental air injections (0.2 L) into the pleural cavity via a cannula that was also used to measure intrapleural pressure (IP). Both CS were tested on each pig in series. Tidal volume (V(T)), respiratory rate, IP, heart rate, mean arterial pressure, cardiac output, central venous pressure, pulmonary arterial pressure, venous and peripheral oxygen saturations (SvO2, SpO2) were recorded. Tension PTx was defined as a mean IP equal to or greater than +1 mm Hg plus significant (20-30%) deviation in baseline levels of the previously mentioned parameters and confirmed by chest x-ray study. PaO2 and PaCo2 were also measured.

RESULTS

PTx produced immediate breathing difficulty and significant rises in IP and pulmonary arterial pressure and falls in V(T), SpO2, and SvO2. Both CSs returned these parameters to near baseline within 5 minutes of application. After vented CS was applied, serial air injections up to 2 L resulted in no significant change in the previously mentioned parameters. After unvented CS application, progressive deterioration of all respiratory parameters and onset of tension PTx were observed in all subjects after approximately 1.4-L air injection.

CONCLUSION

Both vented and unvented CSs provided immediate improvements in breathing and blood oxygenation in our model of penetrating thoracic trauma. However, in the presence of ongoing intrapleural air accumulation, the unvented CS led to tension PTx, hypoxemia, and possible respiratory arrest, while the vented CS prevented these outcomes.

Tension pneumothorax--time for a re-think?

This review examines the present understanding of tension pneumothorax and produces recommendations for improving the diagnostic and treatment decision process.

Is mediastinal shift on chest X-ray of pneumothorax always an emergency?

OBJECTIVE

To determine the incidence of mediastinal shift on chest X-ray due to pneumothorax.

METHODS

A retrospective chart review was undertaken of all patients with pneumothorax presenting to the ED over the period 1 January 1995 to 31 December 1999. The primary outcome was mediastinal shift on initial CXR. The incidence of clinical tension pneumothorax was noted.

RESULTS

There were 176 presentations with pneumothorax in the study period. Two cases of clinical tension pneumothorax were identified and treated prior to CXR. Thirty patients with mediastinal shift on initial CXR, none of which clinically merited emergency needle decompression, were all managed with intercostal catheter (ICC) insertion. Overall, 141 of 176 (80.1%) had an ICC inserted as part of their management. Mean pulse rate (91.8 SD 29.5 vs 86.7 SD 23.6, P = 0.02) and respiratory rate (21.9 SD 14.4 vs 15.1 SD 11.5, P = 0.03) were greater in patients with mediastinal shift on CXR.

CONCLUSION

True clinical tension pneumothorax is an uncommon condition. Radiological evidence of mediastinal shift is more common. No patient in this latter group deteriorated while awaiting X-ray.

Pleural Disease in the Intensive Care Unit

Pleural disease itself is an unusual cause for admission to the intensive care unit (ICU). Pleural complications of diseases and procedures in the ICU are common, however, and the impact on respiratory physiology is additive to that of the underlying cardiopulmonary disease. Pleural effusion and pneumothorax may be overlooked in the critically ill patient due to alterations in radiologic appearance in the supine patient. The development of a pneumothorax in a patient in the ICU represents a potentially life-threatening situation. This article reviews the etiologies, pathophysiology, and management of pleural effusion, pneumothorax, tension pneumothorax, and bronchopleural fistula in the critically ill patient. In addition, we review the potential complications of thoracentesis and chest tube thoracostomy, including re-expansion pulmonary edema.

Tension pneumothorax

The diagnosis of tension pneumothorax has typically been taught as the presence of hemodynamic compromise with an expanding intrapleural space air mass. This may occur quickly or gradually, depending on the degree of lung injury and respiratory state of the patient. Experimentally, tension pneumothorax is a multifactorial event that manifests a state of central hypoxemia, compensatory mechanisms, and mechanical compression on intrathoracic structures. Studies using animal models suggest that over hypotension is a delayed finding that immediately precedes cardiorespiratory collapse. Recognition of early signs and symptoms associated with tension pneumothorax, e.g., progressive hypoxemia, tachycardia, and respiratory distress, can alert medical personnel to the need for rapid decompression before physiologic decompensation.

