Spirometry is one of the most readily available and useful tests for pulmonary function. It measures the volume of air exhaled at specific time points during complete exhalation by force, which is preceded by a maximal inhalation.
The most important variables reported include total exhaled volume, known as the forced vital capacity (FVC), the volume exhaled in the first second, known as the forced expiratory volume in one second (FEV1), and their ratio (FEV1/FVC). These results are represented on a graph as volumes and combinations of these volumes termed capacities and can be used as a diagnostic tool, as a means to monitor patients with pulmonary diseases and to improve the rate of smoking cessation according to some reports.
Lungs provide life-sustaining gas exchange by way of introducing oxygen for metabolism and eliminating the by-product carbon dioxide. Air inspired will pass through the oropharynx to the trachea, which is a membranous tube covered by cartilage bifurcating at the carina as two bronchi at the level of C6. After passing the trachea, the air enters the right and left bronchi, which divide to give several million terminal bronchioles that end in alveoli. The alveoli and surrounding vessels provide a surface where the gas exchange takes place.
Apart from being a key diagnostic test for asthma and chronic obstructive pulmonary disease, spirometry in indicated in several other places, as listed below:
Spirometry has proved itself as an accessible utility to assess lung function. However, it may not be for every patient, and care must be taken in some cases, where it may be absolutely or relatively contraindicated.
The first requirement for spirometry is physical space in order for the patient to be positioned comfortably. The minimum space recommended is a 2.5* 3m room with 120 cm side doors.
Spirometers are classified into closed-circuit and open-circuit spirometers. Closed-circuit spirometers are further sub-classified into wet and dry spirometers, which consist of a piston or a bellow acting as an air collecting system and a supported recording system that moves at the desired rate.
Open-circuit spirometers, which are more commonly used at present, do not have an air collecting system and instead measure the airflow, integrate the results, and calculate the volume. The most commonly used open-circuit spirometer is the turbine flow meter, which records the rate at which turbines turn and derives the flow measurement based on proportionality. Pneumotachographs are another example, which measures the airflow by measuring the pressure difference generated as the laminar flow passes through a certain resistance. Hotwire spirometers, in which a hot metal wire is heated, and the air used to cool it is used to calculate the flow, are also an example of open-circuit spirometers. Ultrasound spirometers can be based on any of the aforementioned open-circuit spirometer principles.
The minimum specifications for a spirometer are the ability to measure a volume of 8L with an accuracy of ±3% or ±50ml with a flow measurement range of ±141 sand a sensitivity of 200ml/s. It is recommended that the spirometer can record at 15 s of the expiration time for the forced maneuver.
The personnel performing the procedure must be familiar with respiratory symptoms and signs. They have to undergo training to understand the technical and physiological background of the tests in order to be competent in performing the techniques of the operation of the device, be able to apply the universal precautions, instruct the patients properly to avoid complications and act accordingly if any of the complications arise. The personnel should be able to identify responses to therapy, the need for initiating therapy, or discontinuing an inefficient one. Continuity of training and periodic retraining is a must for staff in charge of spirometry.
All patients must be informed that they must abstain from smoking, physical exercise in the hours before the procedure. Any bronchodilator therapy must also be stopped beforehand.
The procedure must be carefully explained to the patient focusing on the importance of the patient’s cooperation to provide the most accurate results. The patient’s weight and height must be recorded with the patient barefoot and wearing only light clothing. In the case of chest deformities such as kyphoscoliosis, the span should be measured from the tip of one middle finger to the tip of the other middle finger with the hands crossed, and the height can be estimated from the formula: height = span/1.06. The patient’s age must be recorded. The procedure should be performed with the patient sitting upright wearing light clothing and without crossing their legs. Children can perform the test sitting or standing, but the same procedure should be carried out for the same individual every time.
During the procedure, the back must be supported by a backrest and must not lead forward. Dentures have to be removed if they interfere with the procedure. Manual occlusion of the nares with the help of nose clips helps to prevent air leakage through the nasal passages, although it is not mandatory to occlude nasal passage. The calibration of the spirometer has to be confirmed on the day of the test.
