Respiratory muscle fatigue

The contribution of respiratory muscle fatigue to the development of ventilatory failure has been the subject of considerable interest and has stimulated much research. Experimental studies in dogs have shown respiratory muscle fatigue to be a cause of ventilatory failure in both cardiogenic and septic shock models. In clinical conditions resulting in acute or chronic hypercapnia, respiratory muscle fatigue is believed to occur; however, the specific role of fatigue has been difficult to prove.

Severe diaphragmatic necrosis in 4 horses with degenerative myopathy

Severe diaphragmatic necrosis occurred in horses with degenerative myopathy due to polysaccharide storage myopathy (n = 2), nutritional myopathy (n = 1), and vasculitis (n = 1). Blood gas analysis performed in 1 horse indicated development of respiratory acidosis. Respiratory muscle necrosis can be severe in horses with degenerative myopathy and can lead to respiratory failure.

Glycogen branching enzyme deficiency in quarter horse foals

Seven related Quarter Horse foals that died by 7 weeks of age were examined for glycogen branching enzyme (GBE) deficiency. Clinical signs varied from stillbirth, transient flexural limb deformities, seizures, and respiratory or cardiac failure to persistent recumbency. Leukopenia (5 of 5 foals) as well as high serum creatine kinase (CK; 5 of 5), aspartate transaminase (AST; 4 of 4), and gamma glutamyl transferase (GGT; 5 of 5) activities were present in most foals, and intermittent hypoglycemia was present in 2 foals. Gross postmortem lesions were minor, except for pulmonary edema in 2 foals. Muscle, heart, or liver samples from the foals contained abnormal periodic acid Schiff's (PAS)-positive globular or crystalline intracellular inclusions in amounts proportional to the foal's age at death. Accumulation of an unbranched polysaccharide in tissues was suggested by a shift in the iodine absorption spectra of polysaccharide isolated from the liver and muscle of affected foals. Skeletal muscle total polysaccharide concentrations were reduced by 30%, but liver and cardiac muscle glycogen concentrations were normal. Several glycolytic enzyme activities were normal, whereas GBE activity was virtually absent in cardiac and skeletal muscle, as well as in liver and peripheral blood cells of affected foals. GBE activities in peripheral blood cells of dams of affected foals and several of their half-siblings or full siblings were approximately 50% of controls. GBE protein in liver determined by Western blot was markedly reduced to absent in affected foals, and in a half-sibling of an affected foal, it was approximately one-half the amount of normal controls. Pedigree analysis also supported an autosomal recessive mode of inheritance. The affected foals have at least 2,600 half-siblings. Consequently, GBE deficiency may be a common cause of neonatal mortality in Quarter Horses that is obscured by the variety of clinical signs that resemble other equine neonatal diseases.

Dissecting aneurysm of the carotid artery as a cause of respiratory distress in adult cattle

A two-and-a-half-year-old Friesian cow and a five-year-old Charolais cow developed severe respiratory distress and palpable swellings to the left of the larynx as a result of a dissecting aneurysm of the common carotid artery. Neither cow responded to medical treatment. The underlying pathogenesis of the condition was uncertain, but direct trauma to the carotid artery was a possible contributory factor. Aneurysms of the common carotid artery should be considered when swelling occurs in the region of the larynx or when respiratory distress is due to laryngeal compression.

Acute respiratory failure caused by leukaemic infiltration of the lung of a dog

A seven-year-old crossbred male dog with a suspected leukaemic condition was referred for investigation and treatment. A bone marrow aspirate revealed an acute myeloid leukaemia. Combination chemotherapy was administered and the dog initially improved, but 18 days after the initiation of therapy its body condition deteriorated and the animal developed acute respiratory distress. On postmortem examination, extensive leukaemic pulmonary infiltrates were evident.

Fatal pulmonary fat embolism during humeral fracture repair in a cat

An 11-month-old cat was referred following a road traffic accident with non-weightbearing forelimb lameness and tachypnoea. A diaphragmatic rupture was repaired two days after trauma. Six days post trauma, a further operation was performed to repair the fractured humerus. As an intramedullary pin was advanced into the bone to achieve stabilisation, acute respiratory arrest occurred. Manual extrathoracic cardiopulmonary resuscitation was immediately instigated, but the cat did not recover. Postmortem examination identified a high degree of embolic fat occluding pulmonary capillaries. A massive embolic shower was established as the cause of death.

