Physiological aspects of intermittent positive pressure ventilation

Mean airway pressure and response to inhaled nitric oxide in neonatal and pediatric patients

Inhaled nitric oxide (iNO) can improve oxygenation and ventilation-perfusion (V/Q) matching by reduction of shunt (Qs/Qt) in patients with hypoxemic lung disease. Because the improvement in V/Q matching must occur by redistribution of pulmonary blood flow, and because high airway pressure (Paw) increases physiologic dead space (Vd/Vt), we hypothesized that high Paw may limit the improvement in V/Q matching during iNO treatment. iNO 0-50 ppm was administered during mechanical ventilation. Mechanical ventilator settings were at the discretion of the attending physician. Qs/Qt and Vd/Vt were derived from a tripartite lung model with correction for shunt-induced dead space. Data from 62 patients during 153 trials were analyzed for effects of Paw and iNO on Qs/Qt and Vd/Vt. Baseline Qs/Qt was slightly increased at Paw 16-23 cmH2O (p < 0.05), while Vd/Vt increased progressively with higher Paw (p < 0.002). Therapy with iNO significantly reduced Qs/Qt (p < 0.001) at all levels of mean Paw, reaching a maximum reduction at 16-23 cmH2O (p < 0.05), such that Qs/Qt during iNO treatment was similar at all levels of Paw. During iNO treatment, a reduction in Vd/Vt occurred only at Paw of 8-15 cmH2O (p < 0.05), and the positive relationship between Vd/Vt and Paw was maintained. These differential effects on Qs/Qt and Vd/Vt suggest that both high and low Paw may limit improvement in gas exchange with iNO. Analysis of gas exchange using this corrected tripartite lung model may help optimize ventilatory strategies during iNO therapy.

Effect of manual hyperinflation on haemodynamics in an animal model

BACKGROUND AND PURPOSE

Manual hyperinflation is a physiotherapy technique that improves static compliance and mobilizes secretions, but has the potential to alter haemodynamic function. The aim of the present study was to investigate the effects of manual hyperinflation on haemodynamic function in a healthy animal model, without the usual confounding effects inherent in an heterogeneous intensive care population.

METHOD

The study used a within-subjects design, in an animal research theatre. Nine healthy sheep (eight Border Leicester, one Merino, mean weight 39.5 kg, standard deviation (SD) 1.6 kg) completed the study. The sheep were induced (thiopentane 15-20 ml), intubated, ventilated and surgically instrumented for an arterial line and pulmonary artery catheter. Anaesthesia was maintained by 1.5% halothane/oxygen. Manual hyperinflation was delivered for two minutes with a Mapleson C circuit, using a peak inspiratory pressure of 35 cmH2O and an inspiratory:expiratory ratio of 2:1.

RESULTS

Mean tidal volume during manual hyperinflation was 294% (SD 22%) of the ventilator tidal volume. A paired Student's t-test demonstrated that cardiac output (thermodilution method) decreased significantly (p < 0.05) and systemic vascular resistance increased significantly (p < 0.01) after manual hyperinflation. A repeated-measures analysis of variance (ANOVA) and a least-significant difference pairwise comparison revealed that mean arterial pressure and pulse pressure decreased significantly during (p < 0.01) and increased significantly (mean arterial pressure, p < 0.05 and mean pulse pressure p < 0.001) after the technique. Pulmonary artery pressure also increased significantly during manual hyperinflation (p < 0.01). There were no significant effects on right atrial pressure, pulmonary artery occlusion pressure or heart rate.

CONCLUSION

Significant haemodynamic changes occurred in this animal model. The increased intrathoracic pressure, applied for an increased period during inspiration, decreased cardiac output with compensatory vasoconstriction evident by the increased systemic vascular resistance and mean arterial pressure. The results suggest that there may be a decrease in cardiac output after increased positive pressure in subjects with normal cardiac and respiratory function.

Weaning from long-term mechanical ventilation: a nonpulmonary weaning index

OBJECTIVE

Despite the extensive investigations in the area of weaning, clinicians are still struggling with the question of when to begin the process of weaning. Clinical weaning indices designed to predict the weaning potential are most frequently based on pulmonary factors. However, many physiological, respiratory, and mechanical factors also have impact on weaning, but are often overlooked. We suggest a new "nonpulmonary weaning index" (NPWI), which assesses the influence of factors such as blood albumin and total blood protein on the weaning success.

METHODS

We assess the information value of 17 clinical and paraclinical indices in a retrospective study covering 151 patients on a long-term (at least 7 days) mechanical ventilation. The most informative of those 17 indices are used in the formulation of NPWI. Its threshold differentiates the successful from the unsuccessful weaning trials.

RESULTS

From all 17 indices the most significant are: total blood protein, blood albumin, PaO2, hematocrit, lactate, the ratio PaO2/FiO2, hemoglobine, and RUE. The proposed index uses only two of them: blood albumin and total blood protein. It is easily calculated and can easily be tracked in time. It has high sensitivity and specificity.

CONCLUSIONS

The results of this study suggest that in the decision whether to attempt weaning from long-term mechanical ventilation, more attention should be paid to the nonpulmonary factors.

