Why mechanical ventilation?
By Scott May

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Artificial ventilation has physical and biochemical factors contributing to lung injury and hemodynamic side effects. Precisely controlling the peak inspiratory pressure (PIP), tidal volume (VT) and blood pH helps prevent further damage to the pulmonary system and aids in maintaining a hemodynamic stable patient. Mechanical ventilators should be utilized when logistically safe, operationally understood and available for transport. These are some benefits of using a mechanical ventilator.


How do you prefer to administer ventilation?
  • 1. Mechanically
  • 2. Manually

The pulmonary system operates naturally on a negative transrespiratory pressure gradient (Prs):
(Prs) = pressure at the airway opening (Pao) - alveolar pressure (Palv) (negative)
The diaphragm drops, lowering alveolar pressure below atmospheric pressure. Air then moves along this pressure gradient to inflate the lungs. At the end of inspiration, the respiratory muscles relax; alveolar pressure is higher than atmospheric pressure, allowing passive exhalation.

Adding an artificial airway to ventilate the pulmonary system changes the pressure gradient to a positive system (the measured peak inspiratory airway pressure on the ventilator or pressure manometer):
(Prs) = pressure at the airway opening (Pao) (positive) alveolar pressure (Palv)
A positive pressure system is needed to deliver air down the alveolar pressure gradient. Pressure now needs to be applied to the lungs to open the airways and then push the diaphragm down, the opposite of normal respiration that lowers the diaphragm first then opens the airways.

The positive pressure gradient alters the lung-thorax compliance (CLT) (normal CLT 0.1 L/cmH2O) and places the pulmonary system at increased risk of lung injury from positive pressure (normal CLT ventilated patient 0.035-.055L/cmH2O). The positive pressure gradient affects the airway's epithelium chemically and physical, causing damage or adding insult to an already damaged pulmonary system.

The clinical symptoms from the biochemical cascade may not be clinically seen for over 72 hours. The airway resistance (Raw) will increase (Raw = △PV ) and compliance (CL) (Cl = △V (liters)△P (cmH20)) will decrease. This increases the risk of respiratory failure or further complications to the pulmonary system.

The second side effect is positive pressure transferred to the pleural space. Mean airway pressure (P aw) is a good measurement of the pressure transferred to the pleural space (Paw = (Ti x PIP)+(Te x PEEP)Ttot). A mechanical ventilator is able to measure the mean airway pressure or will provide the information to calculate mean airway pressure. Changes in pulmonary pressures will affect circulation.

Increase pressure in the pleural space decreases venous return and left ventricle stroke volume. Paw should be less then central venous pressure (1 CM H2O = 0.735 mmHg). Compensation for the positive pressure change in the pleural space will be increase cardiac output (CO) by heart rate (HR) or stroke volume (SV), (HR X SV = CO) and increase tone of the venous blood vessels. The patient, unable to compensate, will become hemodynamically unstable. Regulating the pressure will minimize hemodynamic side effects, aid in controlling the damaging side effects of positive pressure ventilation on the lungs and help further protect the injured lung.

Tidal volume (VT) is another factor that causes insult to the pulmonary system (V X IT = VT).The pressure used during ventilation and the time constant (Cl X Raw) of the respiratory system regulates the amount of volume delivered. When using a pressure manometer while ventilating a patient, the pressure being utilized is measured and not the volume. Large volumes cause overdistention of the lower airways, causing capillary airway injury in the alveoli membrane. This type of injury leads to pneumothoraces, noncardiac pulmonary edema and interference with the alveolar air-blood interface.

The effects of tidal volume on the injured lung were demonstrated in the ARDS network trial in 1994. The use of smaller tidal volumes was shown to improve survival rates in patients with ARDS. A mechanical ventilator is able to measure and regulate volume, reducing the affects of volume trauma to the pulmonary airway.

The chemistry of the blood has many effects on the cardiopulmonary system. The body's many chemical reactions optimally operate in a tight pH range influenced by carbon dioxide. The carbon dioxide production rate is based on the metabolic rate of the body. Hypocarbia causes blood vessels in the pulmonary system to dilate, and the cerebral blood vessels to constrict. The above change increases the risk of ventilation-perfusion (V/Q) imbalance in the lungs and constricts the cerebral blood supply. Hypocarbia leads to an alkalotic state, increasing the chance of tetanic contractions and cardiac rhythms.

Hypercarbia has the opposite effects on the pulmonary blood vessels and cerebral blood flow. Hypercarbia constricts the pulmonary blood vessels and dilates the cerebral blood vessels. The pulmonary constriction causes ventilation perfusion mismatch. The increased blood flow to the cerebrum alters cerebral blood flow (CPF), and the cerebral system will compensate if capable (CBF = cerebral prefussion pressurecerebral vascular resistances ).

Hypercarbia leads to acidosis or lowering of the blood pH, thus depressing the myocardial function of the cardiac muscle, leading to a decrease in cardiac output. A mechanical ventilator is able to maintain a set minute ventilation (VE) by controlling specific parameters related to respiratory rate and tidal volume (Ve = F x Vt). Stabilizing the pH aids in the stabilization of a patient's respiratory system and hemodynamic status.

When a mechanical ventilator is not utilized, at least a minimum of a pressure manometer, ETCO2 monitor or TCO2 monitor, and peep value should be utilized during transport.

Peak inspiratory pressure, tidal volume and blood pH stabilization are basic concepts of mechanical ventilation. A mechanical ventilation strategy should be based on the patient's clinical condition, pulmonary status and hemodynamic goals. Mechanical ventilators should be utilized when logistically safe, operationally understood and available for transport.

Scott May, RRT-NPS, C-NPT, works for the Children's Mercy Critical Care Transport Team in Kansas City, Mo. May has more than 15 years of neonatal/pediatric transport and mechanical ventilation experience.


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