The patient must place the mouthpiece in their mouth, and the technician must ensure that there are no leaks, and the patient is not obstructing the mouthpiece. The procedure is carried out as follows:
Spirometry has proved to be a crucial tool in diagnosing lung disease, monitoring patients for their pulmonary function, and assessing their fitness for various procedures. With further research, solid evidence can arise for the role of spirometry in assisting patients in quitting smoking. The American College of Physicians guidelines does not recommend spirometry testing for patients undergoing nonthoracic surgery. There, of course, are exceptions if the patient has preoperative asthma or COPD.
Recent evidence also supports the use of spirometry in nonthoracic surgeries. A recent retrospective observational study found that lower preoperative spirometry FVC may predict postoperative pulmonary complications in high-risk patients undergoing abdominal surgery. In another retrospective observational study, the authors found that %VC (FVC/predicted VC) may be a predictor for postoperative pneumonia in patients undergoing colorectal cancer surgery. More studies are needed, but spirometry may be an important tool in identifying nonthoracic surgical patients that are at high risk of postoperative pulmonary complications.
Lung volumes are essential to understand when evaluating a patient for surgery or evaluating a patient with preexisting lung disease. Tidal volume (TV) is the amount of air that can be exhaled or inhaled in one respiratory cycle. Normal tidal volume ranges from 6 to 8 ml/kg. Inspiratory reserve volume(IRV) is the forcible amount of air inhaled after normal TV. Expiratory reserve volume (ERV) is the amount of forcible air exhaled after exhalation of a normal TV. Residual Volume (RV) is the amount of air in the lungs after maximum exhalation. Both RV and functional residual volume (FRC) can not be measured by spirometry. RV can be indirectly calculated from the FRC and ERV.
Lung capacities are the summation of lung volumes. Total lung capacity (TLC) is the summation of TV, IRV, ERV, and RV. This represents the maximum volume the lungs can accommodate. Vital capacity (VC) is the summation of TV, IR, and ERV. It represents the total air exhaled after maximum inhalation. Functional residual capacity (FRC) is the residual volume plus expiratory reserve volume. It is the volume of air remaining in the lungs after normal exhalation.
These static lung volumes and capacities can diagnosis obstructive and restrictive lung patterns. Restrictive lung disease results in reduced lung compliance and a reduction in lung volumes and capacities. TLC is reduced greater than 80% or below the 5th percentile of the predicted value. Both FEV1 and FVC are reduced, but FVC is reduced more than FEV1. Therefore, the FEV1/FVC ratio is greater than 80%.
One of the most common causes of restrictive lung disease is obesity. Obese patients have a reduction in FRC, which becomes worse when moving from upright to a supine position. The weight of the chest wall pushes down on the lungs. The weight of the abdominal contents pushes against the diaphragm and base of the lungs worsening the restrictive pattern. Other restrictive lung processes are chest wall diseases(scoliosis, chest trauma), and neuromuscular disorders (Myasthenia Gravis, Guillain-Barré syndrome). Obstructive lung disease is a disproportionate reduction in the maximum airflow from the lungs compared to the maximum air that can be displaced from the lung. This can be confirmed by an FEV/VC ratio below the 5th percentile of the predicted value. The RV/TLC ration will increase irrespective of whether VC increases or decreases. The TLC will either increase or stay the same.
Complete spirometry exams will identify FEV1, forced vital capacity (FVC), vital capacity (VC), residual lung volume (RV), maximum voluntary minute ventilation (MMV), and total lung capacity (TLC). One parametric that is highly indicative of postoperative complications is predicted postoperative FEV 1(ppo FEV 1). Predicted postoperative FEV1 <30% are at a higher risk of postoperative pulmonary complications after thoracic surgery.
Spirometry is an apparatus used to assess pulmonary function for diagnostic or monitoring purposes. The procedure must be explained thoroughly to the subject patient by competent personnel who underwent training under supervision by a specialist mentor and will undergo periodic retraining in order to ensure that the results obtained are as accurate as possible and the complications are kept to a minimum. The results are interpreted by a pulmonologist, and the consultation an interprofessional group of specialists is recommended.
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