Strategies for mechanical ventilation

With the advancement of veterinary critical care medicine, an increasing number of veterinary patients are being supported with positive-pressure ventilation. Animals with potentially reversible ventilatory failure (PaCO2 > 60 mmHg) caused by neuromuscular disease or pulmonary parenchymal disease or with pulmonary parenchymal disease causing hypoxemia (PaO2 < 60) despite supplemental oxygen are candidates for ventilatory support. The equation of motion for the respiratory system is defined and is used to describe the potential interactions between the patient and the ventilator. Commonly used modes of ventilation are described in terms of control and phase variables. The intent of this report is to aid clinicians in choosing an optimal ventilatory strategy for each patient that will best achieve the desired physiologic goals with minimal detrimental side effects.

Weaning from mechanical ventilation

Patients that require positive pressure ventilation to maintain sufficient alveolar ventilation or pulmonary gas exchange may eventually reach a point in the course of their care wherein mechanical ventilation is no longer necessary. This process of transferring the work of breathing from the ventilator back to the patient is referred to as ventilator weaning. The term "ventilator weaning" may be used to refer to all methods by which this transfer of workload may be accomplished. In many patients, particularly those with short-lasting or readily correctable causes of respiratory insufficiency (e.g., general anesthesia), the discontinuation of positive pressure ventilation may be easily achieved. Indeed, in patients awakening from general anesthesia, the axiom "awake enough to blink, awake enough to breath" may prove to be a sufficient guideline. However, in those patients requiring long-term mechanical ventilatory support, the process can prove to be both frustrating and exceptionally challenging. It is of crucial importance to identify those patients that may be successfully weaned because of both the financial impact of prolonged intensive care unit hospitalization and the risks imposed on the patient by the process of positive pressure ventilation. To be able to predict which patients may be ready to be weaned from the ventilator requires an understanding of the balance between the work of breathing (ventilatory load) and the ability of the patient's respiratory pump to meet those needs (ventilatory capacity). The management of patients experiencing difficulty during the weaning process requires that the clinician recognize imbalances between ventilatory load and capacity and to correct these imbalances once identified.

Risk of adverse effects in pneumonic foals treated with erythromycin versus other antibiotics: 143 cases (1986-1996)

OBJECTIVE

To determine whether foals with pneumonia that were treated with erythromycin, alone or in combination with rifampin or gentamicin, had a higher risk of developing adverse effects, compared with foals treated with trimethoprim-sulfamethoxazole (TMS), penicillin G procaine (PGP), or a combination of TMS and PGP (control foals).

DESIGN

Retrospective study.

ANIMALS

143 foals < 240 days old.

PROCEDURE

Information on age, sex, breed, primary drug treatment, total days of treatment with the primary drug, and whether the foal developed diarrhea, hyperthermia, or respiratory distress was obtained from the medical records. Relative risk (RR) and attributable risk (AR) were calculated to compare risk of adverse reactions between foals treated with erythromycin and control foals.

RESULTS

Only 3 (4.3%) control foals developed diarrhea; none developed hyperthermia or respiratory distress. Foals treated with erythromycin had an 8-fold risk (RR, 8.3) of developing diarrhea, compared with control foals, and increased risks of hyperthermia (AR, 25%) and respiratory distress (AR, 15%).

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggest that use of erythromycin to treat foals with pneumonia was associated with an increased risk of diarrhea, hyperthermia, and respiratory distress, compared with use of TMS or PGP.

Approach to the patient in respiratory distress

Respiratory distress is a very common presenting complaint in emergency practice. It is essential that the clinician rapidly determine the underlying cause of the clinical signs using physical examination findings and nonstressful diagnostic tests. Oxygen therapy will often stabilize a patient, allowing for a more complete physical examination and diagnostics, including thoracocentesis, thoracic radiographs, and blood collection for laboratory analysis. The disease processes that cause respiratory distress can be grouped according to anatomic location: the airways, pulmonary parenchyma, pleural space, or thoracic wall. The choice of diagnostic and therapeutic techniques will be dependent on the suspected anatomic origin of disease. Techniques useful in diagnosing airway disorders include oral examination, cervical and thoracic radiographs, fluoroscopy, and bronchoscopy. Therapeutic techniques include intubation and tracheostomy. For parenchymal disease, thoracic radiographs, echocardiography, ultrasound of the thorax, and transtracheal or endotracheal wash can be useful. When the disease process is in the pleural space, thoracocentesis can be both diagnostic and therapeutic. Chest tube placement may be necessary for continuous removal of air or fluid from the pleural space. Monitoring of the respiratory patient can involve serial physical examination, pulse oximetry, and arterial blood gas analysis. It is essential to minimize stress on patients with respiratory distress because decompensation can occur easily, leading to respiratory arrest.

