Blood supply

The functional blood supply to the lungs for gas exchange is via the pulmonary arteries, which arise from the right ventricular outflow tract. The blood then returns to the left atrium via the pulmonary veins. The pulmonary arteries lie dorsal and lateral to the bronchi while the veins are ventral and medial. The terminal alveoli have a dense capillary bed to facilitate gas exchange. The nutritional blood supply to the lungs is provided by the bronchial vessels, which arise from the thoracic aorta.

Innervation

The innervation of the lungs is autonomic. Bronchiolar constriction occurs with parasympathetic stimulation of bronchiolar smooth muscle via the vagus nerves. There is also a parasympathetic supply to the bronchioles and alveoli. Sympathetic stimulation via the vagus and stellate cardiac nerves (the right and left pulmonary plexuses) leads to bronchiolar dilatation. The pulmonary plexuses also carry sensory fibers from the lung parenchyma and vasculature.

Regional lymph nodes

Lymphatic drainage from the lungs is to the left, right, and medial tracheobronchial lymph nodes lying to the left, right, and caudal to the tracheal bifurcation, respectively. Subsequent drainage is to the cranial mediastinal lymph nodes ventral or medial to the trachea at the level of the first two ribs.

Clinical signs

Cats with pulmonary disease do not commonly present with obvious signs of respiratory distress unless the condition is at an advanced stage. Early clinical signs may be vague and non-specific and not pathognomonic for a particular disease. Clinical signs specifically relating to the respiratory tract include dyspnea, wheezing, tachypnea, coughing, epistaxis, and hemoptysis.

Diagnostic imaging

A full radiographic assessment of the lungs should ideally include left and right lateral projections. However, if the patient is unstable it is important that emergency procedures (e.g., thoracocentesis) are performed prior to radiography as the stress of handling and positioning for radiography could precipitate respiratory arrest. Inflated views taken under anesthesia are always preferable if possible as this provides additional information, such as a higher likelihood of identifying the presence of small primary or metastatic nodules. However, sedation and/or anesthesia are only advisable in patients who are clinically stable. If the patient is dyspneic a single dorsoventral radiograph may be the only possible view obtainable in the initial stages of assessment. However, it is always important to note that one limitation of radiography is that it is not always possible to correctly identify the anatomical location of disease or the primary disease focus (e.g., bronchial, interstitial, alveolar).¹

Advanced imaging techniques such as ultrasound, computed tomography (CT), and scintigraphy may identify abnormalities not visible on radiography (e.g., lung metastases) and may also further facilitate surgical planning e.g., by identifying affected lung lobes to guide surgical approach.²

Some abnormalities may be seen with ultrasound, such as consolidation, atelectesis, and pulmonary masses.² However, the technical difficulties of ultrasound of the lungs caused by the inability of ultrasound waves to pass through air limit its usefulness to lesions located in an entire lung lobe or at the periphery of lung lobes adjacent to the thoracic wall.^3,4

CT is the preferred imaging modality for pulmonary disease and the CT findings in normal cats and those with pleural, pulmonary, and mediastinal diseases have been well described (Box 47-1).^2,5,6 CT can be successfully performed without chemical restraint in cats by using a positioning device,⁷ but it is more usual to perform CT under sedation or general anesthesia. There will inevitably be some movement blur during CT as there is for radiography and MRI, but this can be minimized by hyperventilation prior to scanning and then undertaking the scan during the expiratory pause (Figs 47-2 and 47-3).⁶ Aspiration of any pleural fluid or air that may be present prior to CT will improve pulmonary parenchymal visualization by allowing increased lung inflation,⁸ while the administration of intravenous contrast agents can also improve the detection of pulmonary lesions.^7,8 CT provided additional correct diagnoses in 28% of cats undergoing conscious CT compared to conscious radiography, as well as further additional information in 74% of cats. Overall, 40% of cats undergoing both techniques had a significant different disease or prognosis following CT imaging (Box 47-2).

