Obesity and the Respiratory System

Last week of PHYL3004 content! That went by quickly...

In this post, we will be talking about the effect of body fat on obesity. In particular, we will be talking about subcutaneous fat. (Visceral or intra-abdominal fat presents its own problems, as mentioned here.) Body fat distribution varies a bit between males and females- males tend to have more belly fat ("apple shape"), whereas women tend to have more fat around the hips ("pear shape"). Since fat adds mechanical load to the respiratory system, opposing the contraction of respiratory muscles, it is important to know how and why obesity impacts the respiratory system.

Before continuing further, it might help to brush up on mechanics of breathing and lung volumes if you don't remember from previous semesters.

Effect of obesity on lung volumes

In obesity, TLC (total lung capacity) decreases. Most of this decrease occurs in FRC (functional residual capacity), particularly in ERV (expiratory reserve volume).

Mechanism(s) for reduced respiratory system compliance and respiratory muscle function

In obesity, lung and chest compliance decrease for several reasons. Fat directly decreases compliance, and increased pulmonary blood flow, which is associated with high BMI, also decreases compliance. As such, lung function changes in obesity are similar to those in restrictive lung diseases, where compliance is reduced. FVC (forced vital capacity) and FEV1 (forced expiratory volume in 1 second) both decrease in both obesity and restrictive disease. However, in obesity, the FEV1/FVC ratio usually remains unchanged, whereas this ratio may increase in restrictive disease (as FVC may decrease to a greater extent than FEV1).

Respiratory muscles, such as the diaphragm, can be negatively impacted by obesity. A large abdominal mass will push the diaphragm upwards, stretching it in the process. If the diaphragm is stretched beyond its optimum length, force will decrease (I've discussed why here).

Interrelationship between obesity, lung volume, airway resistance and airway hyperresponsiveness

Obesity tends to decrease lung volume (as discussed earlier) and thus decrease the total radius of the airways. Since resistance is inversely proportional to airway radius, a decrease in radius causes an increase in airway resistance, which in turn increases difficulty in breathing, particularly during exercise. As such, tidal volume may fall in extremely obese individuals (though it generally remains unchanged).

Low lung volumes have many more negative effects. Breathing at low lung volumes causes alveoli to collapse. This is known as "atelectasis." According to the LaPlace equation (pressure = (2*tension)/(radius)), reducing the alveolar radius increases pressure and tendency to collapse.

Yet another negative impact of low lung volumes is that when lung volume is low, the amount of load placed on airway smooth muscle is also low. As decreased afterload increases shortening (see here), muscle force is more likely to become greater than the opposing load, resulting in narrowing. (If muscle force was smaller than opposing load, bronchodilation woudl result instead.) This phenomenon is also referred to as "airway hyperresponsiveness," and can be quantified using PC20 (see here). Weight loss has been shown to improve PC20.

Changes in upper airway function in obese subjects and the association with obstructive sleep apnoea

The pharynx is more prone to collapse in patients with high BMI. The pharynx, which is quite floppy compared to the nasal cavity (which is surrounded by bone) and the trachea (which is surrounded by cartilage), is supported by the genioglossus muscle (tongue) and other pharyngeal muscles. These muscles provide dilatory pressure that keeps the pharynx open, opposing negative luminal pressures and positive tissue pressures. In obesity, the tissue pressure increases, making it more likely that the pharynx will collapse. The luminal pressure that causes pharyngeal collapse is known as the Pcrit, or critical closing pressure. A lower Pcrit is better.

Pharyngeal muscles are mainly driven by reflexes. These muscles respond to negative luminal pressures (i.e. pressures that pull the lumen in), rising carbon dioxide and falling oxygen levels. Their activity is reduced during sleep, which is why obstructive sleep apnoea can be a problem in some patients, particularly in obese patients. Patients with obstructive sleep apnoea may be helped with CPAP machines, which maintain high airway pressure. Unfortunately, they are also uncomfortable to use.

The decreased lung volumes in obesity can also impact the pharynx. When lung volume decreases, tracheal tension, which is the tension of the trachea pulling on the pharynx, also decreases. As such, the compliance and tendency to collapse of the pharynx also increase.

Only one more lecture to go for this unit!