Page 60 - JSOM Winter 2024

against the visceral surface during ventilation. The interaction intrathoracic pressure increases right atrial pressure. Venous
between the visceral and parietal pleura is seen as ‘lung sliding’ return to the right atrium falls due to the increase in pressure
on ultrasound. There is normally no air in this pleural space. gradient between the systemic venous system and the right
Air in the pleural space (pneumothorax) is always pathologic. atrium. The fall in venous return (cardiac preload) diminishes
It is important to note that in normal spontaneous breathing, cardiac output which results in hypotension and impaired per-
the pressure in the intrapleural space (intrapleural pressure fusion. This form of shock is referred to as obstructive shock as
[IPP]) is below ambient atmospheric pressure throughout the it results from an obstruction of cardiac filling. A temporizing
ventilatory cycle. Intrapleural pressure is –5cmH O at rest, therapy is to increase venous return to the heart by increasing
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approximately –8cmH O during inspiration, and returns to venous pressure with either fluid or blood administration. As
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–4cmH O during expiration. This produces a resting tidal this approach is only temporizing, the obstructive shock must
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volume of approximately 7mL/kg of body mass. During deep be addressed to resolve the hemodynamic effects.
inspiration, the IPP can reach much larger negative pressures
as inspiratory reserve volume is utilized. In spontaneously breathing patients with a tension pneumotho-
rax, the decreasing total lung capacity and inspiratory capacity
A pneumothorax occurs when there is either an injury to the results in diminished functional alveolar volume. The loss of
lung parenchyma and visceral pleural resulting in air egress functional alveolar volume may be compensated for by physi-
from the alveolar spaces or when there is an injury to the chest ological responses that attempt to increase minute ventilation.
wall with a breach of the parietal pleura, both result with the These responses include increasing the intrathoracic pressure
ingress of air into the pleural space. A simple pneumothorax gradients during inspiration by taking deeper breaths and in-
is present when there is air in the pleural space that does not creasing respiratory rate (rapid, deep breathing). It should be
result in significant physiological or hemodynamic sequelae. noted that the continued generation of negative intrathoracic
This air in the intrapleural space does not dramatically change pressure in the spontaneously breathing patient may mitigate
the IPP from negative to positive. Simple pneumothorax can the compromise of venous return to the heart by maintaining a
result in various degrees of lung collapse but still allow the low or negative pressure inside the right atrium. From a phys-
affected lung to expand to some degree during inspiration. As iological standpoint, it is difficult to create significant positive
such, while it needs to be treated, a simple pneumothorax is intrapleural pressure in a spontaneously breathing patient un-
not immediately life threatening. less positive pressure is generated during some phase of the
respiratory cycle. Theoretically this may occur in the patient
A tension pneumothorax occurs when the volume of air in during the active expiratory phase, in which positive pressure
the pleural space becomes large enough that the physiological is created by a Valsalva or positive pressure expiration. The re-
impact is severe and potentially life-threatening. In this setting, sult can be the expanding of the tension pneumothorax, which,
negative IPP becomes increasingly positive to the degree that it in turn, will continue to decrease this compensatory process
begins to collapse the lung. If this increase in volume and pres- resulting in decreasing alveolar recruitment, impairment of air
sure continues, the resultant intrathoracic pressure compresses exchange, and increasing hypoxia. If left untreated the patient
the inferior vena cava, and increases right atrial pressure, and will succumb to this process which manifests as a clinical spec-
this decreases venous return to the heart and cardiac output. trum of respiratory failure followed by cardiac arrest.
The progressive hemodynamic and respiratory compromise
from a tension pneumothorax may result in a precipitous de- Animal studies and case presentations suggest that there is a
cline of the patient’s physiology, progressing to respiratory and difference in presentation between spontaneously breathing
cardiac arrest. and positive pressure ventilated patients with a tension pneu-
mothorax. Spontaneously breathing tension pneumothorax
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patients experience greater clinical impact on the respiratory
Definitions
system with late circulatory compromise and positive pressure
Simple Pneumothorax ventilated tension pneumothorax patients present with greater
The presence of air within the pleural space. impact to the circulatory system, as the respiratory distress el-
ement is often overcome with ventilation. This ventilation, in
Tension Pneumothorax turn, delivers greater pressure with each breath, often resulting
The presence of air within the pleural space with progressive in abrupt circulatory failure.
pressure changes that compromise both the respiratory and
cardiovascular systems. In summary, it is imperative that the clinical medic under-
stands the basic concepts of physiology presented. There are
Ventilation, from either a bag valve mask or a ventilator, clinically important distinctions in the evolution of tension
changes the pressure dynamics from negative inspiratory pres- pneumothorax based upon whether the patient is breathing
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sure during inspiration to positive pressure. This may increase spontaneously or is mechanically ventilated with positive pres-
the rate and volume of air moving into the pleural space. The sures. The ability to distinguish these features is the key to
ventilation also diminishes venous return. This is clinically successful management of this process.
manifested by an increased resistance to manual bagging, in-
creased airway pressures, decreased delivered tidal volume on Clinical Presentation and Diagnosis
a ventilator, hypoxia, progressive tachycardia, hypotension,
and eventually circulatory collapse. Examination
Classically, a description of the signs and symptoms of tension
Central venous pressure is normally in the range of 6–12mmHg pneumothorax may have been described as a suggestive mech-
and right atrial pressure is normally around zero. During anism of injury, combined with the presentation of chest pain,
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the development of a tension pneumothorax, the increasing shortness of breath, dyspnea, hypoxemia, and hypotension. In
58 | JSOM Volume 24, Edition 4 / Winter 2024