Reversal of the pattern of respiratory variation of Doppler inflow velocities in constrictive pericarditis during mechanical ventilation

BACKGROUND

Spontaneous inspiration causes a characteristic decrease of the mitral valve (MV) and pulmonary venous (PV) flow velocities obtained by Doppler echocardiography in patients with constrictive pericarditis (CP). This has been explained by the decrement it causes in the intrathoracic pressure. Positive pressure ventilation (PPV) causes an increment of intrathoracic pressure with mechanical inspiration. Therefore the pattern of respiratory variation produced during PPV may differ from that seen during spontaneous breathing.

OBJECTIVE

Our goal was to describe the effect of PPV on the pattern and magnitude of respiratory variation of MV and PV flow velocities in CP.

METHODS

We performed intraoperative pulsed Doppler transesophageal echocardiography on 15 patients (13 men, mean age 52+/-15 years) with CP after general anesthesia and before sternotomy and pericardial stripping. The peak velocity and time-velocity integral (TVI) of the mitral inflow E and A waves and the PV systolic and diastolic waves were measured at onset of inspiration and expiration for 3 to 6 respiratory cycles. Respiratory phase was monitored with a heat-sensitive nasal thermistor. The percent change in Doppler flow velocities from mechanical inspiration (INS) to mechanical expiration (EXP) was calculated with the formula %change = INS - EXP / INS x 100.

RESULTS

The peak velocity of the mitral inflow E wave was significantly higher during mechanical inspiration than expiration (57 +/-14.5 versus 47+/-13.9 cm/s, P<.001). This represented a percent change of 18%+/-7.9% from expiration to inspiration. The mean TVI of the mitral inflow E was also higher during mechanical inspiration than expiration (P = .02). The peak velocity of the PV D wave was higher during mechanical inspiration than expiration (39+/-17.8 versus 28+/-14.7 cm/s, P<.001). This represented a mean percent change of 28%+/-13.8%. The mean value of the TVI for the PV D wave was also significantly greater during mechanical inspiration than expiration (P <.05).

CONCLUSIONS

Positive pressure ventilation reverses the pattern of respiratory variation of the MV and PV flow velocities in CP. The percent change in the peak velocities of the MV and PV flows produced by PPV is the same range reported in CP during spontaneous breathing.

Physiological changes occurring with positive pressure ventilation: Part one

Critically ill patients requiring mechanical ventilation are subject to a variety of complications and adverse effects associated with positive pressure ventilation. An awareness of the major physiological effects is important, as recognition, prevention and appropriate treatment of complications is critical to optimizing patient outcome. Pierson (1990) suggests that complications due to ventilation occur with greater frequency than is generally appreciated, and can be a response to suboptimal ventilatory management as a result of poor communication and lack of understanding. This may also adversely affect patient comfort, morbidity and outcome. This can be avoided by improved knowledge of both the function of ventilators and the adverse effects associated with mechanical ventilation. Intensive care nurses face the challenge of caring not only for critically ill patients but also for complex machinery. To improve the delivery of care to the patients, an understanding of the machines and their adverse effects is essential. This increases the nurses' confidence and allows them to focus on the patients and associated problems while maintaining safe and informed care. With the introduction of mechanical ventilation, major physiological changes occur, for example airway resistance and intrathoracic pressures are increased and lung mechanics are altered. The following article provides explanation as to how and why mechanical ventilation produces these changes, and highlights areas where they occur.

Coronary artery bypass grafting using two different anesthetic techniques: Part 2: Postoperative outcome

The aim of the present investigation was to study the effects of intraoperative and postoperative epidural pain management during and after coronary artery bypass grafting (CABG) on the recovery time, postoperative pulmonary and cardiac parameters, visual analog scale (VAS) scores, and sedation scores (SS) compared with patients anesthetized with general anesthesia (GA) whose postoperative pain was relieved with intermittent intravenous (IV) administration of nicomorphine. Fifty-four patients were studied postoperatively after uncomplicated CABG. In the thoracic epidural analgesia (TEA) group (n = 27), intraoperative analgesia was based on high TEA in combination with GA. In the GA group (n = 27), IV anesthesia with high-dose sufentanil and midazolam was used. Postoperative pain management in the GA group consisted of intermittent IV administration of nicomorphine, 0.1 mg/kg every 6 hours, whereas for the TEA group continuous high TEA with 0.125% bupivacaine plus sufentanil, 1:1,000,000 (0.05 mL/cm body length/hr) was used. Patients in the TEA group awakened earlier (148 [34] minutes vs 335 [51] minutes), resumed spontaneous respiration earlier (326 [79] minutes vs 982 [52] minutes), and were extubated earlier (463 [79] minutes vs 1140 [58] minutes). VAS score, SS, and postoperative PaO2 were significantly (P less than or equal to 0.01) better in the TEA group. The incidence of tachycardia (15 vs 2 patients) and postoperative myocardial ischemia (12 vs 4 patients) was higher in the GA group. It is concluded that intraoperative and postoperative pain treatment with epidurally administered bupivacaine plus sufentanil improved the recovery time, as well as pulmonary and cardiac outcome after CABG, when compared with IV postoperative pain treatment after intraoperative GA with sufentanil and midazolam.

Ventilation and ventilators--an update

In the five years which have passed since the previous review, the literature has been concerned more with the ways in which ventilators may be applied to patients and the effects of differing patterns of ventilation than with the design philosophy of the ventilators themselves. This account should be read in conjunction with that of 1982 [1].