Biopsy and laboratory analysis

Fluid obtained by bronchoalveolar lavage (BAL) should always be examined cytologically for evidence of neoplastic or inflammatory cells and for evidence of active infection but samples are more likely to be diagnostic for neoplastic disease when there are large lesions involving central rather than peripheral lung.¹⁶ The limitations of BAL include failure to sample a representative area of disease, low cellularity, the difficulty sometimes encountered differentiating neoplasia from epithelial hyperplasia, the presence of marked inflammation obscuring and hindering identification of other cells, and the inability to analyze cells in relation to airway architecture.¹ In addition, many different underlying diseases, including neoplasia and septic or non-septic inflammation can all lead to neutrophilic inflammation being present, thereby sometimes making it difficult to make a definitive diagnosis by cytology alone.¹ Hemosiderosis on tracheal wash samples is common but a non-specific finding.¹⁷ Any fluid obtained by either BAL or tracheal wash techniques should always be submitted for culture, although there may not be a correlation between culture results from BAL and lung tissue obtained by biopsy if aspiration of oral and pharyngeal bacteria into the lower airways has occurred during the sampling procedure, so every care should be taken to avoid this if possible.¹

Where radiography and cytology from BAL fail to establish a diagnosis, obtaining a biopsy for histologic analysis may be useful. Transthoracic fine needle aspiration can be performed blindly after a lesion has been identified on diagnostic imaging or, more preferably, using ultrasound, fluoroscopic or CT guidance.4,18,19 Ultrasound-guided aspiration can either be performed conscious or with sedation,⁴ but undertaking a biopsy procedure necessitates deep sedation or anesthesia. Aspirates can certainly help differentiate neoplastic from non-neoplastic disease and therefore guide clinical decision making, but biopsy is often required to determine exact tumor types.⁴ A solid tissue sample is also more likely to be diagnostic for diseases affecting pulmonary architecture, e.g., fibrosis, emphysema, bronchiectasis.²⁰ Contraindications for obtaining biopsies include coagulopathies, pulmonary bullae, severely compromised pulmonary function, and pulmonary hypertension.⁴ Potential complications encountered after a biopsy procedure include pneumothorax or pulmonary hemorrhage,^4,19 and it is important to counsel owners in this regard before such a procedure is performed.

In most instances, obtaining a tissue sample for histology requires open thoracotomy (see Chapter 41), but where available, thoracoscopy in the cat is considered a less invasive technique than thoracotomy for examining the thoracic cavity and obtaining tissue for histologic evaluation²¹ and should always be considered (see Chapter 42).

Primary pulmonary neoplasia

Primary pulmonary neoplasia is rare in cats, with a reported annual incidence in one study of 2.2 per 100 000 cats,²² while a further study reported an incidence of pulmonary neoplasia of 0.38% of cats examined at necropsy.²³ Affected cats are generally middle to old-aged and tumor development is independent of feline immunodeficiency virus or feline leukemia virus infection.²⁴ No predisposing causes of feline pulmonary neoplasia have been identified.²⁴ Most tumors encountered are malignant and of epithelial origin (98–100%), of which adenocarcinomas are the most common (85–100%).^10,12,25 Adenocarcinomas are not easily differentiated by topographic nomenclature (bronchial versus bronchiolar versus bronchioloalveolar) due to the extent of disease at diagnosis, and some are classified as adenosquamous carcinomas if there is significant squamous differentiation.²⁶ It is therefore more common to describe them as differentiated or undifferentiated, which is more useful clinically as it relates to prognosis.¹⁰ Undifferentiated adenocarcinomas appear to occur most commonly (43–69%) and are more invasive and metastatic than the less frequently reported differentiated adenocarcinomas.^10,25,26 Other epithelial malignancies such as anaplastic carcinoma and squamous cell carcinoma appear to be less common, while sarcomas and benign tumors are rarely reported.^10,27,28 Other reported pulmonary tumors in cats are listed in Box 47-3. Additionally, there has been a report of a mediastinal mass in a cat, which was consistent with bronchogenic adenocarcinoma. The author suggested that it originated from an accessory embryonic lung.²⁹

The reported clinical signs of cats with pulmonary neoplasia are summarized in Table 47-1. Interestingly, respiratory signs are reported to only occur in 33–62% of cats,^{10,12,25,27,28} and in some cats lameness was the only presenting complaint. Lameness may occur from metastatic spread to the digits, appendicular bones or skeletal muscles or, more rarely, hypertrophic osteopathy.^34,39,47 Invasion of the esophagus may cause gastrointestinal signs such as vomiting or regurgitation.^24,27

Significant changes on serum biochemistry and hematology are unusual.²⁵ Paraneoplastic conditions are occasionally recognized, including hypercalcemia, thrombocytopenia and leukocytosis, and are most likely due to humoral factors produced by the tumor.^41–43 Such paraneoplastic syndromes may resolve after tumor excision,⁴¹ but specific further medical management is still required in some cases.

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