Postoperative management of the emergency surgery small animal patient

Hypovolemia, hypothermia, and hypotension are common postoperative findings that predispose the critically ill patient to secondary complications. This patient population is especially vulnerable to sepsis, hypoxia, and immune dysfunction. Careful monitoring is essential for early recognition of potentially life-threatening physiologic derangements. Early and aggressive intervention may help minimize systemic insult before it progresses to acute respiratory distress syndrome, acute renal failure, disseminated intravascular coagulation, or multiple organ failure.

A krimpsiekte-like syndrome in small stock poisoned by Ornithogalum toxicarium Archer & Archer

Krimpsiekte (the syndrome associated with chronic cardiac glycoside poisoning) was purportedly induced by Ornithlogalum toxicarium in the Karas mountains area of Keetmanshoop, Namibia. This chinkerinchee species was previously linked to a condition known as 'kwylbek' krimpsiekte in small stock in the Beaufort West district of the Western Cape Province, South Africa. In a dosing trial, respiratory distress, tachycardia and sternal recumbency were observed in 2 sheep drenched with fresh plant material. A fluorescence polarisation immunoassay (FPIA) detected the presence of a substance that cross-reacted with digoxin antibodies in some of the plant material collected at Keetmanshoop and Beaufort West. This is the first time that apparent cardiac glycoside poisoning was induced by a southern African chinkerinchee species. The presence of the cardiac glycoside-like substance in O. toxicarium requires further chemical verification.

Respiratory failure attributable to moxidectin intoxication in a dog

A 5-month-old 22-kg (48.4-lb) sexually intact male Collie was examined after ingesting a moxidectin-containing deworming medication. The dog was comatose and had respiratory arrest after progressively worsening lethargy, ataxia, and seizures. Exposure was confirmed by isolation of moxidectin from a biopsy specimen of adipose tissue, using liquid chromatography-mass spectroscopy methods. Treatment included use of intermittent positive-pressure ventilation, activated charcoal and cathartic administered enterally, nutrients administered via nasogastric tube, and intensive supportive care. The dog was weaned from a ventilator on day 6 after ingestion and was discharged on day 10. The dog was considered clinically normal during examination 24 days after ingestion. On the basis of the dog reported here and toxicologic data provided by the manufacturer of the deworming product, some Collies may have increased susceptibility to products containing high doses of moxidectin.

Comparison of laboratory test results before and after surgical removal of heartworms in dogs with vena caval syndrome

OBJECTIVE

To compare results of laboratory tests in dogs with vena caval syndrome before and after surgical removal of heartworms.

DESIGN

Longitudinal uncontrolled clinical trial.

ANIMALS

51 dogs with vena caval syndrome.

PROCEDURE

Heartworms were removed from the area of the tricuspid valve and pulmonary arteries via venotomy and by use of flexible alligator forceps. Blood samples were obtained before and 10 days after removal of heartworms. Red and white blood cell counts were determined, using an automated cell counter. Biochemical tests were performed, using a dry chemical method.

RESULTS

45 dogs survived the procedure, and 6 died or were euthanatized after surgical treatment. After surgery, RBC count and total protein, albumin, calcium, and sodium concentrations increased, and total bilirubin, ammonia, BUN, creatinine, uric acid, and potassium concentrations decreased in dogs that survived. Creatine kinase, L-lactate dehydrogenase, and amylase activities decreased, whereas alkaline phosphatase and gamma-glutamyltransferase activities and total cholesterol concentration increased. Alanine transaminase activity decreased in 27 dogs but increased in 3 dogs. Changes in test results in dogs that did not survive. Significant differences were found in RBC count, gamma-glutamyltransferase activity, and total protein, total cholesterol, BUN, and total bilirubin concentrations before and after removal of heartworms.

CLINICAL IMPLICATIONS

Hepatic and renal functions improve rapidly after surgical removal of heartworms, presumably because general and pulmonary circulation is normalized. However, cholestasis may develop, and dogs that survive may need additional treatment to preserve hepatic function.