The pathophysiology of tension pneumothorax in ventilated swine

It remains unclear as to whether the cardiovascular collapse observed in tension pneumothorax (TP) is strictly a mechanical pressure-related phenomenon or secondary to hypoxemia. This study describes the pathophysiologic changes associated with a surgically induced progressive TP in a ventilated swine model. With a balloon occlusion catheter surgically placed into the pleural space, progressive volumes of pneumothorax were created in six anesthetized pigs on positive-pressure ventilation. Air was introduced into the right hemithorax in 100-mL increments every 4-5 min, with measurements of heart rate (HR), mean arterial pressure (MAP), central venous pressure (CVP), mean intrapleural pressure (MIP), oxygen saturation (O2%), arterial blood gas (ABG), and cardiac output (C.O.). With the induced progressive TP, results showed that O2% measures decreased immediately and continued to decline throughout the experiment to levels below 50% prior to cardiovascular collapse. The MAP and HR remained relatively stable until approximately 57% total lung capacity progressive TP (600 mL) was reached. At this point, a significant decline in MAP and increase in HR was noted, indicating tension physiology. The C.O. showed a small but significant decrease after 200 mL of air was injected, with a progressive decline after this point. At > 97% total lung capacity TP, lethal cardiovascular collapse occurred in all animals and was associated with an abrupt drop in C.O., HR, and MAP. There was a concurrent equalization of MIP with CVP at the point of collapse. Arterial blood gas measures correlated with O2% trends during the trials. We conclude that the findings of this study support the alternative hypothesis that significant hypoxemia occurs early and precedes hypotension in ventilated animals with TP. Occlusive mechanical compression, suggested by equalization of MIP and CVP, is probably a late event.

Changes in bronchial and pulmonary arterial blood flow with progressive tension pneumothorax

We studied the effects of unilateral tension pneumothorax and its release on bronchial and pulmonary arterial blood flow and gas exchange in 10 adult anesthetized and mechanically ventilated sheep with chronically implanted ultrasonic flow probes. Right pleural pressure (Ppl) was increased in two steps from -5 to 10 and 25 cmH2O and then decreased to 10 and -5 cmH2O. Each level of Ppl was maintained for 5 min. Bronchial blood flow, right and left pulmonary arterial flows, cardiac output (QT), hemodynamic measurements, and arterial blood gases were obtained at the end of each period. Pneumothorax resulted in a 66% decrease in QT, bronchial blood flow decreased by 84%, and right pulmonary arterial flow decreased by 80% at Ppl of 25 cmH2O (P < 0.001). At peak Ppl, the majority of QT was due to blood flow through the left pulmonary artery. With resolution of pneumothorax, hemodynamic parameters normalized, although abnormalities in gas exchange persisted for 60-90 min after recovery and were associated with a decrease in total respiratory compliance.

Anesthesia for patients with diaphragmatic hernia and severe dyspnea

Problems facing a patient with severe dyspnea secondary to diaphragmatic herniation are hypoxia, hypercarbia and respiratory acidosis, and cardiovascular instability. It is easy to precipitate a crisis in these patients during anesthetic induction as a result of stress, bad positioning, induction of pneumothorax, or inappropriate anesthetic technique. These patients require a smooth, stress-free perianesthetic period with preoxygenation, positioning with the affected side down, rapid intravenous induction, endotracheal intubation, and mechanical ventilation. Maintenance with isoflurane is preferred, and nitrous oxide should be avoided. Close monitoring of the cardiovascular and pulmonary systems is essential. Recovery from anesthesia should include oxygen supplementation, pleural drainage, and local analgesia if required.

