Overview Of Mechanical Ventilation: Cardiothoracic Surgery

Introduction

The technique of moving the air in and out of the lungs is called Ventilation. Mechanical Ventilation is a life sustaining procedure of providing artificial respiration using a mechanical device known as the Mechanical Ventilator. It is a common model in ICUs and its advent of usage publicized the dawn of contemporary medical care units. The usage has increased in both the aspects of researches and clinical perspective in the past years.

History

The noted Greek physician and scientist name Galen lived in the 2^nd century AD, played a major role in introducing the importance of anatomy in the understanding of the disease. Though he made many advances, his dissections were limited to animals and assumed that the organs of humans and animals were identical. He even studied respiration and taught that breathing was important to maintain the circulation (i.e. the physical act of breathing caused the heart to beat). For almost in next 1,500 years, there were essentially no advances made in the understanding of ventilation, nor that matter in any of the sciences; There is a good reason that a major part of this era was called the Dark Ages. Later, Andreas Vesalius has bought changes in the mid-16^th century.

Vesalius was born in Brussels and became Professor of Anatomy in Padua and incurred the wrath of the church because of his dissection of human cadavers and many of his findings contradicted Galen’s teachings and in 1543, he had worked on a treatise on anatomy entitled De Humani Corporis Fabrica, which likely had the first definitive reference to positive pressure ventilation as we know it today.

Robert Hook who coined the term “cell” to described the biological organisms had performed an ingenious experiment to examine Galen’s hypothesis that the movement of the lungs was required for the circulation. It is chief to know that, as Hook stated, it was till then not clear to physicians why people breathed and why people became pulseless. Due to the lack of stimulation people were unconscious and so was the belief.

In 1774, Joseph Priestly and Willhelm Scheele discovered oxygen and subsequently, Lavoisier discovered the importance of oxygen in respiration, thus answering the question posed by Hook a century earlier as to the “genuine use of respiration.”

Mechanical Ventilation:

This is a device used for inflating the lungs artificially by the positive pressure. There are two types namely

1. Negative Pressure
2. Positive Pressure

Negative Pressure Ventilators:

In the late 19^th century, ventilators based largely on (currently) accepted physiological principles were developed. Essentially, ventilation was delivered using sub-atmospheric pressure around the body of the patient replacing or augmenting the work being done by the respiratory muscles. In 1864, Alfred Jones invented one of the first such body-enclosing device iron lungs called as “Spirophore”. The negative pressure was generated by encasing the body of the patient in an iron cylinder. One problem with these devices was, it was too difficult to nurse because it was difficult to get access to the patient’s body. To address this problem, Peter Lord patented a respirator room, in which the patient lay with her head outside the room; inside, huge pistons generated pressure changes, which caused the air is moving into and out of lungs. The ventilator room had a door so that the medical staff could enter the ventilator to care for the patients. The ventilators used were extremely expensive, so James Wilson had developed a ventilation room in which multiple patients could be treated. One such room was used at Children’s Hospital, Boston, for several epidemics.

Later in 1926, Wilhelm Schwake patented for the designing of the pneumatic chamber. He worked on the precise matching of the ventilator and the patient’s breathing pattern in designing.

Positive-pressure Ventilation

The revival of polio marked a watershed in the history of mechanical ventilation. Bjorn Ibsen, an anesthesiologist who had trained in Boston in Beecher’s lab, realized that polio and its symptoms (excessive sweating, hypertension, and total plasma CO₂) were not caused by renal failure but by respiratory failure. He had then recommended tracheostomy and positive pressure ventilation. Lassen, who was the hospital’s chief physician, initially rejected this approach but soon relented when Ibsen demonstrated its efficacy.

Initiation of Positive-pressure Ventilation

Indications: Indications for ventilatory support

• Acute respiratory failure
• Prophylactic ventilatory support
• Hyperventilation therapy

Acute Respiratory Failure

• Hypoxic Lung Failure (Type I)
• Ventilation/perfusion mismatch
• Diffusion defect
• Right-to-left shunt
• Alveolar hypoventilation
• Decreased inspired oxygen
• Acute life-threatening or important organ threatening tissue hypoxia

Acute Respiratory Failure Indications

• Acute hypercapnic respiratory Failure (Type II)
• CNS Disorders
• Reduced drive to breathe: depressant drugs, brain or brain stem lesions, hypothyroidism
• Increased drive to breathe: increased metabolic rate (↑CO₂ production), metabolic acidosis, anxiety associated with dyspnea.

Acute Respiratory Failure Indications

• Acute hypercapnic respiratory Failure
• Increased work of breathing
• Pleural occupying lesions
• Chest wall deformities
• Increased airway resistance
• Lung tissue involvement
• Pulmonary vascular problems
• Hyperinflation
• Postoperative pulmonary complications

Essential components in Mechanical Ventilation

• Patient
• Artificial airway
• Ventilator circuit
• Mechanical Ventilator
• A/C or D/C power source
• O₂ cylinder or Central oxygen supply

Artificial Airways

• Tracheal intubation
• Nasal
• Oral
• Supraglottic airway
• Cricothyrotomy
• Tracheostomy

Goals of Mechanical Ventilation

• Adjust alveolar ventilation pH, PaCO₂
• Improve oxygenation, access with pulse oximetry
• Decrease work of breathing

Terms Related

• Respiratory rate (f): Number of breaths per minute 10-20 bpm
• Tidal Volume (VT): Volume of air inhaled/exhaled during each respiratory cycle (7-12 ml/kg)
• Minute Ventilation (VE): Volume of air expired per minute (VE=VT×f; 6-8 L/min)
• A fraction of inspired oxygen (FIO₂): the amount of Oxygen delivered to the patient and could range from 21% (room air)-100%

Complications of Positive Pressure Ventilation

Pneumothorax: Pleural pressure increases and collapses the lung causing pneumothorax.

Volume-pressure trauma/barotrauma: The lung injury that occurs when large tidal volumes are used to inflate non-complaint lungs (e.g. ARDS) and results in alveolar fracture and motion of proteins and fluid into the alveolar spaces.

Alveolar Hypoventilation: The Alveolar Hypoventilation causation is by inappropriate ventilator setting, leakage of air from ventilator tubing or around ET tube and tracheostomy cuff, lung secretions or obstructions and low ventilation-perfusion ratio.

Alveolar Hyperventilation: Respiratory alkalosis occurs if the respiratory rate and tidal volumes are set too high.

Ventilator-associated Pneumonia: Because ET or tracheostomy tube bypasses normal upper airway defenses.

Sodium and water imbalance: Fluid retention occurs after 48-72hrs of PPV. It is associated with decreased urinary output increased sodium retention. Fluid balance change is because of the decreased cardiac output.

Neurologic System: Increased intrathoracic pressure impedes venous drainage from the head. This increases the cerebral blood volume and causes an increase in intracranial pressure.

Gastrointestinal System: The ventilated patient is at the risk of developing stress ulcers and GI bleeding.

The Future

New novel physiological approaches, included with new biological insights and engineering approaches, will continue in improving the delivery of mechanical ventilation as we move further into the 21^st century. Such advances will certainly welcome as the number of patients requiring mechanical ventilation is expected to increase substantially.