Complications of balloon catheterization associated with aberrant cerebral arterial anatomy in a horse with guttural pouch mycosis

A 3-year-old Quarter Horse gelding was treated for left guttural pouch mycosis by ligation and balloon catheterization of the left internal carotid artery. Catheter advancement was shorter (10 cm) than the normally reported distance (13-15 cm), but was observed endoscopically during placement as it coursed within the internal carotid artery through the guttural pouch. The horse developed a persistently abnormal respiratory pattern after catheter placement, failed to gain consciousness, developed pulmonary edema, and died 5.5 hours postoperatively. Postmortem examination revealed an aberrant left internal carotid arterial course with location of the embolectomy catheter at the junction of the basilar and caudal cerebellar arteries. Brainstem neuronal necrosis and alveolar and interstitial pulmonary edema were identified on histological examination. Angiography may be used to identify aberrant branching patterns. Failure to identify and occlude aberrant branches may result in fatal epistaxis and Brainstem lesions.

Associations between physical examination, laboratory, and radiographic findings and outcome and subsequent racing performance of foals with Rhodococcus equi infection: 115 cases (1984-1992)

OBJECTIVE

To determine whether physical examination, laboratory, or radiographic abnormalities in foals with Rhodococcus equi infection were associated with survival, ability to race at least once after recovery, or, for foals that survived and went on to race, subsequent racing performance.

DESIGN

Retrospective study.

ANIMALS

49 Thoroughbreds and 66 Standardbreds admitted to 1 of 6 veterinary teaching hospitals between 1984 and 1992 in which R equi infection was positively diagnosed.

PROCEDURE

Results of physical examination, laboratory testing, and thoracic radiography were reviewed. Indices of racing performance were obtained for foals that recovered and eventually raced and compared with values for the US racing population.

RESULTS

83 (72%) foals survived. Foals that did not survive were more likely to have extreme tachycardia (heart rate > 100 beats/min), be in respiratory distress, and have severe radiographic abnormalities on thoracic radiographs at the time of initial examination than were foals that survived. Clinicopathologic abnormalities were not associated with whether foals did or did not survive. Forty-five of the 83 surviving foals (54%) eventually raced at least once, but none of the factors examined was associated with whether foals went on to race. Racing performance of foals that raced as adults was not significantly different from that of the US racing population.

CLINICAL IMPLICATIONS

R equi infection in foals is associated with a decreased chance of racing as an adult; however, foals that eventually go on to race perform comparably to the US racing population.

Acute fulminating myasthenia gravis in five dogs

Acute fulminating myasthenia gravis (MG) was diagnosed in 5 dogs. Acute fulminating generalized MG in dogs is characterized by sudden onset of megaesophagus and frequent regurgitation of large volumes of fluid. Generalized muscle weakness can worsen and lead to recumbency within days. Despite appropriate supportive care, weakness is not alleviated by rest. Respiratory failure caused by aspiration pneumonia and loss of strength in muscles involved with respiration is a common cause of death. In dogs with acute onset of regurgitation, MG should be considered as a differential diagnosis. Clinicians should be aware of the risk of rapid progression to quadriparesis if aspiration pneumonia develops.

Surfactant protein-B and lung function in surfactant-treated preterm lambs

We asked if the amount of SP-B (range 37-410 micrograms/ml) in surfactants used to treat preterm lambs at 123 days of gestation correlated with postnatal lung function or the SP-B content of surfactant recovered by alveolar washes after 10 h ventilation. Ventilation was initiated using a low tidal volume strategy to minimize early lung injury. There were small increases in compliance for the lambs treated with surfactants containing more than 37 micrograms/ml SP-B and an increased lung volume for lambs treated with a surfactant containing 385 micrograms/ml/ml relative to the surfactant containing 37 micrograms/ml SP-B. The amount of SP-B in the surfactant used for treatment correlated linearly with the amount of SP-B in the surfactant recovered from the lambs (r = 0.95, p < 0.001). Albumin leak from the vasculature to the airspace was low as was total protein in alveolar washes, indicating minimal lung injury. The SP-B content of surfactant (from 37 to 410 micrograms/ml) had minimal effects on postnatal lung function over a 10-hour study period in lambs ventilated in a manner to minimize lung injury.

The relation of ventilatory failure to pulmonary, respiratory muscle and central nervous system disturbances in calves with an experimentally produced pneumonia