Pneumothorax

This article reviews the classification, etiopathogenesis, and treatment for the various forms of pneumothorax. Traumatic and nontraumatic pneumothoraces are discussed. New theories on the etiology and treatment of primary spontaneous and secondary pneumothorax are mentioned.

Pulmonary and hemodynamic function in dogs during exercise: effects of lung autotransplantation

An airtight facemask/mouthpiece assembly has been devised to facilitate the performance of a wide range of pulmonary function tests during treadmill exercise in dogs. Using this appliance, data were obtained from 6 normal dogs and 3 of the same dogs after left lung autotransplantation. All measurements were made during awake, resting conditions and again after 5-7 minutes of moderate exercise. Resting values for pulmonary function tests, hemodynamic parameters, blood gases and pH from both pulmonary and systemic arterial blood samples did not differ significantly between normal dogs and those studied after left lung autotransplantation. During treadmill exercise, cardiac output doubled and pulmonary vascular resistance decreased comparably in both groups of dogs. Heart rates in both groups rose to approximately 22 b/min and blood gases and pH remained normal. These results indicate the facemask/mouthpiece assembly permits normal ventilation during treadmill exercise. In addition, these data support the view that pulmonary autotransplantation per se need not impose obligatory defects in ventilatory and hemodynamic function despite the increased demands of treadmill exercise.

Thoracic trauma

The physiologic equilibrium of chest injury patients is frequently precarious, and mild stress during examination and treatment may precipitate acute decompensation and death. This is particularly true with the respiratory system, where the normally large respiratory reserve capacity may be rapidly lost. Accurate assessment of the nature of the thoracic injury and the severity of that injury must be determined in order to formulate a therapeutic plan. Many thoracic injuries, such as pneumothorax, pulmonary contusions, or rib fractures, will be self-limiting. Other conditions must be recognized for their potentially lethal nature and dealt with aggressively, and these include cardiac tamponade, tension pneumothorax, and esophageal perforation. By performing a systematic evaluation of the patient and confirming or denying the presence of all possible types of thoracic injury, the veterinarian may avoid overtreatment of self-limiting lesions and recognize and aggressively treat those with potentially fatal outcomes.

Effects of breathing 10 per cent carbon dioxide on the pulmonary circulation of human subjects

The effects on the pulmonary circulation of breathing 10% CO 2 for 10 to 20 min were studied in five eucapnic and 11 convalescing hypercapnic patients to recreate the CO 2 tensions which they had experienced during respiratory failure. Right heart catheterization permitted measurements of pulmonary arterial and wedge pressures and obtaining samples of mixed venous blood. Breathing CO 2 increased mean pulmonary arterial pressures from 33 to 50 mm Hg (52%); pulmonary arterial wedge pressures were unchanged, and cardiac output increased only 22%. In three hypercapnic and two eucapnic subjects reduction of blood hydrogen ion levels by rapid infusion of 120 to 135 mEq NaHCO 3 during CO 2 breathing did not lower pulmonary arterial pressure significantly, nor raise cardiac output. Neither vascular pressures nor cardiac outputs changed during oxygen breathing. Larger increases in cardiac output, which were produced by exercise, raised pulmonary artery pressure only half as much as breathing 10% CO 2 did. Therefore, pulmonary vascular resistance (PVR) that was elevated to an average of 4.8 mm Hg/L/min at rest while breathing air was increased to 6.5 mm Hg/L/min during CO 2 breathing ( P <0.01). In contrast, PVR was unchanged (4.6 mm Hg/L/min) during exercise. This difference between PVR while breathing CO 2 and during exercise was statistically significant ( P <0.02). Restoration of the pH of the blood toward normal by the infusion of bicarbonate during breathing of CO 2 raised the pulmonary arterial pressure. The five eucapnic subjects showed similar changes during CO 2 breathing and exercise, although their base-line values were significantly different. The differences were due, at least in part, to lower Paco 2 . This evidence suggests that CO 2 acts on pulmonary arterioles and capillaries that are exposed to alveolar gases to increase the pulmonary vascular impedance.