Critical care—state of the art

Two experimental designs were used to study the mechanism of the decreased cardiac output associated with the use of positive end-expiratory pressure (PEEP). In the first study of nine dogs the application of 15 cm H2O PEEP led to a decrease in cardiac output (CO) from 2.68 +/- 1.05 to 2.01 +/- 1.26 liters/min (+/- SD) (p less than 0.05) concomitant with an increase in transmural central venous pressure of 5.2 +/- 0.9 to 8.4 +/- 2.7 mm Hg (p less than 0.05) and a slight increase in transmural left atrial pressure of 6.8 +/- 3.3 to 7.3 +/- 3.6 mm Hg (p less than 0.1). These data are consistent with altered ventricular performance. In a second study nine pairs of dogs were cross-circulated. Application of 15 cm H2O PEEP to one member of the experimental pair led to a decrease in the CO of the other member from 2.71 +/- 0.98 to 2.21 +/- 0.81 liters/min (p less than 0.001). This decrease returned toward baseline with the removal of PEEP (p less than 0.02). Results indicate that one mechanism whereby PEEP reduces the cardiac output is through the action of a humoral agent.

Humorally Mediated Alterations in Cardiac Performance as a Consequence of Positive End-Expiratory Pressure

Our survival depends on the production of energy in the form of adenosine triphosphate by the process of oxidative metabolism. This in turn depends on a continual supply of substrates, particularly oxygen and glucose, to vital tissue beds such as the brain and myocardium. In a critically ill patient, the supply of oxygen may be compromised while at the same time metabolic demand is increased. Inadequate oxygen delivery may become the factor mitigating against recovery or even survival of the patient (Schumacker and Cain, 1987). In a patient with brain injury, cerebral metabolic reserve is already reduced by the primary insult and subsequent oxygen lack is a potent cause of avoidable secondary brain injury (Chesnut et al., 1993). This chapter will address fundamental physiological principles of oxygen delivery from the atmosphere to the cells, the pathophysiological effects of head injury on oxygen delivery and principles of resuscitation and the maintenance of cardiopulmonary homeostasis.

17 Respiratory and cardiovascular support

For patients requiring PEEP during ventilation, e.g., patients with respiratory distress syndrome, performing traditional tests of lung function is a complicated problem. The pulmonary mechanics of these patients can change rapidly with potential for a negative impact on oxygenation. In an effort to address these problems, we designed a system with a three-way valve that permits the patient to be switched from the ventilator to a data collection system for the assessment of changes in the disease state. The short periods required for data collection (20 to 30 sec) do not interfere with patient care and are well within the limits of safety. A unique feature of the valved system is that it serves to maintain positive airway pressure during data collection. Tests show that, within the frequency range of interest, the mechanics of the measurement system are not altered by the DC pressures applied to the speaker and the valve itself does not contribute significantly to the serial impedance of the system.

Instrumentation for estimating pulmonary function in patients on positive end-expiratory pressure. Author's replies

Choosing ventilation mode and settings IPPV remains the mainstay of ventilation therapy for patients with no spontaneous respiratory activity. These will be patients undergoing routine anaesthesia, comatose patients or sedated patients with respiratory failure. The use of PEEP is indicated when oxygenation is inadequate or in order to reduce high F iO 2 levels. The level of PEEP applied is chosen as a compromise between achieving satisfactory arterial blood gases and minimizing cardiovascular depression. Ventilator rates are adjusted during IPPV to achieve the required minute ventilation with given tidal volumes. Limiting factors include the absolute values of expiratory times in relation to the RC constant for the individual respiratory system, and the airway pressures produced with the given tidal volumes. In severe cases of asthma, controlled hypoventilation may be acceptable in order to allow full deflation of the lungs and avoid excessively high airway pressures. If full deflation of the lungs is not allowed, an ‘auto-PEEP' effect is produced as with the use of inverse ratio IPPV or with high ventilation frequencies as in HFV. Improved oxygenation in acute severe asthma with reduced hyperinflation and pleateau airway pressure, has been reported with the use of high inspiratory flows and low tidal volumes, in order to maximize expiratory times at a constant respiratory rate (Tuxen and Lane, 1987). Severe unilateral lung problems or mechanical mismatch may require the use of differential lung ventilation, possibly with mixed modes, in order to achieve satisfactory gas exchange. Such conditions include the occurrence of a unilateral bronchopleural fistula, a large unilateral bulla or severe unilateral collapse or consolidation. HFPPV or HFJV techniques in the past have been applied in extremecases of lung pathology where refractory hypoxaemia is a problem, or gross leaks from bronchopleural fistula have made IPPV ineffective. Certainly in cases of upper airway disruption the ability of HFJV to maintain ventilation in the absence of a sealed delivery circuit via a catheters makes it a technique of choice. The ability of HFJV to deliver high minute volumes also enables it to cope in the presence of large shunts. However, in cases of severe hypoxaemia, although HFV modes may offer improved oxygenation by promoting non-tidal mechanisms of gas transport at the higher frequencies (e.g. HFOV), or by taking advantage of grossly altered lung frequency responses, the consistent superiority of any one mode or combined mode has yet to be proven. Mixed spontaneous and mechanical modes of ventilation find their mainapplication in patients with spontaneous respiratory activity or in weaning from ventilation. IMV is commonly used in patients with an inadequate respiratory drive but who can generate adequate tidal volumes. While MMV may be more suitable for patients with variable respiratory drive, this mode still requires further evaluation. Pressure support appears to be more suitable for patients with adequate drive but poor respiratory effort, i.e. in conditions giving rise to low vital capacities and tidal volumes. During weaning from mechanical ventilation, the choice of mode and CPAP or sPEEP levels has to be made in a context that may not necessarily place P aO 2 or P aCO 2 level as the primary criteria. The mechanical ability of the patient to cope with the work of breathing may be a major problem and a compromise between gas exchange and work of breathing capability may have to be reached. Additional factors such as nutrition and patient psychology can also become important during prolonged weaning. CPAP and sPEEP techniques may also have a prophylactic role enabling patients with moderate respiratory dysfunction to avoid endotracheal intubation and/or mechanical ventilation. Accurate and adequate monitoring, including the measurement of respiratory mechanics, is essential for the optimum choice and adjustment of ventilation therapy.

T.E. Oh

Papers

Ventilation―which mode?