To explore the pathophysiology of respiratory failure in an experimental pneumonia, a Pasteurella haemolytica broth culture was injected intratracheally into 12 calves, which were then studied over a period of 10 h. Measurements were made of inspired minute ventilation (VE), ventilatory pattern [inspiratory time (TI), expiratory time (TE), respiratory rate (RR) and tidal volume (VT)], transdiaphragmatic pressure (Pdi), occlusion pressure at the airway opening 100 milliseconds after onset of inspiration (Pawo100ms), arterial blood gas tensions and pH and recorded diaphragmatic electromyogram (EMGdi) and rectal temperature (Tr). On and after the third hour after inoculation, the animals varied in respect of clinical signs, Tr, RR, VE, Pawo100ms/EMGdi, and arterial gases and pH. In benign cases, diminished alertness, laboured respiration and fall of arterial oxygen pressure (PaO2) worsened up to 7 h after inoculation, but then progressively improved, VE being maintained at approximately 150% baseline throughout the study (10 h). Neither arterial carbon dioxide pressure (PaCO2) nor pH was altered. Moderate cases resembled benign cases in respect of laboured respiration, VE, PaO2 and PaCO2; however, pH was at first maintained at preinoculation levels, but declined thereafter. In severe cases, the animals were drowsy between hours 3 and 7, and became comatose between hours 8 and 10; in contrast to both benign and moderate cases: (1) RR was reduced by hour 5, (2) there was no trend towards recovery of PaO2 and pH, (3) VE, Pdi, Pawo100ms and Pawo100ms/EMGdi were severely decreased, and (4) PaCO2 increased. These results suggest that pneumonia does not alter ventilatory neuromuscular pump function in calves, unless concomitant cardiovascular collapse occurs. It is not clear whether fatal ventilatory failure is caused mainly by deterioration in ventilatory muscle fibre processes or structures, altered central nervous system adjustment of ventilatory timing, or cardiovascular dysfunction. However, inspiratory pressures fall when excitation to the diaphragm is still growing, which suggests peripheral respiratory muscle fatigue.

Tracheal resection and anastomosis for an intratracheal chondroma in a ball python

A 9-year-old female ball python was evaluated for lethargy and respiratory compromise. Radiographic and endoscopic examination revealed a 1-cm intratracheal mass occluding the tracheal lumen. A partial tracheal resection and anastomosis was performed to remove the mass. On histologic examination, the mass was determined to be an intratracheal chondroma. Eosinophilic cytoplasmic inclusions observed within the tissue were calcium deposits. Electron microscopy was used to differentiate these from viral inclusion bodies often associated with neoplasms in reptiles. Endoscopic evaluation of the trachea 5 weeks after surgery revealed complete healing and minimal stenosis at the surgery site. Indications of tumor regrowth were not evident. Clinical signs of recurrence of respiratory compromise had not been observed 9 months after surgery.

The impact of the 1993 Braer oil spill on grey seals in Shetland

Signs of acute respiratory distress were reported in moulting grey seals (Halichoerus grypus) hauled out on Lady's Holm, Shetland, following the Braer oil spill in January, 1993. Behavioural observations carried out between 16 January and 13 February 1993 showed that the proportion of animals exhibiting a discharge of nasal mucus was significantly higher than the proportion at a control site in the north (Papa Stour). The proportion of animals affected on Lady's Holm increased for up to one month following the spill. However, the time lag between exposure and peak response was approximately 30 days, longer than may be expected for an acute effect. The proportion of non-specific signs of respiratory distress in unexposed Shetland seals was assessed from observations made between 16 January and 25 January 1994. Symptoms similar to those seen in 1993 were also reported during this period, but the proportion of affected animals was higher in 1993. Symptoms were not observed at a grey seal moult site on the east coast of England in March 1993 and 1994. Grey seals moulting in Shetland during the time of the oil spill may have been acutely affected by exposure to hydrocarbons, but without sufficient baseline data on the occurrence of respiratory distress in grey seals it is difficult to determine the proportion attributable to other causes.

Pulmonary function changes induced by three regimens of bronchodilating agents in calves with acute respiratory distress syndrome

Two aerosolised bronchodilators, one sympathomimetic and one parasympatholytic, were tested either alone or in combination for their ability to improve the pulmonary function of double-muscled calves suffering from acute respiratory distress syndrome. In control animals treated with 0.9 per cent saline the parameters of pulmonary function and signs of clinical distress did not change significantly within the hour following the first treatment. Among the other animals, both at one hour and seven days after the first treatment, the most clinical improvement was observed in the animals treated with both bronchodilators and the least in the animals treated with clenbuterol hydrochloride. One hour after the first treatment the respiratory system compliance of the animals treated with ipratropium bromide and the arterial oxygen tension of the animals treated with both bronchodilators were significantly enhanced. After seven days the resistive parameters, the rectal temperature and the respiratory rate were also significantly improved in the animals treated with ipratropium bromide or both bronchodilators whereas only the respiratory rate and rectal temperature were significantly reduced in the animals treated with clenbuterol hydrochloride.